B Zero acceleration = zero net force?

[paulb203]

From Khan Academy;

“As truck moves with constant velocity down the road, its acceleration is zero.
Therefore, the net force acting on the box must be
zero.”

Which forces are involved in a truck moving with constant velocity (and therefore no acceleration)?

The gravitational force and the normal force cancel each other out.

But the applied force is greater than the frictional force, hence the truck moving, so how do the overall forces end up balanced resulting in a net zero force?

I’m thinking (wrongly, it seems) that the forces are unbalanced, that there is a net force involved, i.e, the applied force minus the frictional force, which is a non-zero amount.

P.S. There will be two friction forces; the friction between the tyres and the road; and the friction between the air and the truck overall. So the sum will be; applied force minus the two frictional forces, which will still give us a non-zero amount (which is wrong, it seems!)

 

[weirdoguy]

But the applied force is greater than the frictional force, hence the truck moving

They had to be in the beginnig, so that the truck could start moving with respect to the road, but once it moves, the net force can be zero. Air drag and rolling friction oppose component of gravity paralell to the road (I assume that road is not horizontal, since there is written "down the road", but I have a hangover so my english might not be englishing correctly).

applied force

What applied force? Applied by whom? Applied by what? One of the answers frequently given is "by the engine" but engine is part of the car, we are only interested in external forces.

 

[A.T.]

But the applied force is greater than the frictional force, hence the truck starts moving,

With the added word in bold, this is correct.

But for a truck, the propulsive force is a frictional force too. So your naming here is very confusing.

 

[Herman Trivilino]

But the applied force is greater than the frictional force, hence the truck moving,

No! This the lesson of 1st Law. Being at rest or moving in a straight line at a steady speed are indistinguishable. One appears different from the other only because of the state of motion of the observer.

 

[Nugatory]

since there is written "down the road", but I have a hangover so my english might not be englishing correctly).

That is one of many inexplicable English idioms - "down the road" generally means "along the road" with no implication of elevation change. However, "up the road" usually does imply an elevation change, although it will occasionally be used to mean "approaching along the road".

 

[Nugatory]

But the applied force is greater than the frictional force, hence the truck moving

If the applied force were greater than the frictional force the truck would gain speed. If the applied force were less than the frictional force the truck would slow down and gradually roll to a stop. It does neither, indicating that the frictional force is enough to prevent the applied force from accelerating the truck, the applied force is enough to prevent the frictional force from slowing the truck; the two forces must be equal and opposite.

 

[Ibix]

That is one of many inexplicable English idioms - "down the road" generally means "along the road" with no implication of elevation change. However, "up the road" usually does imply an elevation change, although it will occasionally be used to mean "approaching along the road".

Very dialect dependent, I'd say. In this context I'd have used "along the road" to avoid confusion, but I've heard people use both up and down the road to mean along the road.

 

[Lnewqban]

I’m thinking (wrongly, it seems) that the forces are unbalanced, that there is a net force involved, i.e, the applied force minus the frictional force, which is a non-zero amount.

You could replace the naming of applied and friction forces by propelling and movement-impeding forces.

That moves the focus away from the causes of those forces to the results regarding movement or repose of a studied body.

Examples:
If our truck is moving downhill or uphill, we must consider the influence of a component of the weight in the direction of its movement.
While moving at slow speed, there is no aerodynamic resistance to talk about.

Copied from:
https://en.wikipedia.org/wiki/Newton's_laws_of_motion#First_law

"Translated from Latin, Newton's first law reads,
Every object perseveres in its state of rest, or of uniform motion in a right line, except insofar as it is compelled to change that state by forces impressed thereon."

If our truck is not accelerating or not moving at all, you can rest assured that the summation of all the acting forces and components of forces aligned with the direction of the actual or potential linear movement equals a zero amount.

 

[paulb203]

They had to be in the beginnig, so that the truck could start moving with respect to the road, but once it moves, the net force can be zero. Air drag and rolling friction oppose component of gravity paralell to the road (I assume that road is not horizontal, since there is written "down the road", but I have a hangover so my english might not be englishing correctly).



What applied force? Applied by whom? Applied by what? One of the answers frequently given is "by the engine" but engine is part of the car, we are only interested in external forces.

Thanks.

Yes, elsewhere (Khan Academy) shows an example of this. A man kicks a swivel chair. It moves across a floor. But once he has kicked it, once his foot is no longer in contact with it, no longer applying a force, it continues to move across the floor. So the applied force ACCELERATED the chair. But the applied force was no longer required once the chair was moving. A friction force did, after a short distance of travel, cause the chair to decelerate (to negatively accelerate) and to soon come to rest (relative rest). Without any friction it would have continued to move, until it hit a desk, or a wall, or whatever. Is that correct?

Q. If it hit a wall and stopped what kind of force is that? Is that a pushing force, regards Newton's 3rd law (the chair pushes the wall, the wall pushes the chair)?

Regards the applied force. What about the driver pushing the accelerator (gas pedal)? Or the road pushing the tyres (is that Newton's 3rd law again; the tyres push the road, the road pushes the tyres)?

 

[paulb203]

With the added word in bold, this is correct.

But for a truck, the propulsive force is a frictional force too. So your naming here is very confusing.

Thanks.

I think I'm beginning to get the first part, that the unbalanced force is required to get the object moving, to accelerate it; and after that any unbalanced force is to counter friction (if we want to keep the object moving).

As for propulsive force; is this Google result accurate;

"Propulsive force is the force that causes an object to move, or change its motion. It can be created by pushing or pulling. "

You said it was a frictional force too, in this instance; what did you mean by that?

 

[paulb203]

No! This the lesson of 1st Law. Being at rest or moving in a straight line at a steady speed are indistinguishable. One appears different from the other only because of the state of motion of the observer.

Thanks.

I find this difficult to grasp. Khan gives an example. They compare a delivered box, on your doorstep, with another box, in the delivery truck driving away from your doorstep, at constant velocity.

The box on your doorstep, in the reference frame of you and your house, is at rest. And the box in the truck, in your reference frame, is moving away from you at constant v.

But from the reference frame of the truck, the box in the truck is at rest. And the box on the doorstep, in the truck’s reference frame, is moving away from the truck at constant v.

I might be getting this, a wee bit, when I imagine sitting in the truck, feeling at rest in relation to the other passenger/s, and the box/es, etc, in the truck, and watching the box on the doorstep disappear gradually into the distance.

But on the other hand I’m thinking/feeling that the box in the truck, more specifically, THE TRUCK ITSELF is moving in a different sense; it’s moving by virtue of a propulsion caused by burning fuel, by using energy, by internal combustion, pistons, rotating wheels, etc; whereas the box isn’t doing any of this, or having anything like it, done to it. It’s just kinda sitting there. And I'm thinking/feeling that the box on the doorstep just APPEARS TO ME to be moving away, but it's actually me that's moving away from it
Nb; I’m not disputing the physics, I’m just finding it difficult to grasp.

P.S. Does this have anything to do with the rotation of the Earth etc? Or would the above apply if the Universe consisted solely of the Earth, and the Earth didn't rotate?

 

[paulb203]

If the applied force were greater than the frictional force the truck would gain speed. If the applied force were less than the frictional force the truck would slow down and gradually roll to a stop. It does neither, indicating that the frictional force is enough to prevent the applied force from accelerating the truck, the applied force is enough to prevent the frictional force from slowing the truck; the two forces must be equal and opposite.

Thanks.
So when we're cruising along the motorway (highway) at constant v, our foot pressure on the accelerator/gas pedal is just sufficient to overcome the frictional forces between the wheels and the road, and the vehicle and the air?

 

[Nugatory]

But on the other hand I’m thinking/feeling that the box in the truck, more specifically, THE TRUCK ITSELF is moving in a different sense; it’s moving by virtue of a propulsion caused by burning fuel, by using energy, by internal combustion, pistons, rotating wheels, etc; whereas the box isn’t doing any of this, or having anything like it, done to it. It’s just kinda sitting there. And I'm thinking/feeling that the box on the doorstep just APPEARS TO ME to be moving away, but it's actually me that's moving away from it

The truck is moving relative to the surface of the earth. The box is not moving relative to the surface of the earth.

It is natural for us people confined the surface of a planet to consider things that are at moving relative to that planet to be “actually” moving and things that are rest relative to the local patch of surface to be “not moving”. But that’s a choice of convenience based on where we are at the moment, not a fundamental difference. The truck needs propulsion because its motion relative to the surface of the earth creates frictional forces between it and the earth, the box doesn’t.

P.S. Does this have anything to do with the rotation of the Earth etc? Or would the above apply if the Universe consisted solely of the Earth, and the Earth didn't rotate?

We don’t know for sure because we don’t have an otherwise empty universe to do experiments in. But that’s how we we would expect it to be.
When I’m trying to catch a thrown ball, I choose to consider the earth under my feet to be at rest while the ball is moving. When I make that choice I am also choosing to ignore the rotation of the earth, that the earth is moving around the sun, the sun is drifting through a rotating galaxy that in turn is moving in intergalactic space…. And I can get away with this because all of these cancel out when I calculate the only thing that matters, namely the speed and position of the ball relative to me.

A historical note: it was Galileo who first realized that “actually moving” is an illusion, what matters is relative motion.

 

[Nugatory]

Thanks.
So when we're cruising along the motorway (highway) at constant v, our foot pressure on the accelerator/gas pedal is just sufficient to overcome the frictional forces between the wheels and the road, and the vehicle and the air?

Yes. And we do it almost unconsciously: “ooops, speeding up, let’s lift off a bit”.

 

[jbriggs444]

[with reference to the propulsive force]

You said it was a frictional force too, in this instance; what did you mean by that?

So when we're cruising along the motorway (highway) at constant v, our foot pressure on the accelerator/gas pedal is just sufficient to overcome the frictional forces between the wheels and the road, and the vehicle and the air?

Let us look more closely at the propulsive force. Say we have a four wheel drive car. The propulsive force is friction from the road on all four tires. That friction acts in the forward direction on the car.

Meanwhile, air resistance is the only force acting on the car in the rearward direction.

Why is friction acting in the forward direction? Because static friction acts to oppose the relative motion that would exist in its absence. With the engine running, the tires would tend to spin rearward against the road. So friction with the road pushes them forward.

The accelerator pedal does not overcome friction between wheels and road. It overcomes air resistance.

Please digest this thoroughly before we open the chapter on "rolling resistance".

 

[Steve4Physics]

@paulb203, it should help if you first understand some simpler (non-rolling) situations. E.g. work out the forces acting on you when:
- swimming through water at constant velocity;
- walking uphill at constant velocity.

 

[A.T.]

and after that any unbalanced force is to counter friction (if we want to keep the object moving)

If it is just countering friction, then it is balanced by friction, not unbalanced.

And again, all horizontal forces from a level road are friction, regardless if they propel or brake.

You should start drawing free body diagrams, because your text descriptions are very confused.

 

[Herman Trivilino]

Thanks.

I find this difficult to grasp.

Think about riding in a jetliner, cruising at a speed of 600 mi/h at an altitude of 30 000 ft. As you rest in your chair you start playing with a ball. You toss it upward and it comes back down and lands in your hand.

An observer watching you from a mountain top with a super telescope would see you toss the ball up and in the fraction of a second it takes for the ball to return to your hand the jet and you move a considerable distance. To the observer the ball travels in a parabola, but to you it travels in a straight line. Who is right? You both are because you are looking at the ball while you are in two different states of motion.

Khan gives an example. They compare a delivered box, on your doorstep, with another box, in the delivery truck driving away from your doorstep, at constant velocity.

The box on your doorstep, in the reference frame of you and your house, is at rest. And the box in the truck, in your reference frame, is moving away from you at constant v.

But from the reference frame of the truck, the box in the truck is at rest. And the box on the doorstep, in the truck’s reference frame, is moving away from the truck at constant v.

I might be getting this, a wee bit, when I imagine sitting in the truck, feeling at rest in relation to the other passenger/s, and the box/es, etc, in the truck, and watching the box on the doorstep disappear gradually into the distance.

But on the other hand I’m thinking/feeling that the box in the truck, more specifically, THE TRUCK ITSELF is moving in a different sense; it’s moving by virtue of a propulsion caused by burning fuel, by using energy, by internal combustion, pistons, rotating wheels, etc; whereas the box isn’t doing any of this, or having anything like it, done to it. It’s just kinda sitting there. And I'm thinking/feeling that the box on the doorstep just APPEARS TO ME to be moving away, but it's actually me that's moving away from it
Nb; I’m not disputing the physics, I’m just finding it difficult to grasp.

Think of the package coasting at a constant speed in a vacuum, or at a nearly constant speed on a very low friction cart or air track.

P.S. Does this have anything to do with the rotation of the Earth etc? Or would the above apply if the Universe consisted solely of the Earth, and the Earth didn't rotate?

It's due to a basic nature of reality.

 

[Herman Trivilino]

A friction force did, after a short distance of travel, cause the chair to decelerate (to negatively accelerate) and to soon come to rest (relative rest). Without any friction it would have continued to move, until it hit a desk, or a wall, or whatever. Is that correct?

Yes. And if there were less friction the chair wouldn't slow down as much. With very little friction, such as sliding on ice, or hovering over a surface like a hovercraft would, the friction might be so little that your measuring instruments detect no decrease in the chair's speed.

Extrapolate this to the case of no friction and you have the chair moving at a steady speed forever.

Edit: Go to YouTube and search for "chair hovercraft".

 

[paulb203]

@paulb203, it should help if you first understand some simpler (non-rolling) situations. E.g. work out the forces acting on you when:
- swimming through water at constant velocity;
- walking uphill at constant velocity.

Thanks, Steve.
I'll give them the swimming one a go first.

Vertical forces. Buoyancy and gravity. Net force of zero.

Horizontal forces. Question, regards applied force. Is it only an applied force when it is one object applying a force to another? A man pushing a box, or a wall pushing a man, for example? If so, when I’m swimming there is no applied force? I’m the cause of my own propulsion (?) or thrust (?).

Is my movement through the water down to Newton’s 3rd law? I push the water backwards, the water pushes me forward?

Where does friction come in?

And if I’m moving at constant v there is no acceleration therefore no net force? So the forward pointing forces must be cancel out by the backward pointing forces? Is my own pushing force cancelled out by the friction (pushing?) force of the water?

 

[paulb203]

If it is just countering friction, then it is balanced by friction, not unbalanced.

And again, all horizontal forces from a level road are friction, regardless if they propel or brake.

You should start drawing free body diagrams, because your text descriptions are very confused.

Thanks, A.T, for your patience, and support.
Is this FBD anywhere close?

Correction; Fg PLUS Fn. Fth PLUS Fd.

And, I'm thinking now that should be Ff, not Fth.

And as well as Fd there should be another Ff on that side.

 

[paulb203]

Think about riding in a jetliner, cruising at a speed of 600 mi/h at an altitude of 30 000 ft. As you rest in your chair you start playing with a ball. You toss it upward and it comes back down and lands in your hand.

An observer watching you from a mountain top with a super telescope would see you toss the ball up and in the fraction of a second it takes for the ball to return to your hand the jet and you move a considerable distance. To the observer the ball travels in a parabola, but to you it travels in a straight line. Who is right? You both are because you are looking at the ball while you are in two different states of motion.


Think of the package coasting at a constant speed in a vacuum, or at a nearly constant speed on a very low friction cart or air track.

It's due to a basic nature of reality.

Thanks, Mister T.

The parabola/straight line (both!) thing is fascinating, and exciting. I love it.
If I throw a ball up now as I sit here on this couch, which is on a rotating Earth, the Earth will have moved by the time the ball returns to my hand; what are the implications there regards straight line v parabola?
Is me on the plane analagous to me on the Earth, so it's a straight line from my perspective? (The Earth is a kind of vehicle, like the plane?). But it could be a parabola from a distant observer (not on the Earth)?

Regards it being due to a basic nature of reality; if the Universe consisted solely of our planet, and let's imagine no atmosphere or sky or anything; just Earth, and it didn't rotate; would the same still apply regards the two boxes? Would the box on the doorstep still be moving away from the truck - in the reference frame of the truck?

 

[paulb203]

Yes. And if there were less friction the chair wouldn't slow down as much. With very little friction, such as sliding on ice, or hovering over a surface like a hovercraft would, the friction might be so little that your measuring instruments detect no decrease in the chair's speed.

Extrapolate this to the case of no friction and you have the chair moving at a steady speed forever.

Edit: Go to YouTube and search for "chair hovercraft".

Thanks.

Really interesting, that the chair would move at a steady speed forever, if there was no friction. Like a kind of perpetual motion? Some force required to accelerate an object, but then, with no friction, it's perpetual motion, or motion until the object disintegrates or the Universe ends?

I've heard there is no such thing as a perfect vacuum, that there is always either a small amount of matter in any region of space, and there is EM radiation, not to mention strange quantum phenomena; would those small amounts of matter, and the radiation (and the quantum phenomena) cause enough friction to eventually slow an object to rest (relative rest) - I'm talking about an object travelling at constant v in space, light years from any other object.

 

[Steve4Physics]

Vertical forces. Buoyancy and gravity. Net force of zero.

Yes

Horizontal forces. Question, regards applied force. Is it only an applied force when it is one object applying a force to another?

Depends on how you are using the word 'object'. Is the air surrounding you an 'object'?

For a given object, I would say an 'applied force' is any external force acting on the object. It doesn't matter how the external force is produced.

A man pushing a box, or a wall pushing a man, for example?

If a man pushes a box, the box is also pushing the man (Newton's 3rd law). The applied force on the box is the contact force from the man. The applied force on the man is the contact force from the box.

How the man and box then move depends on the other applied forces (typically friction) acting on them . If the box is on very rough ground and the man is on slippery ice, maybe only the man will move (backwards).

(Also worth noting is the fact that the energy is coming from the man is irrelevant.)

The same applies to man/wall.

If so, when I’m swimming there is no applied force? I’m the cause of my own propulsion (?) or thrust (?).

There are two horizontal applied (external) forces, both produced by the water:

a) Drag (friction from the water) acting backwards, trying to make you slow down; if you stop moving your arms and legs, drag will decelerate you to a halt.

b) Thrust (forward force of the water on you). This arises because when you are swimming, your arms and legs apply a backwards-directed force on the water. From Newton's 3rd law, the water applies a forwards-directed force on your arms and legs: that force is the thrust.

Is my movement through the water down to Newton’s 3rd law? I push the water backwards, the water pushes me forward?

Yes - exactly!

Where does friction come in?

And if I’m moving at constant v there is no acceleration therefore no net force?

When swimming at constant velocity, the drag and thrust have equal mangitudes. If either changes there will be a net force causing acceleration.

So the forward pointing forces must be cancel out by the backward pointing forces?

For a constant velocity, yes.

Is my own pushing force cancelled out by the friction (pushing?) force of the water?

Your pushing force (force applied by you to the water) is always equal in magnitude to the thrust of the water on you. At constant velocity this is the same magnitude as the friction.

If you then push a bit harder, the water pushes a bit harder (thrust increases); the net force on you increases and you accelerate.

But drag increases with speed so the net force quickly comes back to zero and you now have a new, faster, constant velocity.

Minor edits made.

 

[jbriggs444]

Question, regards applied force. Is it only an applied force when it is one object applying a force to another?

I consider both "applied" and "reaction" as meaningless noise words. A force is a force. It does not matter whether a force arises because of something I did or because of something you did. It does not matter whether the force comes from an identifiable rigid object or from a fluid.

By contrast, "external" has meaning. For an external force, you must have identified a system of interest. An external force is one that acts between something within the system and something outside the system.

Is my own pushing force cancelled out by the friction (pushing?) force of the water?

Your hands will (typically) be moving backward through the water. The water will be pushing forward on them. Your body will (typically) be moving forward through the water. The water will be pulling backward in it. If you are swimming at a constant average speed then the two will balance out on average.

If you are a proficient swimmer, you may be using your hands as water foils and gaining some thrust from lift rather than purely from drag.

 

[A.T.]

Thanks, A.T, for your patience, and support.
Is this FBD anywhere close?View attachment 357475

Correction; Fg PLUS Fn. Fth PLUS Fd.

Yes, add the vector arrows above all force symbols, and use + between them.

And, I'm thinking now that should be Ff, not Fth.

And as well as Fd there should be another Ff on that side.

You can split those 4 general forces into as many parts as you want, depending on the question and information you have. But this is the general force balance for a vehicle at constant velocity.

 

[Herman Trivilino]

Really interesting, that the chair would move at a steady speed forever, if there was no friction.

That's all there is to it. Don't overthink it and get lost in the weeds.

 

[Herman Trivilino]

Is my movement through the water down to Newton’s 3rd law? I push the water backwards, the water pushes me forward?

Yes, except that since you are moving at a steady speed in a straight line there is no need to go looking for a force that makes that happen. When you're at rest you don't go looking for a force that makes that happen. Being at rest or moving in a straight line at a steady speed are equivalent. Equivalent! They only appear to be different because of the observer's state of motion.

The water pushes forward on the parts of your body you use to propel yourself (your hands, arms, feet and legs) and it also pushes backwards on you (drag force). To an observer at rest on the shore you are moving forward. Precisely the same thing happens to you if the water is moving and you are propelling yourself against the current fast enough so that to an observer at rest on the shore you are not moving.

 

[paulb203]

Thanks a lot, guys.
Loads for me to think about.

 

[sophiecentaur]

What applied force? Applied by whom? Applied by what? One of the answers frequently given is "by the engine" but engine is part of the car, we are only interested in external forces.

That's a bit nit-picky. The engine produces a force (torque) which is transmitted as a force to the tread on the tyres which, via the friction, pushes the road backwards. (etc. etc.)

Many of the above posts seem to have a problem with the word 'friction'. There seems to be a subconscious need for friction to be a loss mechanism. In the case of an ideal road wheel / tyre all the ForceTimes Distance relative to the footprint produces a traction with no loss. Only when you have some slippage is there any FTD and that there is loss of energy / wasted Work.

I consider both "applied" and "reaction" as meaningless noise words. A force is a force.

Agree. There are so many discussions about this sort of situation and someone introduces a causal chain and a 'who dunnit?' question. It's the same with Work By and Work Done. It's just Work with a sign which depends on your measurement frame.

 

[A.T.]

It's the same with Work By and Work Done. It's just Work with a sign which depends on your measurement frame.

The sign of work doesn't just depend on the reference frame, but also on whether the value represents work done by A on B, or by B on A. These are two completely different issues, that can affect the sign of work.

 

[sophiecentaur]

These are two completely different issues

Maybe but they are both viewed in an intuitive way.

but also on whether the value represents work done by A on B, or by B on A.

It's only by knowing the displacement and the force that you can decide what's on what. In a scenario of say two jet engines pushing against each other you can pass through the condition where the net FTD changes sign. Or are you saying that the frame of reference is involved? Perhaps if you made your statement in a different way, I'd understand what your are saying.

 

[A.T.]

Maybe but they are both viewed in an intuitive way.

That is no reason to conflate them.

It's only by knowing the displacement and the force that you can decide what's on what.

Yes, from these two you can compute work.

Perhaps if you made your statement in a different way, I'd understand what your are saying.

This has been explained to you so many times in previous threads. Just reread them instead of hijacking another one.

 

[berkeman]

This thread has run its course, and after some editing it has been closed. Thanks to all who tried to offer clear explanations.