# Silly question about Newton's laws of motion

1. Feb 20, 2014

### Matt2411

I'm sorry I didn't use the outline provided automatically by the thread. My question does not focus on the maths; sorry if I chose the wrong section (I'm new here).

Anyhow, what I'm asking is: Why does Newtonian Physics state that a moving object with uniform velocity (abiding by the definition of "inert" - not being accelerated; maintaining its state) has balanced forces acting upon it? I can understand why that is the case of a stationary object, but it does not make sense for me when talking about a moving one. The way I see it, if the body is doing distance over time then evidently that can only happen while it's affected by a net force. Say we're talking about a car (I know, the most typical example but oh well); if it's running then it must be because of the force of the gas overcoming the force of friction (and the force of the air? Is that correct?). If it has balanced forces, then it must necessarily stop. Right?

I'm pretty sure I must be thinking about it wrongly. Or maybe I got bad sources.

I would really appreciate some help :) I know it must be something really basic for you guys, but it would really help this amateur learning science as a hobby.

2. Feb 20, 2014

### Andrew Mason

This is not really a silly question. At least it wouldn't have been considered silly about 400 years ago.

The great accomplishment of Galileo and Newton was to show the superiority of physical experiment over philosophical reasoning to determine the laws of physics.

The first law of motion is based mainly on Galileo's experiments. He observed that the laws of physics were the same on a ship moving on calm seas as on land.

If a car is coasting, it will slow down precisely because there are net forces acting on it tending to slow it down.

Welcome to PF, by the way.

AM

3. Feb 20, 2014

Welcome to PF
Newton's first law states that "Every body stays at rest or moves in a constant velocity in a straight line unless an external force acts on it"(External force meaning net force)
So Once an object starts moving,no net force is required to keep it moving.
For example,a car is moving with a constant velocity.So the net forces are zero.
But at the start,the car has to have a non zero net force to accelerate to a certain velocity.After that,the net forces become [STRIKE]equal[/STRIKE] and the car moves with the velocity it had at the time net forces became [STRIKE]equal[/STRIKE]
EDIT:Sorry it should be zero instead of equal...

Last edited: Feb 20, 2014
4. Feb 20, 2014

### Matt2411

Thanks for the welcome guys :)

Yes, I understand that net forces are what would make the car accelerate or decelerate. But I was specifically referring to the moment when the car is doing neither of the two, yet moving in constant velocity. If I understand Newton's first Law correctly, then that car has no net forces acting on it. That sounds counter-intuitive for me, because if the force produced by the gas equals the force of friction (thus having no net forces) then the car should stop, shouldn't it?

My sincerest apologies if I don't explain myself clear enough-

5. Feb 20, 2014

No.If the car is moving when the net force gets to zero,it will stop accelerating or decelerating and move with the velocity it had at the time net force became zero.

6. Feb 20, 2014

Once the car is moving,it would need a deceleration to stop.No net force means it is not decelerating or accelerating.

7. Feb 21, 2014

### jackarms

I like to think of it mathematically too:
$$\Sigma F = ma$$, or $$a = \frac{\Sigma F}{m}$$
Acceleration means change in velocity, so if there's no acceleration, then the velocity must not change. (Just like velocity means change in position, so if there's no velocity, then the position must not change.) From the equation, acceleration can only equal zero if $\Sigma F$ (net force) equals zero (or, alternatively, if the object has infinite mass, but that's another story). So when net force is zero (i.e. all acting forces cancel) then velocity will not change.

Another way to think of it is in terms of energy (if you've gotten there yet). Forces are like energy pumps -- either sucking energy from an object or pumping energy into it. Kinetic energy is one type of energy tied to objects' velocity. If an object has balanced forces on it, then, using this idea, it's energy should not change, since the forces would also balance in their transfer of energy to the object. If the object slowed down, it would lose energy, and so the question would be: where did that energy go? That depends on the situation, but the pathway for the energy would have to be an unbalanced force that disrupts the balance of energy created by net forces.

And, finally, talking about it in a more intuitive, practical scenario, Think of the same car moving at constant velocity along a road -- a real life road, so there's friction, and eventually the car will come to a stop. To prevent this, you have to step on the gas, applying more force to the car. If you apply a very small amount of gas, friction will still slow the car down. You have to ever so slightly increase your force from the gas so that you just barely match the force of friction, at which point you can coast -- equal forces from your foot with the gas and friction on the road.

8. Feb 21, 2014

Good point(s) jackarms

9. Feb 21, 2014

### collinsmark

'Just wanted to point out that forces are vectors with both magnitude and direction. And the direction is important. If the car has one force acting on it in one direction, and another force of equal magnitude acting on it in the opposite direction, then the car has two forces acting on it -- yet the net force is zero. The two forces together sum to zero, since they are equal in magnitude yet opposite in direction. Thus the car moves at a constant velocity.

Sometimes three, four or more forces can all sum together to make a net force of zero.

Last edited: Feb 21, 2014
10. Feb 21, 2014

### Andrew Mason

If,

a) as Galileo observed, the laws of motion are the same on a ship moving uniformly on calm seas as in a stationary laboratory, and

b) it does not take a force to keep a body at rest,

then:

c) it does not take a force to maintain a body in uniform motion.

Unless you can do an experiment to show that Galileo was wrong and that the laws of physics are different in a uniformly moving frame of reference, then logic should compel you do abandon your "intuition".

AM

11. Feb 21, 2014

### Matt2411

Ok, so then I'm having problems accepting that statement. It sounds difficult to concede that a car in uniform velocity is having no net force acting on it.

So then shouldn't it stop? If both forces cancel out then the car should not move. Why do we say it keeps a constant velocity?

12. Feb 21, 2014

### PhotonicBoom

Think of the car as a system. And the 2 forces acting on it are external forces. Your system can still have internal energy in the form of Kinetic Energy right? It would not make sense that by setting up 2 opposite but equal forces to stop your moving system because what you are doing essentially is giving it a bit of energy and removing the same amount of energy from the other end.

13. Feb 21, 2014

### Andrew Mason

Ok. Let's start out with an easy question: Does a car at rest need to have a force to keep it at rest?

Now: Does the same car at rest on a ship that is moving at a uniform velocity on calm seas need to have a force to keep it at rest relative to the the ship?

AM

14. Feb 22, 2014

### Matt2411

No... but isn't that different? In that case, the ship is the body that's moving (the one with energy). But in my example, the car had to exert a force itself. Right?

15. Feb 22, 2014

### Dick

Think about motion in the vacuum of space far from any gravity or anything. There are no external forces. Does a moving object spontaneously come to a halt? Aristotle might have thought so. We don't.

16. Feb 22, 2014

### vela

Staff Emeritus
When you take your foot off the gas pedal, the car slows down because of friction. In terms of forces, what's going on is if we add up all the forces acting on the car, the only one that doesn't cancel with another is the force of friction. So there's a non-zero net force on the car, and it causes the car to slow down. To keep the car from slowing down, you give it some gas. Now when we add up all the forces, we have a new force to include, the force produced by the engine. That force cancels the force of friction so that the net force on the car is 0. When there's no net force, the car moves with constant velocity. You seem to be confusing the net force with the force produced by the engine.

Our everyday experience tells us that to keep an object moving, we have to keep pushing on it. But that's only because friction is always present. If there were no friction, then there'd be no need for the engine to exert a force to keep the car moving at constant speed. You need to unlearn what you already "know", that to maintain a constant velocity requires a force. It doesn't.

17. Feb 22, 2014

### Andrew Mason

But the car at rest with respect to the ship is moving at constant velocity relative to the water and land. As you correctly noted, no force is needed to keep it at rest relative to the ship ie. to keep it in uniform motion relative to the land/water. That is a complete answer to your question. This is essentially how Galileo came to formulate the first law of motion.

AM

18. Feb 23, 2014

### Simon Bridge

@Matt2411: I suspect you have a semantics problem.
In common use, a "force" is "what you have to do to get movement". To keep the car moving, you have to press down on the gas pedal. Pushing a little gets a constant speed and pushing a lot gets you an acceleration.
This makes sense intuitively and it is how we normally use language.

In physics we use a different definition ... a careful and precise one.
I want to stress this: the word "force" in physics does not mean the same as it does in common language.
You are wrestling with this difference and seem to have reach a stage where the two meanings have become mixed up.

The word "force", in physics, is defined as being "the rate of change of momentum".
In practice this means that F=ma is the definition of what physics means by the word "force".
It is the thing you have to have to get an acceleration.

If the acceleration is zero - then the force is zero. By definition.

In physics if the car slows down when you take your foot off the gas, then the gas was providing a balancing force for whatever is now slowing the car down. By definition.

The reason this makes scientific sense is because of what you you find when you try to figure out where the forces are coming from.

The classic experiments were done by Galileo - he rolled balls down ramps and up another ramp, being very careful to make sure there was very little by way of obvious opposing force. Using your idea of force. You can try this yourself.

He discovered that the ball would accelerate down the first ramp at a constant rate, then it went along the flat at a constant speed, then it would decelerate up the second ramp until it reached the same height as it started out at on the first ramp.

What is the force on the ball when it is on the flat? It does not slow down. Common sense dictates that there must be something pushing it but what?

Galileo tried putting the second ramp at different angles, and found the ball went to the same height no matter what. If the second ramp was shallower than the first, the ball had to travel further to get to the same height.
He argued that if the second ramp were at zero degrees angle, then the ball would keep rolling for ever.

Galileo realized that common sense is what tells you that the Earth was flat, and that the Heavens revolved around the Earth. The experiments made sense if he discarded the common-sense idea about how motion happened and we end up with our current definition for force.

I don't blame you for finding it hard to swallow.
All I can say is that you can either take everyone's word for it or do the experiments yourself and see if you can do better.