Energy conservation

Doc Al

You are welcome to ponder the "ultimate" source and meaning of inertia, but first learn a bit of physics.


Somewhere you've picked up the misconception that a force is required to keep you moving forward. Some ancient Greeks thought this, but we've since discovered otherwise.

You seem nominally interested in physics, but not enough to pick up a textbook and learn what we already know. Constant velocity does not require a force!
Nope. Again: Learn (or at least read about) Newton's laws. Learn what we mean by a "force". A force requires an agent: something doing the pushing or pulling. If you are moving at a constant velocity either all the forces acting on you have canceled out or there are no forces acting on you. (For the latter, imagine a spaceship coasting in outer space, away from all masses.)

What's relevant here is whether the body is accelerating or not.

newTonn

What would have been the scenario if human beings stopped thinking out of box(from what they learned till then).?

Tell me why you require a force to stop an object moving at uniform velocity?
With your force you have to cancel some other force.what is the other force.it is inertia.if you try to find an equation to find out that force,you will end up with my conclusion.

Ok.A force require an agent.yes did i told you no?
To acheive a constant velocity,you require a force .isn't it?Agents role is finished there.
shall i explain?
consider a golf ball (at rest-relatively).
you hit it with the club(sorry if i am right-club is the stick i mean)
Now the ball is accelerating positively ,then negative acceleration and ultimately comes to a hault.
If you see any intervals,you can see there is a force acting on the ball.
But you cannot say,the club was hitting on the ball throughout the way.
Learning is something and understanding is something else.
Please don't undermine peoples.There is something to learn from every layman.

Doc Al

You should open the box and see what's in it. But first you must find the box.

If you are asking why the world is the way it is, I can't answer that.
There is no force (at least no net force) acting on an object moving at uniform velocity. So there is no "other force" that you have to cancel.
Your conclusion is based on misunderstanding.


No!

I have no idea what this example is supposed to tell us. If you wish to accelerate the golf ball, you must exert a force on it. True. So?
Might I dare suggest that there is something to learn from studying basic physics?

newTonn

THE CLUB IS NOT HITTING THE BALL THROUGHOUT THE WAY OF BALL.BALL IS MOVING BECAUSE AGENT(CLUB) EXERTED A FORCE ON IT IN PAST,BUT THE FORCE REMAINS UNTIL THE END OF EVENT(UNTIL FORCE DIMINISHES AND BODY COMES TO REST WITH THE EFFECT OF GRAVITY).This has to be true if there is no gravity as well.
Now if you couldn't understand what my example is telling you..

(We can wake up a man who is sleeping.But we cannot wake up a man who is pretending so.)

Doc Al

Using bold caps does not help.

As soon as the club loses contact with the ball, it no longer exerts a force on the ball. If there are no other forces acting on the ball--such as gravity and air resistance--the ball will continue in a straight line at constant speed forever. But there are other forces acting on the ball. And when the ball hits the ground, the ground and grass exert other forces on the ball, eventually bringing it to rest. Once the ball is at rest, the net force on it is zero. So what?

Can't you hear the alarm clock ringing? Time to wake up!

russ_watters

It's over there!! You can lead newTonn to a box, but you can't make him open it?

Nancarrow

Except that it isn't true.

Er, sorry, I meant:

EXCEPT THAT IT ISN'T TRUE.

Ariste

Using your golf ball example:

You seem to think that once the ball is in the air, there must be some force acting on it to keep it moving. Unfortunately, Isaac Newton has told us that this isn't true.

The club exerts a force on the ball and accelerates it. It does work on the ball. This work is the transfer of energy from the club to the ball. After this work is done, the ball contains kinetic energy, with a magnitude of 1/2mv^2.

Work is then done by various forces to stop the ball. This work is done against the energy that was transferred to the ball by the club, not some force that is keeping the ball in motion. The ball, according to the law of inertia, will stay in motion until some net force acts upon it. In this case, air resistance and friction (once the ball hits the ground) will cause the ball to stop moving. Again, the work done by air resistance and friction is done against the work the club originally did on the ball, not against any force that has continued to act on the ball throughout its flight.

Here's where you're wrong: the force does not remain with the ball throughout its flight. The energy transferred to the ball by the force remains with the ball until it stops moving. The club does work on the ball, transferring energy to it. Once the ball is in the air, there is no force acting on it in the direction that it was originally hit. The forces of friction and air resistance now perform work against this moving body. This work is done to counteract the work that was originally done by the club - against the kinetic energy that the ball now contains. The work of air resistance and friction is not against any continuing force on the ball.

And just to be clear, gravity does not bring the ball to rest. It brings the ball down. Friction and air resistance bring the ball to rest.

K.J.Healey

Remember also that when you hit a golf ball, the ball accelerates WHILE the club is touching the ball. As soon as it leaves the face of the club, it WILL NOT SPEED UP. It will maintain its velocity forever and ever and ever. Unfortunately, there are natural FORCES that will CHANGE THE VELOCITY of the ball. Thats what a force does, by definition. It CHANGES VELOCITY. F = MA where A is the CHANGE IN VELOCITY. So this ball that would go forever and ever at some constant velocity V unfortunately gets pulled on by Gravity and Air resistance. Which Change its Velocity V.

If you are applying a SINGLE FORCE on an object, it will forever ACCELERATE and its velocity will always increase (assuming no speed limit of light, but don't even ask about that yet).

You have to realize that THAT is what a force is.

rcgldr

Because any change in velocity will require a force, whether it's to increase or decrease the velocity.

The other force is the "reactive force" of an accelerating or decelerating object, which correpsonds to inertia. This acceleration or deceleration can be linear (rate of movment) or angular (rate of rotation). When forces applied to an object do not cancel each other out, the object's velocity and/or rate of rotation changes. This "reactive force" is always equal to the imbalance in forces applied to an object, regardless of the inertia of an object. Inertia (mass times linear and angular movement) isn't considered to be a force, just a factor into how an object will react to external forces (with mass determining the rate of change versus applied forces).

newTonn

Could anybody draw a freebody diagram of the ball ,just after the club hits the ball.

newTonn

Please coool and try to solve this problem with a freebody diagram(at any instance after the club hits ball).and get confirmed that without showing the force exerted by club,the ball should move in opposite direction.(because all other forces you are mentioning here are acting on the opposite direction).
Forget about the ball.
If you found a body which is accelerating with respect to you,Do you say a force is acting on the body?(we dont know wheather it was hit by something,pulled by something etc)

russ_watters

Sorry, newTonnn, there is a time interval there. The ball is in contact with the club for a finite and measurable amount of time, during which it accelerates.

russ_watters

Assuming you mean after the ball leaves the club-face, the ball will then be in motion with only the force of gravity acting on it. The club is no longer accelerating it.

Yeah, the golf ball accelerates really quickly when hit by a golf club.

Ariste

No, no. You're not understanding.

After the ball leaves contact with the club, it has a pretty large velocity in what we'll call the forward direction. Yes, all other forces acting on the ball after it leaves the club are acting in the opposite direction, but these only act in response to the ball's movement relative to them. Through air resistance and friction, which act in the opposite of the direction of the ball's flight, the ball is brought to rest.

I think you are asking me why the ball does not begin to go backwards the other way after friction and air resistance bring it to a stop. You're saying that, if all forces acting on the ball after contact with the club (friction and air resistance) are pushing the ball backwards, the ball should slow to a stop and then begin to move backwards. As you can see in everyday life, though, this isn't the case.

Air resistance and friction only act on a body that is in motion relative to the agent of the air resistance or friction. When you push a box across the floor, the frictional force pushes against your push. When the box is just standing still, though, the frictional force doesn't push it backwards, or in any direction at all. Friction only acts against an applied force. Same with air resistance.

In the case of the golf ball, air resistance and friction bring the ball to rest, and once it's at rest, these forces cease acting, and the ball stays at rest. If you want to understand this in terms of free-body diagrams, you'll need to make at least two. After the ball leaves the club, there will be a force directed against the ball's motion (air resistance) and a force straight down (gravity). This diagram will stay pretty much the same until the ball hits the ground, when the frictional force will add to air resistance and create a greater force opposite the ball's motion. Once the ball comes to a rest, however, there will only be two forces acting on the ball in the diagram: the force of gravity straight down, and the normal force straight up. These forces will be exactly balanced, and there will be no other forces because the ball is not moving and thus friction and air resistance will not act on the ball. Since all forces are balanced, the ball will stay at rest.

rcgldr

How about a very slow motion video of the impact? The club doesn't just hit the ball, it compresses the ball about 10% (iron) to 25% (club), and the ball springs back with a reasonably elastic reaction, so the ball leaves the club at a much higher velocity than the club is moving (club at 100mph, ball leaves at 170mph). Many golf broadcasts include "swing videos", such as this one, at the end is a super slow motion close up of the impact. I read that the duration of the interaction is about 5 milliseconds, (1 / 200th of a second), pretty quick. Note that the ball has bounced off the club about the same time that the club has only moved about 1/3rd of it's width over the tee.

http://www.youtube.com/watch?v=XPJtK-DxnV4

As already posted, once the ball leaves the club, then the forces acting on it are aerodynamic drag and gravity, until the ball impacts with the ground (or trap, tree, spectator, ... ).

Ariste

Hey, pretty neat video :) I didn't realize the ball compresses like that.