I push a ball which is at rest and then it starts moving. I have done some work on the ball. Assume that the plane is frictionless. The ball would continue moving, based on the principle of inertia, of course. But is inertia something like energy?

I’ve read that when we push a ball, our energy is stored inside it. What does "stored" mean here?

When I apply a retarding force on the ball, why doesn’t it stop immediately (as there is no force being applied from the other side, which can balance my force)? Please, if possible do not take inertia into count!

  • $\begingroup$ Because it requires a force to stop. A force generates acceleration and constant velocity means zero acceleration. So if you push a ball and let go, the ball will necessarily continue with zero acceleration but not necessarily zero velocity. $\endgroup$
    – Horus
    Commented Oct 2, 2015 at 16:56
  • $\begingroup$ @Horus Offcourse! But my question is different. Please have a look again. $\endgroup$ Commented Oct 2, 2015 at 16:57
  • $\begingroup$ Well if it stops immediately then it is an instantaneous stop resulting in infinite acceleration which means infinite force. $\endgroup$
    – Horus
    Commented Oct 2, 2015 at 16:59
  • $\begingroup$ Well thats the question . Why doesnt it stop immediately? Does the ball “now” contains a special energy? $\endgroup$ Commented Oct 2, 2015 at 17:00
  • 2
    $\begingroup$ I'm afraid now I do not understand what you're trying to ask. I have already told you why one cannot stop a ball instantaneously. $\endgroup$
    – Horus
    Commented Oct 2, 2015 at 17:03

1 Answer 1


Nothing is stored, actually.

Here, on Earth we are used to the fact that things "naturally" stop moving after a while (if we do not make them continue the movement somehow) "by themselves". But this is only apparent, as there is always certain force that prevents movement. This force is gravity often coupled with friction - gravity pulls the ball and the surface prevents further movement down (toward the center of Earth), but also - since the ball is "squeezed" by the gravity and the surface - it prevents the movement forward (parallel to the surface). It's like squeezing the ball inside a tube that's smaller in diameter (or rather between two surfaces).

That's the situation we are used to. We consider it normal, because we observe such behaviour everyday, since the childhood.

However, if there is no force opposing the movement (which we never experience on Earth) the ball will just keep moving. Because ... why shouldn't it? If there is nothing on the way, and no force working on the ball, it will just maintain the state it is in, i.e. movement.

This is what is known under the name of Newton's First Law of motion.

Why doesn't a force stop moving the ball immediately? If you place a wall on the way of a moving ball it will stop immediately. What about a force, like gravity?

Well, movement is always relative (to something). When you say a ball is moving, you usually assume it is moving with respect to Earth. So when you are sitting on a bench in a park and put a ball beside you, you will say it is not moving. But when you are on a train and have a ball with you, and put it beside you ... is it moving or not? Right, the answer depends on what you assume as the point of reference - the train or, say, a tree you are just passing by.

So, back to the question, everything depends on the relative movement of the observer. If he moves along with the ball and suddenly a force starts exerting its influence, he will say the ball started moving toward the force immediately. But if he sees the ball as moving (and the initial movement of the ball will be away from the source of the force), then he will see things differently - he will say it slowed down first, and then, after a momentary halt, begun to move toward the source with acceleration.

  • $\begingroup$ That solved many of my queries but what about this part - When i apply a retarding force on the ball, why doesn’t it stop immediately? ( as there is no force being applied from the other side, which can balance my force ) $\endgroup$ Commented Oct 2, 2015 at 17:19
  • $\begingroup$ Forces accelerate mass. Unless acceleration is infinite, it will take time to stop. $\endgroup$
    – BowlOfRed
    Commented Oct 2, 2015 at 17:19
  • $\begingroup$ It's like when you have a bucket filled with water, and drill a hole at the bottom of it. It will take some time before all the water pours out; the more water there is in the bucket, the longer it takes to get empty. But if your question is why doesn't the water vanish as soon as you drill a hole, then my answer is because that's how nature works. $\endgroup$
    – Mehrdad
    Commented Oct 2, 2015 at 17:26
  • $\begingroup$ @AaryanDewan, I edited my answer $\endgroup$ Commented Oct 2, 2015 at 17:30
  • $\begingroup$ There are errors in this answer. A moving ball does 'store' kinetic energy. That's why a wrecking ball can destroy a wall: it performs work on it. A wall doesn't 'immediately' stop a ball moving: even in the base of the strongest wall, the ball experiences a strong deceleration during a brief period of time. $\endgroup$
    – Gert
    Commented Oct 2, 2015 at 17:38

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