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Let's say 10 kg block is sliding on a frictionless surface at a constant velocity, thus its acceleration is 0.

According to Newton's second law of motion, the force acting on the block is 0:

$a = 0$

$F = ma$


So let's say that block slid into a motionless block on the same surface, the motionless block would move.

Wouldn't the first block need force to be able to move the initially motionless block? I understand that it has energy due its constant velocity, but wouldn't it be its force that causes the displacement?

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Related: and links therein. – Qmechanic May 7 '14 at 21:24
There was no force on the block until it came into contact with the motionless block. – Immortal Player May 8 '14 at 1:18
up vote 2 down vote accepted

Here's a slightly different but equivalent way to think about it.

Forces describe interactions between two objects. If two objects are interacting, they exert forces on each other. If two objects are not interacting, they do not exert forces on each other. Thus, an object doesn't "carry around" a force with it. A force is not a property of an object, just as dmckee explains. Instead, we describe interactions between two objects using the more-abstract concept of force.

In your block-hits-other-block scenario, it's tempting to ask where did the force come from if colliding object had $F_\text{net}=0$? But when forces are viewed as interactions, it becomes more apparent that the force didn't come from anywhere within one of the objects. There simply wasn't an interaction before they collided, so we wouldn't ascribe the existence of a force force.

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The zero force related to zero acceleration is not a property of the object, it is a statement about the forces acting on the body. That is your title should not read "has no force" but "is subject to no net force".

If a body has a non-zero, but constant, velocity then we know that the total of all the forces applied to it is zero (from Newton's Laws).

We also know that is has non-zero "momentum", and when it collides with another object some (or all) of that momentum can be transferred to the other object. During the collision the body is subjected to new forces and the net force is no longer zero meaning that it will accelerate.

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When the body is subjected to new forces during the collision, what are those forces or where do those forces come from? – mzee99 May 7 '14 at 20:55
They are the same kinds of forces that prevent a book from falling through a table. We often call them "contact forces". At one level you can think of them as coming into being because atoms and molecules in a solid want to maintain their approximate distance from one another so that the body resists being deformed. – dmckee May 7 '14 at 21:33
@mzee99: You should read the wiki page on normal force. – Immortal Player May 8 '14 at 1:24
@dmckee: Sometimes can we say that a body accelerates by the virtue of its property? Say for example, I am running. I am not accelerated by gravity or any other force which has law. – Immortal Player May 8 '14 at 1:35

When first block which is already in motion slides into a motionless block then its momentum changes...momentum=mass*velocity and force=rate of change of momentum so as momentum changes force exerts on both object in opposite direction with same magnitude so their momentum changes and so velocity also changes....

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If a body is moving with constant velocity, acceleration is zero. So net force acting on it will be also zero. But the body has energy due to its constant motion.

Take the case of a freely falling body: it reaches its terminal velocity (120mph) when gravity = air resistance (drag). So it doesn't have net forces acting on it. But it hurts if it falls on us. It is because it has a momentum, $p=m v$. the momentum is imparted to the second body which makes it accelerate. So even if the body is moving with constant velocity, it can impart its momentum to other body.

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