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?


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.


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.

  • $\begingroup$ When the body is subjected to new forces during the collision, what are those forces or where do those forces come from? $\endgroup$ – mzee99 May 7 '14 at 20:55
  • 2
    $\begingroup$ 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. $\endgroup$ – dmckee --- ex-moderator kitten May 7 '14 at 21:33
  • $\begingroup$ @mzee99: You should read the wiki page on normal force. $\endgroup$ – Immortal Player May 8 '14 at 1:24
  • $\begingroup$ @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. $\endgroup$ – 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....


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.


You are confusing the roles of force and energy.

An applied force is an effect that if unopposed will cause a body to accelerate.

Energy is the property that gives the ability to apply force.

If you have two motionless blocks on a surface, neither has energy and neither can affect the other.

If either or both blocks are moving on the surface they have kinetic energy. If the blocks collide they apply equal and opposite forces to each other during the collision, in accordance with Newton's laws, and generally there will be a redistribution of energy and momentum between them, depending upon their relative masses, angle of incidence etc. If a moving perfectly elastic block hits an identical stationary one head-on, all of its energy and momentum is transferred by the collision to the stationary one. The previously stationary block now carries all of the ability to apply force, and the originally moving block no longer has that ability.


I suspect the confusion arises from choosing reference frames. Let's try different views on the same scenario.

(1)Let the system consist of a block 1 that is sliding on frictionless surface with no external forces acting on it before the collision. (Here I assume that normal force and weight do cancel out, and thus are not included in 'external forces' part.)

When it collides with standing block 2, the forces act on a system. Thus forces are no longer zero and momentum changes.

Very similar scenario happens when we let block 1 stand and block 2 collide:

(2)Let the system consist of a block 1 that is stationary on frictionless surface. Since it is not accelerating, sum of forces acting on it are zero. When block 2 collides with it, forces act on block 1 and momentum is no longer conserved.

(3)Finally consider system of block 1 that is moving on frictionless surface and block 2 that is standing in line of motion of block 1. Assume no external forces act on a system. Therefore: $$\vec F^{ext}=0\implies \vec v=const.\qquad\text{conservation of momentum applies, let mass of block i = }M_i$$ $$M_1\vec v_o=M_1\vec v_1 + M_2\vec v_2$$

Therefore in order to find out how the system behaves, first and foremost we need to define that system.


Let's for reference say $A$ moves with a constant velocity and hits $B$ (which is at rest).

When $A$ is moving with some constant velocity, it has no force acting on it (because there is no change in velocity w.r.t time, so hence $a=0$ and $F=0$).

But when $A$ collides with $B$ [Assuming the collision is elastic].

$A$ comes to rest! So the whole velocity of $A$ is now transferred to $B$. But remember $B$ was at rest and now it has gained some velocity. Therefore there is a change in the $B$'s velocity w.r.t time which causes it to gain some acceleration, which imparts a force on it.


Newton’s 2nd law is often written as F = ma. However, it is also written as

F = rate of change of momentum of a body or

F = dp/dt where p is momentum of the body (p= mass X velocity)

A mass moving with uniform velocity possesses momentum that is product of its mass and velocity. When it collides with another object, it passes on part of its momentum or full momentum. As such the body with which it collides gains momentum. Thus there is a change of momentum of the 2nd body. Change in momentum of a body over a time “t” constitutes “force” per equation given above

Thus a body moving with a uniform velocity (zero acceleration) is capable of exerting force.

You may also look at the situation wherein the velocity of the first object reduces on impact. A reduction in velocity over time period constitutes an Impulse. Thus upon impact the body has impressed force on the 2nd body due to its own change in velocity. More the change in velocity more the force

Watch this video from The Science Cube that I have made to understand better

Newton's 2nd Law of Motion

What is impulse?

  • $\begingroup$ I don't see how this really answers the question. $\endgroup$ – Kyle Kanos Dec 29 '17 at 10:56

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