What moves an object? Momentum or Kinetic Energy?

Question

What exactly makes an object move? Momentum is $\vec p= mv$ where $m$ is mass of an object and $v$ is the velocity of an object, and as a result of this when I fire a bullet the bullet may travel much faster but the gun will recoil or move in opposite direction with the same momentum.

This seems well and good and we can say momentum makes objects move, but since kinetic energy is $\frac{1}{2}mv^2$ where $m$ is mass of an object and $v$ is the velocity of an object. That let us assume the mass of the bullet is $10$kg (very large bullet!) and is accelerated or is velocity is $1000$ $m/s$ which should mean that the kinetic energy is $5,000,000$ joules of Kinetic Energy and momentum is $10000$ $kg$ $m/s$. That said let us assume the mass of the gun is $100$ $kg$ then for the momentum to be conserved here then the velocity of the gun that would recoil backwards would be $100$ $m/s$ so its kinetic energy is $500,000$ joules which is 10 times less than the bullet.

I'm confused here, because of newtons 3rd law - "Every action has an equal and opposite reaction" so I think momentum moves an object but however kinetic energy is the energy of the movement so shouldn't both (gun & bullet) have equal kinetic energies?

Why do you think that they should have equal kinetic energies? Not attacking here, I just want to understand where you're coming from. — NeutronStar, Aug 27 '14 at 18:42
Newton's third law is about forces, not conservation of momentum or energy. — HDE 226868, Aug 27 '14 at 18:46

score 6 · Answer 1 · answered Aug 27 '14 at 19:39

The answer to your title question is "neither". An object that moves has both momentum and kinetic energy, but it acquired these because a force was applied for a certain length of time.

Now $F \cdot \Delta t$ has the same dimensions as momentum (and in fact, $F \Delta t = m \Delta v$ so there is a direct relationship between impulse which is $force * time$, and momentum which is $mass * velocity$). But in the end it's impulse that moves objects, and momentum is what they acquire as a result.

Just to clarify - if you have two objects colliding, one can transfer momentum to the other; but the mechanism for the momentum transfer is again a force between them.

score 3 · Answer 2 · edited Apr 13 '17 at 12:39

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I'm with Floris that the correct answer is "neither", but I'd like to to take the explanation in a slightly different direction.

The notion that something is "moving an object" carries an unstated assumption that the natural state of an object is to be at rest. That is one of those things that is both obvious and wrong.¹

The natural state of an object is to continue moving however it was moving before.

It's changes to an objects motion that need explaining, not it's continuing to move as it did before.

¹ On the other hand holding this belief puts in the company of Aristole, so it's not all bad.

edited Apr 13 '17 at 12:39

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answered Aug 27 '14 at 22:00

dmckee --- ex-moderator kitten

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You make a valid point. "Moving" should be interpreted as "accelerating", perhaps. Of course when you add an inertial frame of reference in which the object is initially at rest the distinction becomes imperceptible. Thanks! – Floris Aug 28 '14 at 03:48
@dmckee, This question might not be a duplicate, if Floris ' comment is true, and it is true. It is a rather interesting question asking what really makes a body accelerate. It might be worth considering to reopen it – – bobie Aug 28 '14 at 06:26

What moves an object? Momentum or Kinetic Energy?

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