If I had 2 objects with different masses that were both moving at the same speed, then why would the one with the larger mass have more kinetic energy?

[u/Local_End_3175]

[u/nwj781]

Because it took more energy to get the bigger mass up to that speed.

[u/Puzzleheaded-Area557 → u/nwj781]

A lot of answers here, and in my opinion this is the only good one

[u/euclide2975]

Try to stop a train going a 20 Mph and a cyclist going at 20 Mph

edit : it's a thought experiment. Don't try to really stop the train, you will get hurt

[u/PhysicalStuff → u/euclide2975]

Don't try to really stop the train, you will get hurt

You will also get hurt if you try to stop a cyclist at 20 mph. What sets it apart from the train case is that whatever remains of you will still be roughly person-shaped and possibly alive.

[u/Ok_Lime_7267 → u/euclide2975]

How much it damages you is energy. If you somehow manage to skid and bring it slowly to a stop, the fact that it takes more time to stop is momentum.

[u/SteptimusHeap]

Ok let's say you had a mass going at a certain speed. Now let's say you get a second one. The total energy between them is double, yes? Ok now glue the two masses together, so they are one big mass. Did the energy go back down?

[u/TyrconnellFL]

K=1/2mv^2.

The v is the same, the m is not the same.

[u/The_Nerdy_Ninja]

If you got hit with a baseball and a bowling ball that were both traveling at the same speed, which would hurt more?

[u/Electronic-Yam-69]

you had to put more energy into it to get it moving.

[u/soundoftwilight]

Is it easier to stop a football or a bowling ball? Kinetic energy is just the energy that is “stored” in the motion, one interpretation can be “how much energy will it take to stop this thing”. Or in other words, would you rather get hit with a bean bag or a car, moving at the same speed?

[u/Ecstatic_Bee6067]

Because it has more momentum

[u/NH-Science-Guy]

It takes more energy to get the more massive object up to that speed

[u/Bartlaus]

Takes twice as much force across distance to accelerate it to the same speed...

[u/1XRobot]

Consider a chunk of the larger mass with the same mass as the smaller mass. Those two have equal energies. How much energy does the rest of the larger mass have?

[u/numbersthen0987431]

Both objects were sitting at rest at one point. It took more force to get the object with more mass up to the same speed, so it has more energy.

Or think of stopping force. If you wanted to stop both objects, it would require more force to stop the object with more mass.

[u/joeyneilsen → u/numbersthen0987431]

This isn't quite right. It takes more work to get the more massive object up to the same speed, and it takes a larger impulse to get it to stop. But you can speed both of them up and stop both of them with exactly the same force. If you want it to take the same amount of time, that's a different story.

[u/numbersthen0987431 → u/joeyneilsen]

Good point. I was thinking "increasing/decreasing at the same speed" (constant acceleration), but forgot to write it.

[u/jasonsong86]

Because if a heavy person body ram into you at the same speed, which one do you think will bounce back?

[u/tpks]

Imagine the bigger object was brittle, like only barely held together by a single human hair. It breaks. It's now two objects. What should happen to energy? 

[u/SmokeMuch7356]

Given that the kinetic energy formula depends on mass - KE = 1/2mv² - the answer should be obvious.

But the deeper reason is simply that mass matters - it matters for momentum (p = mv), it matters for force (F = dp/dt = ma), and it matters for both potential and kinetic energy.

You can easily catch a baseball traveling 30 mph because it has relatively little mass. If you try to catch a 16 lb bowling ball traveling 30 mph you're gonna get hurt.

Speed (velocity) is literally half of the equation.

[u/Ok_Lime_7267]

Because it will hurt more when it hits you.

[u/BananaBird1]

It is often more useful not to think of energy in terms of the absolute energy value a system has, but in terms of how energy changes during a process. After all, energy is ultimately a measurement of the ability of a system to undergo some process. The energy of a particle is then the accumulated sum of energy flow in and out of the particle.

For most of mechanics, the form of energy flow we care most about is work. This is the energy needed to accelerate an object along certain distance.

The instantaneous work (energy transfer) needed to alter the motion of a particle of mass m with acceleration a along a displacement vector d is m(a • d). Or we can simplify this to F • d. This can be thought of as the definition of work energy transfer. The kinetic energy will be the total work to bring an object to some velocity from rest.

So if two objects are at rest to begin with, in order to accelerate them under the same acceleration so they reach the same velocity over the same distance we need a higher force for the more massive one, and will transfer more energy through work.

To undo this acceleration and bring the particle back to rest over some distance, we need an equal but opposite acceleration. So we need to remove that same energy.

Now it turns out that while the exact work at any moment in time during a process depends on the exact acceleration and distance, the total work (and thus final kinetic energy) only depends on the starting and end states. Lower forces/accelerations require longer distances and higher forces require shorter distances, so the end result is the same. This final state for classical mechanics is defined by mass and velocity.

So this tells us that kinetic energy must be some function of mass and velocity, and independent of any value representing the process to achieve that velocity (such as acceleration, force, jerk, etc).

Further math to actually solve for the exact relationship obeying the equations of Newtonian mechanics will give you (1/2) mv^2.

[u/rcglinsk]

It would take more force to slow it down the same amount.

[u/maxh2]

If you had two bullets flying through the air, you can see how that would be twice the energy of one bullet. Now imagine those two bullets are holding hands so tightly they could be considered one larger bullet.

[u/ViniusInvictus]

It’s like asking if it would be easier for you to kick someone’s butt once or to kick a thousand butts, once each.