# Thread: Big Rock turned into small Bullet

1. Imaging this. We've got a row of rocks , aligned on a hill. Their masses are in the following order: 100 kg, 50 kg, 25 kg, 12.5 kg, 6.25 kg ... and say we come to something below 1 kg.

Now if we push the 100 kg rock upon the other rocks, the kinetic energy should completely transmitted to the small ones, cause the smallest to act like a bullet, right?

2.

3. No. The collisions between rocks will not be perfectly elastic, and even if they were, it would not work as you think. If a large mass collides with a smaller mass in an elastic collision, it retains most of its momentum and kinetic energy. It is only in collisions of equal masses that the momentum is all transferred. Momentum and energy both have to be conserved in an elastic collision.

4. So that is why a Newton's cradle uses the same size spheres?

5. Yes, that's why

6. A related question Is there a limit to the size of spheres one could use other than production. If you had multiple planets set up in a Newton's cradle would the energy transfer work the same provided they had identical mass and composition

7. A related question Is there a limit to the size of spheres one could use other than production.
On mythbusters, they tried to scale up the Newton's cradle to use wrecking ball sized spheres, but they were unsuccessful in getting the elastic collision that they needed.

8. It works with planets. The slingshot effect sometimes used to accelerate space craft is basically an elastic collision, though they don't physically collide.
Gravity assist - Wikipedia, the free encyclopedia

9. How can you calculate what part of energy is transferred from 1 object to another if comparing size and mass..

Lets say.. 2 objects, both with a radius of 10cm, one is 1kg, one is 10kg.. The 10kg ball will hit the 1kg ball, what speed will both balls have after this..

We know the heavy ball can never move faster then the light ball, as it can not pass it. But it can not transfer all it's speed to the light ball, so it'll still move..

lets say the heavy ball had an initial speed of 10m/s and it has no friction in moving, of bumping.

What would happen?

10. The relevant equations to calculate final velocities in a 1-dimensional elastic collision are here.
Elastic collision - Wikipedia, the free encyclopedia

Plugging the numbers into the forumula from the wikipedia page, I get
KE of m1 before collision = 500
v1 after collision = 8.18
v2 after collision = 18.18
KE of m1 after collision ~334.7
KE of M2 after collision ~165.3

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