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Q & A: speed of bullet, up and down

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Most recent answer: 05/06/2018
Hi, I am confused by the conflicting answers to this one. In a vacuum,if you fire a gun vertically upwards, some people say the initial velocity upwards will be the same as the final velocity downwards when it reach the ground due to law of energy conservation. I don't agree because there are 2 forces acting going upwards, the energy pushing the bullet up and the deceleration from gravity. There is only gravity acting on the downward bullet. Can you please clarify?
- Leonardo (age 45)

There are indeed two forces on the bullet after it leaves the gun, but "energy" is not one of the forces. The two forces causing the velocity to change are gravity and air friction.If gravity were the only force then it would speed up the falling bullet just the same amount that it slowed down the rising bullet. That would leave the speed near the ground the same on the way down as on the way up. One way of thinking of that is that the gravity leaves the kinetic energy of motion plus the potential energy which depend on height) unchanged. When the height is the same, the kinetic energy would be the same so the speed would be the same, whether it's going up or down. The air friction, however, always acts to slow the bullet down. It pulls energy from the bullet's motion and transfers it to heat in the bullet and the nearby air. It causes the falling bullet to have less energy and move slower than the rising bullet near the ground.

Mike W.

(published on 05/06/2018)

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