# Homework Help: I need help

1. Oct 23, 2006

### roxas

okay my question is kinda stupid, and i know i totally sux at physics.
if a bullet is shot upwards, will it fall at an equal or greater force? i know its decelerating as it is shot up, and it would be acclerating as it falls, and so wouldnt it fall at a greater force?

2. Oct 23, 2006

### Hootenanny

Staff Emeritus
Greater or equal force than what?

3. Oct 23, 2006

### roxas

the force when it was shot upwards, before reaching it highest point and falling

4. Oct 23, 2006

### QuantumCrash

At the point when you actually shoot the bullet, the forst will be very great. Afterwards, if you neglect air resistance, the force acting on the bullet is pretty much constant and there is only one: gravity that acts as the weight of the bullet.

5. Oct 23, 2006

### HallsofIvy

I assume you actually mean "speed" rather than "force". As quantum crash said, neglecting air resistance, the only force on a bullet is gravity and that is constant. One way to look at speed is to use conservation of energy. Since the potential energy depends only on height, at a given height going up or down, kinetic energy= total energy- potential energy is the same and so the speed, at that height is the same whether the bullet is going up or down. If we include air resistance, it will "rob" the bullet of energy so at a given height, the bullet coming down will be slower than going up.

6. Oct 23, 2006

### roxas

okay thx

7. Oct 23, 2006

### mineys

Well, technically, neglecting air resistance, as is so popular in physics these days, if the bullet were to collide with an object at the instant it is shot out of the gun, it would apply the same force as if the bullet was falling back to its original position. But hallsofivy is very right. another way to look at it as it has a constant ACCELERATION of -9.8m/s where down is negative. at the highest point of the bullet's trajectory, the bullet has an instantaneous velocity of 0, and then falls for the same amount of time at the same acceleration as it did whilst traveling upwards, and will acheive the exact opposite velocity once it gets to the point from whence it came. Now, obviously in the real world, this is not the case because the bullet would have hit terminal velocity by then, and the acceleration due to gravity would go away.