# Homework Help: Free Fall Gravity Question

1. Nov 5, 2007

### grantP

[SOLVED] Free Fall Gravity Question

question below

Last edited: Nov 5, 2007
2. Nov 5, 2007

### anap40

Vf does not equal 0, it equals -Vi.
The easiest way to look at it is that In free fall the object spends the same amount of time going up as it does going down. Also note that when the object is at the top of its path v=0.
So we know that it is in the air for 3 seconds, therefore at 1.5 seconds it is at its maximum height and v=0.
the change in velovity is 5-0=5 and the accel is 5/1.5 = 3.333m/s^2

3. Nov 5, 2007

### grantP

ok thanks....on another note, how would you figure out an equation from say a helicopter which is 30m in the air and is throw up at 20m/s?

4. Nov 5, 2007

### anap40

an equation to find what, the acceleration?

5. Nov 5, 2007

### grantP

since i dont know the Distance of the obj.'s path (30+d) how would i go about figuring it out?

6. Nov 5, 2007

### grantP

well im looking for velocity of the obj when it hits the ground, and the time as well, and i just assumed i needed to know the distance

7. Nov 5, 2007

### anap40

to do that you can use that fact that the object is moving at the same speed but opposite direction when it passes the same height on the way back down from it hieght. For example if a ball is thrown up at 5m/s when it passes the height that the thrower released it at it will be moving at 5 m/s down. So for the helicopter use the initial velocity that the ball is thrown up at in vf=vi+at

8. Nov 5, 2007

### grantP

k got it

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