Question

Suppose that you toss a rock upward so that it rises and then falls back to the earth. If the acceleration due to gravity is 9.8 m/sec2, what is the rock’s acceleration at the instant that it reaches the top of its trajectory (where its velocity is momentarily zero)? Assume that air resistance is negligible.

Answer 1

Regardless of where the rock is in its flight path, the force of gravity is still acting on it. So your rock, even when it's velocity is momentarily at 0m/s, is still experiencing a downward acceleration of 9.8m/s^2 (assuming air resistance is negligible).

Think about this: If there were no acceleration at that point, there would be no force to cause it to come back down.

Answer 2

so the acceleration of the rock would be 0 ?

Answer 3

Read what I posted again. No it's not 0 :)

Answer 4

if no acceleration then it would not come back down..? so The rock has a downward acceleration of 19.6 m/s2? would it double while coming back down ?

Answer 5

At the top of its flight path, the rock is moving at 0 m/s...for an instant. The only reason it doesn't stay at 0m/s is because the force of gravity is still acting on it.

The acceleration of the rock is a constant 9.8 m/s^2 downward for the rock throughout its flight. It doesn't double...that would require more energy.

Answer 6

oh so it stays the same..would it always stay constant?

Answer 7

If there are no other forces acting on the rock then the only force will be the force of gravity...which will be essentially constant (unless you through it REALLY high...miles).

Answer 8

*throw* I mean

Answer 9

ohh thank you

Answer 10

i get it but not all the way, do you have an example?

Answer 11

This may help:
http://www.physicsclassroom.com/class/vectors/u3l2a.cfm

Answer 12

oh i am starting to get it thank you very much

Answer 13

Ok, good luck :)