# Gravity and Acceleration

future_vet
Here's my last string of questions... for the week at least! :) Thank you all so much for your help.

If we throw a ball up, is its acceleration just before it reaches its highest point slightly higher than g? This would make sense to me, because if it was at g or lower, it would be ready to fall back down, or would be falling down...

If a ball is thrown up and another is thrown down, when they reach the ground, do they both have the same speed? Also, this would make sense to me, because they are both exposed to gravity.

A car traveling at 60 km/h accelerates at 2.0 m/s^2. How much time is required for the car to reach a speed of 90 km/h?
I calculated 15 seconds for that... It makes sense to me, but we never know...

Thanks!

## Answers and Replies

Staff Emeritus
Here's my last string of questions... for the week at least! :) Thank you all so much for your help.

If we throw a ball up, is its acceleration just before it reaches its highest point slightly higher than g? This would make sense to me, because if it was at g or lower, it would be ready to fall back down, or would be falling down...
I wouldn't agree with this. Neglecting air resistance, what forces are acting on the ball when it is in flight? Thus, what is the acceleration of the ball?

If a ball is thrown up and another is thrown down, when they reach the ground, do they both have the same speed? Also, this would make sense to me, because they are both exposed to gravity.
If they're thrown from the same point then, yes.
A car traveling at 60 km/h accelerates at 2.0 m/s^2. How much time is required for the car to reach a speed of 90 km/h?
I calculated 15 seconds for that... It makes sense to me, but we never know...

How did you calculate it? I can't check you're right if I can't see what you've done!

future_vet
I wouldn't agree with this. Neglecting air resistance, what forces are acting on the ball when it is in flight? Thus, what is the acceleration of the ball?

The forces acting on the ball in flight would be gravity?
The acceleration of the ball would be 0?

Staff Emeritus
The forces acting on the ball in flight would be gravity?
Correct, and the graviational force is equal at all points during the ball's flight.
The acceleration of the ball would be 0?

Where did this conclusion come from? (Huge hint: what is the acceleration due to gravity?)

future_vet
How did you calculate it? I can't check you're right if I can't see what you've done!

Ah oops, here it is:
2.0 m/s^2 = (90 000 - 60 000 m/h)/t
t= 30 000/2

And... I can't figure out how I got my answer then... Maybe I forgot about the km vs meters...

...

Staff Emeritus
Ah oops, here it is:
2.0 m/s^2 = (90 000 - 60 000 m/h)/t

Your units here are incorrect. Your conversion to metres was correct, but on the left you have seconds, and on the right you have hours. You need to convert the expressions on the right to m/s before you can calculate t.