Suppose that the gravitational acceleration on a certain planet is only 2.9 m/s2. A space explorer standing on this planet throws a ball straight upward with an initial velocity of 20 m/s.
(a) What is the velocity of the ball 4 seconds after it is thrown?

Question

Suppose that the gravitational acceleration on a certain planet is only 2.9 m/s2. A space explorer standing on this planet throws a ball straight upward with an initial velocity of 20 m/s.
(a) What is the velocity of the ball 4 seconds after it is thrown?

ipwnbunnies · Answer

This is a simple free fall problem.
You can use this simple formula for velocity wrt time:
$$Final velocity = Initial velocity + (a*t)$$

ipwnbunnies · Answer

You're given initial velocity, the acceleration of gravity of the planet, and the time interval. 
You can find the final velocity.
Be careful with your sign for acceleration!

anonymous · Answer

Thank you! Can you explain to me why acceleration is negative in this case though?

ipwnbunnies · Answer

The general convention is upwards is the positive direction, and downwards is the negative direction. Of course, gravity pulls everything downward.
So, a negative acceleration is one that tends to "push" the object in the negative direction