An object is moving initially with a velocity of 4.7 m/s . after 3.9 s the object's velocity is -2.1 m/s . what is the object's average acceleration during this time?
i believe that the vf is -2.1 m/s vi should be 4.7 m/s and the t should be 3.9s but its #confused

Agroup of students were investigating the force of gravity. they began by dropping a foam ball from a height of 3 meters into a bucket of sand. the ball hit the sand in 0.306 seconds. they dropped additional balls of approximately the same diameter, but of different masses. here is the data they collected. based on this experiment and the collected data, what would their conclusion be?

It is really hard for me right know

On mars a rock falls an unknown vertical distance from a resting position and lands in a crater. if it takes the rock 2.5 seconds to fall, how high is the cliff the rock fell from? mars' surface gravity is 3.8 m/s2.