What did Galileo discover when he dropped two cannon balls of different weights from the Leaning Tower of Pisa? A.
The force acting on both of the balls was the same.
B.
The time for both of the balls to reach the ground was the same.
C.
A mass-dependent attractive force was exerted by Earth on the two balls.
D.
Gravity pulls everything downward.
E.
The balls exerted an attractive force between each other as they fell.

Aparticle's position is given by x = 3.00 - 9.00t + 3t2, in which x is in meters and t is in seconds. (a) what is its velocity at t = 1 s? (b) is it moving in the positive or negative direction of x just then? (c) what is its speed just then? (d) is the speed increasing or decreasing just then? (try answering the next two questions without further calculation.) (e) is there ever an instant when the velocity is zero? if so, give the time t; if not, answer "0". (f) is there a time after t = 3 s when the particle is moving in the negative direction of x? if so, give the time t; if not, answer "0".