Acceleration is a change in velocity, where it's a change in magnitude or a change in direction. Gravity is a type of acceleration . When you drop an object, its velocity increases the further it falls. Galileo showed that gravity accelerates all objects by the same amount no matter how much they weigh. That is, if you drop a bowling ball and a golf ball off the roof at the same time, they'd land at the same time.
Air resistance keeps a piece of paper, for example, from falling at the same rate.
Momentum and force Momentum is the mass times the velocity. Momentum, like velocity, is a vector. It has magnitude and direction. A force is a change in momentum. Therefore acceleration can be caused by a force.