1. Acceleration: A force can directly increase the speed of an object. This is called acceleration. Think of pushing a box across the floor. The force you apply causes the box to speed up.

2. Deceleration: A force can also decrease the speed of an object. This is called deceleration or retardation. Imagine applying the brakes in a car. The force of the brakes slows the car down.

Let me know if you'd like more detail on either of these!