A force is a push or a pull, and there are forces at work everywhere! Forces
can make things move or stop moving, make things speed up or slow down, or
just keep things still. One of the most familiar forces is gravity, which pulls
everything down towards the ground. In this chapter, you’ll be fighting against
gravity by constructing a crane and by making a ping-pong ball hover in the air.
You’ll also explore the forces that make a raft stay afloat.