5.2a Every object exerts gravitational force on every other object. Gravitational force
depends on how much mass the objects have and on how far apart they are. Gravity is
one of the forces acting on orbiting objects and projectiles.
5.2b Electric currents and magnets can exert a force on each other.
5.2c Machines transfer mechanical energy from one object to another.
5.2d Friction is a force that opposes motion.
5.2e A machine can be made more efficient by reducing friction. Some common ways of
reducing friction include lubricating or waxing surfaces.
5.2f Machines can change the direction or amount of force, or the distance or speed of
force required to do work.
5.2g Simple machines include a lever, a pulley, a wheel and axle, and an inclined plane.
A complex machine uses a combination of interacting simple machines, e.g., a bicycle.