What are Forces?
Forces are interactions that cause an object to move.
There are several types of forces; these are examples of only a few:
When a person parachutes out of a plane, gravitational force causes them to move towards the mass of the earth. Gravity is a constant force acting on objects on the earth's surface.
Forces that are produced by other objects are called applied force. If I push my books across the table, I am creating an applied force that is acting on my books.
Friction is forced that is caused when a moving object runs against another object. When I slide my books across the table, a friction force exists between the table and my books.