Science Section 3-4 "Connecting Motion with Forces"

A force is a push or a pull one body exerts on another. Examples of force; jump, open or close a door, picking up a pencil

Force does not always change velocity. Forces that are equal in size and opposite in direction are balanced forces.

A net force on an object always changes the velocity of the object. Objects accelerate in the direction of the greater force. A net force acting on an object will change its speed, direction, or both.

Inertia is the tendency of an object to resist any change in its motion. If an object is moving, it will keep moving at the same speed and in the same direction unless an unbalanced force acts on it. The more mass an object has, the greater its inertia is.

Sir Isaac Newton (1642-1727) was able to state laws that describe the effects of forces. Newton's first law of motion is that an object moving at a constant velocity keeps moving at that velocity unless a net force acts on it. If an object is at rest, it stays at rest unless a net force acts on it.  This law is sometimes called the law of inertia.

Friction is the force that opposes motion between two surfaces that are touching each other. The amount of friction depends on two factors- the kinds of surfaces and the force pressing the surfaces together.