In this topic you will learn about how motion is affected.

A force is a push or pull exerted by one object on another. Forces change the motion of objects. For example, a rocket's engine provides a strong push that makes the rocket accelerate. Forces can also change the shape of objects. For example, the force your muscles can apply to clay through your hands can squeeze the clay into different forms.

Friction is a force that opposes the motion of an object sliding over another. Friction acts any time the surfaces of objects are in contact. Friction can be described as sliding friction, static friction, or rolling friction.

When an object moves through a gas, such as air, or any liquid, there is a force that opposes motion. This force is called drag force. As an object moves through the air, the air molecules bump into it (a drag force) and cause the object to slow down. Drag force depends on speed. The faster the speed, the greater the drag force.

Gravity is a force of attraction that exists between all objects with mass. The force of gravity between two objects depends on the masses of the two objects and the distance between the objects. The closer the objects are to each other, the stronger the force of gravity that they exert on each other. The force of gravity between Earth (or any planet, asteroid, or star) and any other object is called the weight of the object.

The combined effect of all the forces acting on an object is the net force. When you weigh something on a spring scale, gravity pulls the object down. The object pulls on the spring, making it stretch. When the pull of the spring offsets the pull of gravity, the spring stops stretching, and the object stops moving.

Anytime two or more forces acting on a single object completely cancel each other out we say that they are balanced forces. You can tell when forces are balanced because the motion of the object is unchanged. It is as if no force was acting on the object. When the forces are balanced, the net force on the object is zero. When several forces are acting on a single object and the forces do not completely offset one another, they are said to be unbalanced forces. The leftover force can be represented as a single push or pull of a certain strength and direction.

The tendency of an object to oppose a change in motion is called inertia. In other words, an object at rest tends to stay at rest, and an object moving in a straight line tends to keep moving that way.