Forces change the way things move. The force of the wind will alter the direction of a hot air balloon. A moving ball, with no forces acting on it, will continue moving in the same direction and at the same speed until a force acts upon it. A force can also change the shape of an object. A giant crusher can change the shape of a car — even your hand can exert a force to shape and mould certain objects. Whenever we find that the speed or direction of a moving object is changing, or the shape is changing, we say that forces are acting to cause these changes.
Forces change motion and shape. The force of a foot kicking a ball speeds the ball up. The force of a parachute on a skydiver slows the skydiver down. The force of a string on a whirling ball constantly changes the direction of motion, keeping it moving in a circle. Combinations of forces applied to materials can stretch, twist, and crush them.
Illustrates the force exerted on a steel ball by the flick of a finger. We can see that the force sets the ball moving, and when the force stops, the ball continues in a straight line at a constant speed, unless another force acts on it. When a magnet is held near the moving ball, it exerts a pulling force on it – changing the direction of the ball. This is because magnets attract steel.