Copyright © 2007 Dorling Kindersley
When you strike a ball with a bat, stretch a rubber band, or lift a suitcase, you are applying a force. All forces are pushes and pulls. Some forces can act over long distances—for example, the force of GRAVITY pulls you down when you jump from a high diving board.
Table 24. FUNDAMENTAL FORCES
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.
Forces are measured by their effects. Spring balances are used to measure the effect of forces. The stronger the force applied to a spring, the more it is stretched. The amount the spring is stretched from its normal size is proportional to the force applied to it. The unit of measurement of force is called a newton. One newton of force increases the speed of a one-kilogram mass by one meter per second every second.
Two forces are balanced when they are equal in size and opposite in direction. If the balanced forces are applied to an object, they will have no effect on its motion in a straight line, but may stretch or compress it. If the forces do not act along the same line, they may cause the object to rotate.
Gravity is the force that acts between all objects with mass. Gravity always attracts, never repels—it is always a pull and never a push. The strength of the force of gravity increases either when mass is increased or when the distance between the objects is decreased.
Mass is the amount of matter in an object. The mass of an object does not change if it is taken from Earth into space. Weight is a force. The weight of an object on Earth is the force acting on it because of Earth’s gravity. On the Moon the object weighs less than on Earth since the Moon’s force of gravity is less.
If you place a block of wood in water, it displaces (pushes aside) some of the water but floats. A force in the water called upthrust acts upward on it to counter the downward pull of gravity. The upthrust equals the weight of the displaced water. When you place a heavy object in water, the weight of the displaced water, which equals the upthrust, is less than the weight of the object, so the object sinks.