FORCES

Everyday Forces

A force is any push or pull that causes an object to move, stop, or change speed or direction. A object starts to move, stops moving, or changes speed or direction only when a force acts on it. Some forces act on objects directly. When you push on a door, you directly apply the force that makes the door open. Other forces, such as gravity and magnetism, act on objects at a distance. A magnet moves objects toward it even if the magnet is not touching them. The force of the magnetic field attracts objects to the magnet.

Friction, Gravity, and Magnetism

Three of the forces that affect objects on Earth every day are friction, magnetism, and gravity. Friction is a force that opposes , or acts against, motion when two surfaces rub against each other. When a baseball player slides into second base, the friction of his legs rubbing on the dirt provides enough force to stop the slide. Can you think of more examples of friction

between two surfaces? Click here

A hockey puck slides over ice and slows.

Roller skating and using brakes and concrete to slow down

Hands rubbing together

Bicycle brakes slowing the tires

Gravitation

Gravitation is another force that affects objects on Earth. Gravitation is the force that pulls all objects in the universe toward one another. This is the force that holds things to the surface of Earth. Even two Ping Pong balls sitting side by side on a table pull on each other with gravitation. The force of gravitation acting on them is smaller than the force of friction keeping them apart, so the balls don’t move toward each other.

Gravitation

Earth and a Ping Pong ball pull on each other with a much stronger force, so they stay together. This is because the strength of the gravitation between two objects depends on the mass of the objects. The total mass of the two Ping Pong balls is small, so the force between them is small. The total mass of a Ping Pong ball and Earth is very large, so the force between them is much larger.

The strength of gravitation between two objects also depends on the distance between the objects. When two objects are close to each other, they are pulled together by a greater force than if they were far apart. The greater the distance between two objects, the less the force of gravitation acts on them.

Earth and the moon have a lot of mass. The moon is kept in orbit around Earth by gravity. However, if Earth and the moon were much farther apart, gravity would not be enough to keep the moon in orbit around Earth.

Keep in mind that the force of gravitation between two objects pulls on both objects. When you toss a ball into the air, gravity pulls the ball toward Earth. Gravity also pulls Earth toward the ball. Earth, however, has so much mass that its movement toward the ball is not enough to be measured. So even though a gravitation force pulls on both objects, the smaller object does most of the moving.

Summary

A force is a push or pull that can make an object move, stop, or change speed or direction. Some forces are direct, while others act at a distance. Where two surfaces rub against each other, the force of friction opposes motion. Magnetism pulls magnets and magnetic objects together. Gravitation pulls objects toward each other. The strength of the gravitation depends on the masses of the objects and how far apart the objects are.

Review

1. What starts an object moving or stops it if it is already moving? Answer

2. Name three forces that affect you every day. Answer3. What force keeps the moon in orbit around Earth?

Answer4. True or False- Friction is a contact force, which

means that two objects or surfaces MUST touch one another.

5. Give an example of friction between 2 surfaces.

A force

Friction, Magnetism, and Gravitation

Gravity

True