Last week we started a new chapter in science.  This week we will go on.

Lesson 2: What Are Forces?

VOCABULARY

Force

Gravity

Weight

Types of Forces

To move a ball, you can throw it, kick it, or hit it with a bat. Any kind of push or pull is a force. You must apply a force to make an object move. An object will keep moving until another force stops it. When you catch a moving ball, the force from your hand stops the ball.

Friction is one force that stops things or slows them down. When two objects rub together, there is friction between them. Most rough surfaces make more friction than smooth surfaces. You can slide farther on the ice than you can on dirt or grass.

Forces also make objects change directions. You apply a force to the handlebar to turn your bike. When you push one end of a seesaw down, the other end comes up.

Some forces do not even have to touch the object. A magnet can be used to push or pull objects made of iron or steel. This is called magnetic force. A magnet can pull a nail.

MAIN IDEA AND DETAILS A magnet picks up paper clips. What force is at work?

Ways That Forces Change Motion

Three things affect the motion of an object.

• the strength of the force

• the direction of the force

• the mass of the object

The stronger the force is, the greater the change in motion. If you toss a ball gently, it doesn’t move fast or far. If you throw a ball hard, the ball moves faster and farther. You use more force on the ball when you throw it as hard as you can.

If an object is not moving and a force pushes or pulls it, the object will move in the direction of the push or pull. When you pull up on an object, it comes up. When you push an object to the right, it moves to the right.

The smaller the mass of an object is, the easier it is to move. It takes less force to pick up a pencil than to pick up a book since a pencil has less mass.

You can add all the forces that push or pull on an object. The sum is called the net force. Suppose you and a friend push the same way on a door. The net force on the door equals your push plus your friend’s push.

MAIN IDEA AND DETAILS Name three things that affect the motion of an object and tell what each one does.

When equal forces act in opposite directions, they cancel each other out. The net force is zero.

Why does the waterfall back down to Earth after shooting up from the fountain?

Gravity

When you throw a ball into the air, you know that it will come back down. Gravity is a force that pulls two objects toward each other. The ball comes back to Earth because Earth’s gravity pulls on it.

Gravity is a very weak force between most objects. You don’t feel the force of gravity between you and your desk. But Earth’s gravity pulls very strongly. This is because Earth is so large.

The force of gravity depends on how much mass an object has. Weight is a measure of the force of gravity on an object. Objects with a large mass weigh more.

MAIN IDEA AND DETAILS Does gravity pull a horse or a puppy harder? How do you know?