Unit 6 – Forces and Motion

6Forces and Motion

In this chapter you will ...

• describe the position and motion of objects.

• identify pushes and pulls.

• distinguish and describe contact and non-contact forces.

• describe how friction, magnetic force and gravity can affect the motion of objects.

How can we describe the position and motion of objects?

How do forces affect motion?

Go Online!

Access interactive content relating to this topic on the NGScience website.

ngscience.com

What is the difference between contact and non-contact forces?

Position and Motion

When describing the world around us it helps to know where things are and how they are moving.

The position of an object is its location. We can describe the position of an object by saying where it is relative to other objects.

Describing Position

Let’s describe the positions of some of the objects in the room. The plant is to the left of the computer. The computer is between the plant and the books.

The clock is above the plant.

The keyboard is in front of the computer.

What are some other ways to describe the positions of the objects?

AB Activity 6.1 Try This!

With a partner, describe the position of your desk relative to other classroom objects.

Go Online!

Observe and describe different objects in motion on the NGScience website. QuickCode: F1T2

Describing Motion

When an object is in the process of changing its position, it is moving. We say it is in motion. We can describe the motion of an object in different ways. We can describe motion by talking about the path along which it is moving. Things can move in a straight line. They may roll, turn or spin. Things can move up and down, back and forth or round and round.

Describe the different ways things move in a playground.

We can also describe the speed of an object in motion. Speed is how fast or slow it is moving.

AB Activity 6.2

Describe the motion of passengers on a roller coaster during a ride. How does the speed of motion change?

Try This!

Choose a ball sport. Describe how forces are used to change the motion of the ball.

Try This!

Forces can make an object change shape. Plan and conduct an investigation to see how different forces can change the shape of a piece of clay.

Forces and Motion

A force is a push or a pull. A push is when you press something away from you. A pull is when you tug something closer to you. Pushes and pulls can change the movement of objects.

A push force applied to a wheelbarrow makes it move.

A suitcase moves when you use a pull force to drag it behind you.

In what ways can forces change the movement of an object?

Sometimes, like when riding a bicycle, we use a push and a pull.

We use pushes and pulls when we vacuum a floor or open and close a door.

What push and pull forces can you observe around you? How do they make things move?

AB Activities 6.3 – 6.5

Force can make things move in different ways.

During a game of soccer, a kick starts the ball moving. As players pass the ball, it speeds up, slows down and changes direction. When the goalkeeper catches the ball, it stops moving.

How do forces change the movement of a ball during a soccer match?

Try This!

With a partner, use forces to make marbles move in different ways. Describe the forces you used and how it affected the motion of the marbles.

As we can see from a soccer match, forces can make an object: • Start and stop moving. • Speed up and slow down. • Change direction.

AB Activity 6.6

Types of Forces

There are different types of forces. Forces that are applied when objects are touching are called contact forces. Hitting a ball and pulling on a rope are contact forces. Some forces can be applied when objects are not touching. These forces act at a distance and are called non-contact forces.

Take a magnet and hold it above an iron nail. The force of the magnet is applied to the nail without touching it. The force of a magnet is a noncontact force.

Friction

Take a wooden block and slide it along the floor. The wooden block moves when you push it. It then slows down and stops.

Friction is a contact force that slows down moving objects. It occurs when two objects are in contact with each other. The friction between the surface of the block and the surface of the floor caused the block to slow down and stop.

What causes the block to slow down and then stop?

Try This!

Given the same force, would a wooden block move further on a wooden floor or carpet? Plan and conduct an investigation to find out.

Think Deeply

What are some examples where friction can be helpful? When can friction cause problems?

Think Deeply

Why are the paper clips attracted to the magnet?

Magnetic Force

What do you notice about the objects attracted to the magnet?

What are some ways magnets are used by people?

Try This!

In small groups, gather some objects in your classroom that you think will be pulled by a magnet. Use a magnet to test each object.

Engineer It!

Design and build a toy car that can move up and down a ramp using magnetic force. A magnet is an object that can pull certain types of metal – it has magnetic force. Magnetic force is a non-contact force. It works over a distance. A magnet does not need to be touching the object it pulls.

AB Activity 6.7

A Closer Look The Poles of a Magnet

Magnets have two poles – a north pole and a south pole. The pulling force of a magnet is strongest at the poles. Magnets can attract (pull) or repel (push away) other magnets. When the opposite poles of two magnets are brought together, the magnets attract each other. When the like poles of two magnets are brought together, the magnets repel each other.

Try This!

Take two bar magnets and put the poles together in different ways. Describe what you observe.

Gravity

Use your finger to trace the path the basketball will take as it leaves the boy’s hands.

All objects attract other objects. This force is called gravity.

The boy used a push force to throw the basketball and the Earth’s gravity pulled the ball down to the ground.

What force causes the ball to move up into the air? What force will cause the ball to fall to the ground?

When you drop a pencil, it falls to the ground. When a skydiver leaps from a plane, they are pulled to the Earth’s surface.

All of the objects on Earth are being pulled downwards towards the center of the Earth by gravity. The Earth’s gravity keeps everything on its surface and stops things from floating away.

AB Activity 6.8 Try This!

In small groups, go outside and throw a ball into the air. Describe how forces, including gravity, affect the motion of the ball.

Think Deeply

In what way is gravity similar to magnetic force?

Science Words

position motion speed force push pull contact force non-contact force friction magnetic force gravity

Review

1. Which best describes position? (a) How far an object has moved. (b) The location of an object. (c) How an object moves. 2. Which best describes speed? (a) The location of an object. (b) The path along which an object moves. (c) How fast or slow an object moves. 3. Which best describes motion? (a) The movement of an object. (b) The speed of an object. (c) The force of an object. 4. What is a force?

5. List two activities where you use a push force. 6. List two activities where you use a pull force.

7. List five ways forces can change the motion of an object. 8. Describe and provide an example of a contact force. 9. Describe and provide an example of a non-contact force.

10. What force pulls the pins to the magnet?

11. True or false. Friction helps things move. 12. What force is pulling the boy to the water?