What Is a Force?

The tennis player uses push forces to throw and hit the ball.

Pushes and Pulls

Whenever an object is in motion, it is because a force acted on the object, causing it to move. A force is a push or a pull. A push is when you press something away from you. You use a push when you jump in the air. You use a push when you throw or hit a ball. The push force starts the ball moving.

A pull is when you tug something closer to you. You use a pull when you open a drawer or drag a suitcase.

The pull causes the objects to move.

There can be more than one force acting on an object. When you use a bow and arrow, you push and pull at the same time.

How are forces used to move the golf ball differently? What effects do forces have on the motion of the golf ball?

Forces can have different strengths. A large, strong force will move an object further and faster than a smaller, softer force.

Compare the logs in the truck to the tree in the cart. Which would require a greater force to move? Why?

Larger, stronger forces are required to move heavier objects. The force required to move the truck is much greater than the force required to pull the cart.

Did You Know?

We can measure the strength of a force using a spring balance called a Newton meter.

AB Activity 11.6

Forces can affect the motion of objects in different ways. When a player kicks the ball off the ground, it starts the ball moving.

Think Deeply

How does the strength of a kick affect the distance the ball travels? Plan and conduct an investigation with your friends to find out. Communicate your findings to other groups.

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

Players kick the ball as they run, moving the ball in a zigzag motion. Passing the ball from player to player causes the ball to speed up, slow down and change direction.

To prevent a goal, the goalkeeper uses a push force which stops the motion of the ball.

AB Activity 11.7

Contact Forces

There are many different types of forces acting around us. Some forces occur when objects are touching. They are called contact forces.

Pushes and pulls from people, animals and machines are example of contact forces. In a baseball game, the pitcher is in contact with the ball when he pushes it forward. To change the direction of the ball, there is contact between the bat and the ball.

A tow truck needs to be in contact with the car to affect its motion.

Judo is a contact sport. Players make contact as they use pushes and pulls to affect each other’s motion.

Discuss the contact forces you observe in daily life. How do the contact forces affect motion?

Take a book and slide it along the classroom floor. What do you observe? When you slide a book, you use a push force to set it in motion. The book slows down and comes to a stop due to a contact force called friction..

Friction is a force that opposes motion. It occurs when the surfaces of objects rub together. Rough surfaces produce more friction than smooth surfaces. Friction is also greater when surfaces press harder together.

Sometimes it is useful to reduce friction. A slide has a smooth surface to reduce friction. A snowboard has a smooth surface to help it move quickly on snow.

What are some examples of when it is useful to increase friction?

As a force that opposes motion, we can increase friction to help things to slow down or stop. The soles of shoes are often rough. This helps to increase the friction between the shoe and the ground and stops you from slipping. A bicycle has rubber brake pads to increase friction when it presses against the wheel rim. This allows the cyclist to slow down or stop moving.

AB Activities 11.8 – 11.9 Try This!

Have you ever seen someone rub their hands together when it is cold? Try it now. What do you observe? What can you infer about friction?

brake pads

Non-contact Forces

Some forces can push and pull objects without touching. These forces can act at a distance and are called non-contact forces.

Place a magnet on a piece of paper and sprinkle iron filings over the magnet. What do you observe? The pattern of the iron filings is the result of magnetic force. A magnetic force can act at a distance.

A magnet is an object with a magnetic force. A magnet can pull or attract objects made of certain metals such as iron and nickel. A magnet cannot attract objects made of plastic, wood, rubber or glass.

A magnet has two poles – a North pole and a South pole. When the like poles of two magnets are brought together, they push or repel each other. When unlike poles are brought together, they pull or attract each other.

Many objects make use of magnets. Some doors used magnets to keep them closed. A maglev (magnetic levitation) train has magnets that repel it from the track. This reduces friction and allows the train to travel at fast speeds. Magnets are also used for medical imaging and storing data on computer disks.

Like poles repel each other.

Engineer It!

Design, model and build a useful tool that uses magnetic force. Describe how the tool works and how it uses magnetic force.

Unlike poles attract each other.

AB Activity 11.10 Go Online!

A maglev train uses magnets to hover above the tracks. This allows it to reach speeds of more than 400 kilometers per hour! Watch a maglev train in action on the NGScience website. QuickCode: P8T1

As the boy moves down the slide his hair stands on end! What force is causing this to happen?

Electrical forces are created when there are unbalanced electrical charges inside an object. Opposite charges attract each other. Like charges repel each other. The plastic slide caused the boy to have an imbalance of like charges in his body. The charges in his hair repelled each other. The electrical force was strong enough to make his hair stand on end.

You can create and observe the electrical force yourself. Rub a balloon through your hair. Slowly pull the balloon away from your head. What do you observe?

Rubbing the balloon through your hair created two opposite static charges. The charges attract one another and make your hair stand up. Notice that your hair and the balloon do not need to be touching to affect each other. Electrical forces are non-contact forces and can act at a distance.

AB Activity 11.11

Think Deeply

Gravity on the Earth’s moon is only one-sixth of the Earth’s gravity. With a classmate, talk about what it would be like to walk, jump and play different sports on the moon. When skydivers jump out of an airplane, they are pulled towards the surface of the Earth. What force is pulling them down? All objects have an invisible force that pulls on other objects. The force is called gravitational force, or gravity.

The strength of an object’s gravitational force depends on it mass. Mass is the amount of matter something is made of. The gravitational force of objects with a small mass, such as a basketball, is too weak to observe.

Compared to a basketball, the Earth has a much greater mass. The strong gravitational force of the Earth affects everything on its surface.

It is the force of gravity that pulls you down when you jump in the air. It is the force that keeps everything on the Earth’s surface. Gravitational force also depends on distance. The closer objects are to each other, the greater the gravitational force.

Amazing Fact!

The object with the most mass in our solar system is the Sun. The gravitational force of the Sun keeps all of the planets and other bodies in the solar system in orbit around it.

AB Activities 11.12 – 11.13 Think Deeply

How does a parachute help a skydiver reach the ground safely?