192

Science Alive for VELS Level 5

What is a force? force is a push, a pull or a twist. A force can change the speed,

direction or shape of an object. For example, when a racquetstrikes a tennis ball, it can force the ball to change speed and

direction. It can temporarily change the shape of the ball too.

A

Types of forceThere are names given

to the many types of force that are around us.

Here is a short description of some of the most

important ones.

Contact forces

Often, forces can act between two objects that are in contact with (touching) each other. Familiar examples of contact force include the force of your hand pulling your shoelaces, the force of your friend’s hand on a bottle top as he unscrews it, and the force of your schoolbag pulling down on your shoulders.

Forces can act between two objects that are in contact with each other.

Other examples of contact force include friction and buoyancy.

➡➡

➡

The air in this tank

enables the diver to

breathe under water.

Contact between

the water and the

flippers pushes

the diver forward

as she kicks.

The push from under

the diver is a force

called buoyancy. To

overcome buoyancy,

divers wear weight

belts.

The weight of the water

pushes down on the diver.

The Earth pulls down on the

diver with the force of gravity.

➡

Friction is a contact force between two surfaces that are sliding, or attempting to slide, over one another. For example, there is friction between the tyres of your bike and the ground when you pedal. Without friction, the tyre would just slip and you would not move forward.

You cannot walk on water, but . . . water does provide an upward force on you when you step in. This upward force is called

buoyancy. It is the force that enables you, and ships, to float. Divers in the water need to wear a heavy belt to be able to sink to the bottom, otherwise they would keep bobbing up to the surface.

Non-contact forces

Forces can occur even between two objects that are not touching each other. These forces are called non-contact forces. Like all other forces, non-contact forces can

 

 

 

 

1939. Forces and motion

✓ le

arn

ing I CAN:

explain what a force islist several types of forcesrecognise and distinguish between contact and non-contact forces.

affect the motion of an object — they can get the object moving, slow it down or stop it altogether.

Gravity is a non-contact force. We can see the effects of the force of gravity acting between an apple and the Earth when the apple drops from a tree. The force of gravity acting on us is often called our weight. Gravitational forces also hold the Moon in orbit around the Earth, and the planets in our solar system in orbit around the Sun.

Magnetic forces can also act at a distance. These forces act between two or more magnets, or between magnets and some substances, such as iron. Magnets have two ends or poles. When two magnets are brought together they either attract (pull) each other or repel (push), depending on the position of the poles of the magnets.

Electric forces hold all of the material around us together. Electric forces even hold together the material we are made from. It is responsible for plastic wrap sealing our sandwiches and for your hair standing on end if you hold onto a Van de Graaff generator.

Action and reaction forcesAll forces act between two (or more) objects, whether they are in contact or not. You never see something being forced forwards all by itself; something else must be present to cause the force.

The damaged car in the photo on the right must have collided with some object, such as another car or a pole. We can describe the force of the object on this car as the action. The object would also have

experienced a force on it from the car in the picture, perhaps causing similar damage. We can call this force the reaction force. Every force has an action on one object and a reaction on the other. The force on either object can be called the action, making the other the reaction. The point to remember is that forces always act in pairs.

This damage must have been caused by another object.

➡

➡

➡

Water pushes on the diver

from every direction.

Go to worksheet 9.1:Deep diving danger

REMEMBER

1. What is a force?2. What are the three possible

results of a force acting on an object?

3. List two types of contact forces and three types of non-contact forces.

4. What is buoyancy?

THINK

5. Look at the drawing on the previous page of the student opening the drink can. We can call the force applied by the boy’s hand to the pull-tab the ‘action’. What is the ‘reaction’? Justify your answer.

6. Air resistance is the force that results as objects move through the air. Is air resistance a contact or a non-contact force? Explain your answer.

 

 

 

194

Science Alive for VELS Level 5

Representing forceshe skysurfer on page 191 would not fall down to Earth if it weren’t for a force. In fact, without forces, the aeroplane he jumped from wouldn’t even be in the

sky. Knowledge of everyday forces can often be a matter of life or death for people who participate in extreme sports. Scientists have helped us understand forces by creating ways to represent them.

T

Drawing forcesScientists use arrows to represent forces. The direction of the arrow shows which way the force is acting. The length of the arrow shows how big the force is. A long arrow represents a larger, stronger force than a shortarrow.

The arrows that represent forces acting on an object should be drawn from the object’s centre of

gravity. All objects, including your body, have a centre of gravity.

Picture a point in your body where

your weight would be concentrated if your body was a

single point. That point is called

your centre of gravity.

Your centre of gravity changes with your position. When standing, your centre of gravity is at about bellybutton height.

Balanced and unbalanced forcesForces act on us all the time: when we are moving, when we are stopped and even when we are sleeping. More than one force is acting on us all the time. The forces acting on us can be balanced or unbalanced.

Unbalanced forces

The arrows describing the forward and backward forces on the kayaker are not the

same length. The forward force comes from the kayaker using a paddle to push

forward. The backward force is the drag from the water slowing the kayak down.

These forces are unbalanced. The forward force is larger than the backward force,

so the kayaker and his kayak move forward faster and faster. Eventually they

cannot move any faster. This is because drag increases when speed increases.

Eventually these two forces balance out and the kayaker moves at a constant speed.

Balanced forces

The arrows describing the up and down forces acting on the kayaker are the same

length. That means that the forces are the same size. But these forces are acting in

opposite directions. The weight of the kayaker and the kayak pushes down, but a

push from the water holds him up. The two forces are balanced and so the kayaker

does not move up or down.

 

 

 

1959. Forces and motion

✓ le

arn

ing I CAN:

explain what a force is

describe balanced and unbalanced

forces

describe the effects of balanced

and unbalanced forces.

The effect of balanced and unbalanced forces

Unbalanced forces can get something moving, like the kayak in the water. Unbalanced forces can increase or decrease the speed of the kayak as well. If the forward and backward forces are balanced, there is no increase or decrease in speed; the kayak moves at a steady speed or stays at rest.

Balancing weights

You will need:

washer fishing line or strong string

50 g masses ruler

marker pen paper

sticky tape 2 pulleys

2 clamps (if the pulleys don’t have them).

• Set up the equipment as shown.

• Use a ruler to find the middle of the string.

Mark this spot on the paper under the string.

• Line up the washer with the central spot marked on

the paper.

• Hang a 50 g mass from each end of the string.

Support the two masses until they are both secured

to the string.

• Gently let both masses go at the same time.

1. What happens to the position of the washer?

2. Are the forces on the washer balanced or

unbalanced?

• Repeat the process. Begin with the washer in the

centre of the string. This time, secure one 50 g mass

at one end and two 50 g masses at the other.

• Gently release the masses at the same time.

3. What happens to the position of the washer when

the masses are not equal?

4. Were the forces balanced or unbalanced in this part

of the experiment?

Washer

in centre

ClampClamp

Hanging

masses

Hanging

masses

Pulley Pulley

Paper

REMEMBER

1. How is the size of a force represented in a

diagram?

2. Imagine a moving object. List three things that an

unbalanced force could change about the object

and its motion.

3. What is the name of the force that slows down

movement in water?

THINK

4. There are four forces acting on the person in this

diagram.

(a) Which forces are balanced?

(b) Which forces are unbalanced?

(c) Is the rider’s speed increasing, decreasing or

constant?

5. Redraw the diagram from question 4 without the

force arrows. Instead, draw force arrows that

show the forces acting when the bike rider is

slowing down.

OBSERVE

6. Choose two objects around you. Use arrows to

draw the forces you think are acting on the

objects. (Hint: You can tell if forces are balanced or

unbalanced by looking at the object’s motion.)

Go to worksheet 9.2:

Balance in sport