1 of 9© Boardworks Ltd 2010. 2 of 9© Boardworks Ltd 2010 What are forces?

1 of 9 © Boardworks Ltd 2010


What are forces?


Push and pull


Force

• Forces come in pairs• Forces have a magnitude (measured in

Newtons) and a direction• Forces are often represented with arrows

that show the direction and magnitude

5N, north (up) Magnitude: 5NDirection: north (up)


All forces involve interactions between objects. There are several different types.

Types of forces

What forces are acting during this rocket launch?

All types of forces can occur whether objects are still or moving.

Gravity and magnetism are forces that can act over distances.

Friction and upthrust are forces that involve direct contact between objects.


There are two wind machines applying forces to the ice skater. The forces acting on the skater are equal in magnitude and opposite in direction.

Balanced forces

The forces are balanced, so they cancel each other out. The skater does not move. Balanced forces do not cause a change in motion.


Unbalanced forces

Unbalanced forces are forces that are not equal in size or are not opposite in direction

Unbalanced forces lead to a change in speed or direction.

What if only one wind machine is blowing on the skater? The forces acting on her are no longer balanced so she will start to move to the left. Her speed will change – this is called acceleration.


Balanced Forces

The ship is floating on the water, the forces are balanced. Which forces are acting on the ship?

Gravity (Water’s) UpthrustBecause Gravity and Upthrust are equal the

ship stays afloat.


Balanced Forces

The vase is resting on the table. It is not moving, therefore the forces must be balanced.

Which forces are acting on the vase?

Gravity Table’s Upthrust

Have a look around the classroom and name some of the balanced forces in action on different objects.


Unbalanced Forces

The man and the parachute are slowly falling to the ground.

Which forces are acting on the parachute?

Which force is bigger? How can you tell?

Gravity is pulling the man to the ground

Air resistance is pushing upwards

Gravity is the bigger force. We can tell this because the direction of movement of the man and the parachute is the same as the direction of the force of gravity.


Unbalanced Forces

The same forces are acting on the elephant but without a parachute there is much less air resistance.

Gravity is pulling to the ground

Air resistance is pushing upwards

Gravity is a much bigger force. Because there is a bigger difference between the force pulling downwards and the force pushing upwards the elephant falls much more quickly than a man with a parachute.


Unbalanced Forces

The paperclip is jumping up to the magnet.

Which forces are acting on the paper clip? Which is the greater force?

The magnetic force is pulling the paperclip upwards

Gravity is pulling the paperclip to the ground

The magnetic force is greater than the force of gravity therefore the paperclip moves in the direction of the biggest force.


Unbalanced Forces

5 N, right + 10 N, right =15N, right


Forces

Each team is exerting a force – what happens if the pulling force produced by each team is equal?

What would happen if the team on the left were able to produce a bigger pulling force than the team on the right?


Force pairs


500 N 500 N

Imagine a car traveling at a constant speed of 50 km/h.

Balanced and unbalanced forces

The engine provides sufficient force to balance all the frictional forces that are acting to decrease the speed.


A crosswind acting on the car produces a sideways force.

The crosswind causes the direction of the car to change – this happens because the sideways forces on the car are not balanced. The car will veer sideways.

Balanced and unbalanced forces

If the car turns right so that the wind is now behind the car, what will happen to the speed?

100 Ncross wind

500 N 500 N


The resultant force is calculated by finding the difference between opposing forces in each direction.What is the resultant force on this truck?

The sum effect of more than one force is called the resultant force.

A resultant force of 100 N is accelerating the truck.

Resultant force

500 N400 N


Balanced and unbalanced forces – summary

and the object is stationary, it will remain stationary

and the object is moving, it will continue to move at the same speed and in a straight line.

If the forces are unbalanced, four things can happen:

If the forces on an object are balanced:

In other words, the object will continue to do what it is already doing without any change.

The speed can change. It can start moving. It can stop moving. It can change direction.


20 N

1. What is the resultant force on the satellite?Resultant force = 20 N –

10 N

= 10 N down

The satellite will accelerate downwards.

Resultant forces – question 1

5 N 5 N


5 N

2. What is the resultant force on the bird?

5 N 5 N

The forces acting in each direction horizontally are equal in size, so there is no resultant force in this direction.

Resultant force = 5 N – 5 N = 0 N


The vertical forces are not balanced, the bird will accelerate in a downwards direction.

Resultant force = 5 N – 0 N = 5 N down


The vertical forces are equal in size and opposite in direction so there is no resultant force in the vertical direction.

The horizontal forces are not balanced, so the yacht will accelerate to the right.

10 N

13 N

3. What is the resultant force on the yacht?

10 N

10 N

Resultant force = (20 N +10 N) – 13 N

= 17 N right


20 N

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