© Boardworks Ltd 2005 1 of 31 KS4 Physics Forces.

© Boardworks Ltd 20051 of 31

KS4 Physics

Forces


Forces

Types of Forces

Balanced and Unbalanced Forces

Summary Activities

Contents

Newton’s Laws


A force is a push or a pull. A force cannot be seen but you can see how a force affects an object.

What is a force?


What type of force?


When two objects or materials do not need to be touching for a force to have an effect, it is a non-contact force.

Examples:

When two objects or materials need to be touching for a force to have an effect, it is a contact force.

Examples: friction air resistance

gravity electrostatic magnetic

Contact and non-contact forces

Non-contact forces act over a distance. Are these non-contact forces attractive, repulsive or both?


Friction is a type of force that always acts in the opposite direction to which an object is moving and slows it down.

Whenever there is friction between two objects, heat is generated and their surfaces eventually wear away.

The effect of friction can be reduced by using a lubricant.

Oil is a common lubricant that is used in car engines and bike gears to reduce friction effects.

Effects of friction


If you rub your hands together they get warm. There is resistance to the rubbing motion.

What is the name of this resistive force called?

What causes this force?

Your hands might look smooth, but on a microscopic level they have rough surfaces. So when you rub your hands together you feel the resistive force of friction.

What is friction?

It is called friction.


Friction always acts in the opposite direction to which an object is moving.

An object will only start to move if the forces applied to it are greater than any frictional forces.

What is the direction of friction acting on each moving ball?

In which direction does friction act?

friction

friction

friction

friction


Gravity is an attractive force that exists between all masses.

The larger the mass, the greater the gravitational attraction.

The greater the distance between masses, the smaller the gravitational attraction.

The Earth has a large mass and so produces a strong gravitational force.

The Moon is kept in orbit around the Earth by the pull of the Earth’s gravity.

Gravitational attraction


The pull of the Earth’s gravitational force on an object is called weight.

The Moon also has a gravitational force.

Why is the weight of an object on the Moon less than the weight of the same object on the Earth?

The Moon is smaller than the Earth and so the pull of the Moon’s gravity is weaker than the pull of the Earth’s gravity.

This means that the weight of the object is less on the Moon.

Gravity and weight


What is the difference between mass and weight?

Mass is the amount of matter that makes up an object.

The mass of an object is always the same, wherever it is in the Universe.

The units of mass are kilograms (kg).

Weight is a force due to the pull of gravity on an object.

The weight of an object will vary depending on where it is in the Universe.

The units of weight are newtons (N).

1 kgmass = 1 kg

weight = 10 N

Mass and weight


Mass and weight – true or false?


Forces

Types of Forces


Summary Activities

Contents

Newton’s Laws


The direction of each arrow shows you the direction of each force.

The size of each arrow can be used to compare the sizes of the forces.

A force diagram uses arrows to show the forces acting on an object.

What is the force diagram for this falling object when it first starts to fall?

air resistance

weight

Force diagrams


What forces are acting on Mel’s computer?

The computer is pulled downwards by the force of gravity and causes it to have weight.

The table exerts an equal and opposite force pushing upwards on the computer. This is called the normal force.

Forces on still objects

reaction force

These forces are balanced so the computer does not move.

What forces are acting on Mel as she works at her computer?

weight


Forces and motion

If the forces on an object are balanced, the object will continue to do what it is already doing without change.

If the object is stationary, it will remain stationary.

If the object is moving, it will continue to move at the same speed and in the same direction.

If the forces on an object are unbalanced, two things about the object can change:

The speed – the object may speed up or slow down.

The direction of motion.


If an object is stationary and unbalanced forces act on it, what will happen to the object?

If an object is moving and unbalanced forces act on it, what can happen to the object?

Unbalanced forces and motion

The speed of the object can change. It might speed up or slow down.

The direction of the object can change.

The object will start to move – its speed and direction have changed.


Balanced or unbalanced forces?


Friction and movement


Forces

Types of Forces


Newton’s Laws of Motion

Summary Activities

Contents


Newton’s Laws of Motion

1st Law – An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force.

2nd Law – Force equals mass times acceleration.

3rd Law – For every action there is an equal and opposite reaction.


An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force.

11stst Law of Motion Law of Motion (Law of Inertia) (Law of Inertia)


Inertia is the tendency of an object to resist

changes in its velocity: whether in motion or

motionless.

These pumpkins will not move unless acted on by an unbalanced force.


Once airborne, unless acted on

by an unbalanced force (gravity and

air – fluid friction), it would

never stop!

Unless acted upon by an

unbalanced force, this golf ball

would sit on the tee forever.


Why then, do we observe every day objects in motion slowing down and

becoming motionless seemingly without an outside force?

It’s a force we sometimes cannot see – FRICTION!


There are Four main types of Friction:

Sliding Friction: Ice Skating Rolling Friction: Bowling Fluid Friction (air/liquid): Air or water resistance Static Friction: initial friction when moving an

object


Newtons’s1st Law and You

Don’t let this be you. Wear seat belts! Because of inertia, objects (including you) resist changes in their motion. When the car going 80 km/hour is stopped by the brick wall, your body keeps moving at 80 m/hour.


Newtons’ 2nd Law of Motion

The net force of an object is equal to the product of its mass and acceleration, or F=ma.


The net force of an object is equal to the product of its mass and

acceleration, or F=ma.

Newton’s 2nd Law


Newton’s 2nd Law proves that different masses accelerate to the earth at the same

rate, but with different forces.

We know that objects with different masses accelerate to the ground at the same rate. However, because of the 2nd Law we know that they don’t hit the ground with the same force.


For every action, there is an equal and opposite reaction.

Newton’s 3rd Law


According to Newton, whenever objects A and B interact with each other, they exert forces upon each other. When you sit in your chair, your body exerts a downward force on the chair and the chair exerts an upward force on your body.


There are two forces resulting from this interaction - a force on the chair and a force on your body. These two forces are called action and reaction forces.


Flying gracefully through the air, birds depend on Newton’s third law of motion. As the birds push down on the air with their wings, the air pushes their wings up and gives them lift.


Forces

Types of forces


Summary activities

Contents

Newton’s Laws


Multiple-choice quiz

© Boardworks Ltd 2005 1 of 31 KS4 Physics Forces.

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