2. Laws of Motion v1.1

1 of 42 © Boardworks Ltd 2007



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

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

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

air resistance

weight

How can forces be represented?

The forces acting on any object can be shown using a force diagram.

The length of each arrow is proportional to the size of the force.

Forces are measured in newtons (N)


thrust

What forces are acting on this fish as it swims at a constant speed?

friction

Thrust is the forwards force created by the fish.

Upthrust is the upwards force on the fish caused by the water around the fish. This is sometimes called buoyancy.

weight

upthrust

What forces act on a swimming fish?


What forces are acting on this accelerating boat?

What forces act on a moving boat?

thrust

friction weight

upthrust

airresistance

Air resistance is friction caused by movement through air.


Identifying forces


What are resultant forces?

There are usually several different forces acting on an object. The overall motion of the object will depend on the size and direction of all the forces.

The motion of the object will depend on the resultant force. This is calculated by adding all the forces together, taking their direction into account.

50 N 30 N

Resultant force on the crate = 50 N – 30 N

= 20 N to the left


Calculating resultant forces



What is friction?

What is friction and why is it an important force?


Friction is a resistive force that slows things down and tries to stop objects sliding past each other.

What is friction?

What would happen if friction didn’t exist?

friction pulling force

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


Friction – useful or a problem?


What causes friction?

Friction occurs between two objects because the surfaces of those objects are rough, and contain bumps and hollows.

This roughness means that a force is needed to move the two objects over each other.

Even objects that appear very smooth, such as polished metal or ice, have a rough surface if viewed under a microscope.


Different types of friction

Friction is most obvious when it acts between two solid objects, but it also acts between solid objects and gases, and between solid objects and liquids.

Friction caused by an object moving through air is called air resistance.

Friction caused by an object moving through a liquid, such as water, is called drag.


How does friction change?


Friction always tries to stop things moving or slow things down. What effect does this have?

Reducing the effects of friction

Friction creates heat and eventually wears down surfaces, which can be very damaging.

Lubricants are used to reduce friction in machinery and so help protect surfaces. In car engines, oil is used as a lubricant.

Bearings and rollers can also be used to reduce friction, for example, in wheels


Reducing the impact of air resistance

Friction caused by air resistance affects the speed and fuel consumption of vehicles such as cars, bikes and aeroplanes.

Vehicles are specially shaped, or streamlined, to enable air to flow past them as easily as possible, therefore reducing air resistance.

In a similar way, ships and boats have streamlined hulls to reduce the drag effects of water.

Which of these cars is the most streamlined?



Introducing balanced forces


What is Newton’s first law?

If the forces on an object are balanced, the object will continue to do what it is already doing:

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 resultant force acting on an object is zero, all the forces are said to be balanced.

This forms the basis of Newton’s first law of motion, which states:


Terminal velocity of a skydiver


Velocity–time graph of skydiver



Introducing unbalanced forces


If the resultant force acting on an object is not zero, all the forces are said to be unbalanced.

This forms the basis of Newton’s second law of motion, which states:

What is Newton’s second law?

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

the speed of the object may change – it may either increase or decrease

the direction of motion may change.


How is movement calculated from force?

The resultant force acting on an object is related to the object’s mass and acceleration. These three factors are linked by the following equation:

Resultant force is measured in newtons (N).

Mass is measured in kilograms (kg).

Acceleration is measured in metres per second per second (m/s2).

force = mass x acceleration


How do we use Newton’s second law?

A car has a mass of 1,000 kg. What force must the car’s engine supply to cause an acceleration of 2 m/s2?


= 1,000 x 2

= 2,000 N


Using a formula triangle

A formula triangle helps you to rearrange a formula. The formula triangle for force (f), mass (m) and acceleration (a) is shown below.

x

Cover the quantity that you are trying to work out, which gives the rearranged formula needed for the calculation.

So to find force (f), cover up f…

…which gives the formula…

f = m x a


How do we use Newton’s second law?

A lorry has a mass of 12,000 kg. What acceleration is caused by a force of 10,000 N?


= 0.83 m/s2

= 10,000

12,000

force

mass

acceleration =


F = ma calculations



What forces are acting on Mel’s computer?

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

These forces are balanced so the computer does not move.

What forces support objects?

weight

reactionforce

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

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


A force cannot exist on its own – there is always a second force acting against it.

This forms the basis of Newton’s third law of motion states, which states:

What is Newton’s third law?

These pairs of forces that act between two objects are sometimes called action–reaction pairs.

If object A exerts a force on object B, then object B exerts an equal but opposite force on object A.


Action–reaction pairs


How many pairs of balanced, unbalanced and action–reaction forces can you spot?

Balanced and unbalanced forces



air resistance – A frictional force that acts against an object moving through air.

balanced – A pair of opposing forces that are the same size and which have no effect on an object’s speed or direction.

drag – A frictional force that acts against an object moving through liquid.

friction – A force between two touching objects that tries to oppose movement.

reaction force – The opposing force created by an object in response to another force.

Glossary (1/2)


resultant force – A single force that has the same effect as all the forces acting on an object added together.

terminal velocity – The velocity of a falling object reached when its weight and air resistance are balanced and it no longer accelerates.

unbalanced – A pair of opposing forces that are different in size and which cause an object to change its speed or direction.

upthrust – An upwards force acting on an object in a liquid or a gas.

weight – The force created by the gravitational attraction on a mass, measured in newtons (N).

Glossary (2/2)


Anagrams


The effect of resultant forces


Examples of the first law?


Multiple-choice quiz

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2. Laws of Motion v1.1