Force, Work, Power and
Pressure
Chapter 27
Learning intentions
By the end of this chapter you should be able to
• Explain force, work, power and pressure
• List four examples of forces
• Explain the moment of a force
• Explain levers and what they are used for
• Carry out simple calculations for work, power and
pressure
What is Force?
Force: Causes a stationary object to move
Unit of force = Newton (N)
Types of Forces:
• Force of gravity
• Magnetic force
• Electric force
• Force of friction
Sir Isaac Newton
Forces on a car
Types of Forces
Force of Gravity
• Objects fall to the ground due to gravity
Magnetic force
•If a magnet is placed
near a piece of metal
it will attract the
metal causing it to
move
Types of Forces
Electric force
• Static electricity attracts small pieces of paper
• When you rub a plastic pen against the sleeve of your
jumper, you can lift small pieces of paper with your pen.
activity
• Try to demonstrate static electricity (you can watch the videos for
ideas)
Force of Friction
Friction: A force that opposes the sliding motion
of an object when in contact with another
object
• Rougher surfaces create a greater frictional force
when in contact with another object
• Smoother surfaces create less frictional force when in
contact with another surface (e.g. walking on ice)
Activity
• Watch the video
Advantages of Friction
• Friction is needed to stop a car or bicycle
• Friction occurs between the brakes and the brake pads)
• Friction allows you to walk on the ground
• Friction occurs between the shoes and the ground
• Friction between car tyres and the ground allows
cars to drive around corners at a greater speed
Disadvantages of Friction
• Shoes become worn because of the friction
between the shoes and the ground
• If a person is walking a lot they may get blisters due
to the friction between their feet and the shoes
• Friction between a moving car and the air results in
more fuel being used (air resistance)
How to reduce friction:
Lubrication: Used to reduce the friction
between two surfaces in contact
Examples of lubricants:
•Oil
•Liquid polish
•Soap
How to reduce friction:
Examples of lubricants used to reduce friction:
• Oils are used in engines to reduce friction of the
moving parts
• Human joints (where two bones meet) have a
liquid to reduce the friction between the bones
• Squeaky hinges on doors (caused by friction) can
be fixed by using a liquid polish as a lubricant
How to reduce friction:
Synovial fluid in joints
Weight and mass in science
Weight and Mass:
What is the difference?
Weight (force)
Mass
Weight is the amount of
matter in an object and the
force of gravity on the
object
Weight is measured in
Newtons (N)
Mass is the amount of
matter in an object
Weight of an object can
change
Mass of an object does not
change
Mass is measures in grams
or kilograms
Weight and Mass:
How to convert mass into weight
Weight (Newtons) = Mass (kilograms) x 10
What is the weight of an object of mass 6.8kg?
Weight = Mass x 10
Weight = 6.8 x 10
Weight = 68 N
Mass (kilograms)= Weight (Newtons) ÷ 10
What is the mass of an object of weight 250N?
Mass = Weight ÷ 10
Mass = 250 ÷ 10
Mass = 25 kg
The effect of force on a spring:
Hooke’s Law: The extension of a spring is directly proportional to the force
causing the extension
Experiment
To investigate the relationship between the extension of a spring and the
applied force
Equipment: Spring, weights, metre stick, retort stand
Method:
1.
2.
Place a spring to a retort stand and measure the length of it
Attach a weight to the spring and measure the extension of the spring
3.
Repeat by adding more weights to the spring and record the new
extensions
The effect of force on a spring:
Result:
The effect of force on a spring:
Conclusion:
• The graph has a straight line through the origin
• The extension of a spring is directly proportional to the force
Force: causing a stationary object to move
Force causes movement in a circular manner
Moment of a force: When an object moves in a circular pattern due to
the force applied to it
It tells us how much spin or circular motion a force can cause
How to Calculate Moment of a Force
Moment of a force (Nm) = Force (N) x Distance from fulcrum (m)
Fulcrum or axis: the point at which the object turns
Moment of a force calculations
1.
A force of 7N is applied to a piece of cardboard. The distance from the
fulcrum to the force is 0.8m. Calculate the moment of the force.
Moment of a force (Nm) = Force (N) x Distance from fulcrum (m)
Moment of a force = 7N x 0.8m
Moment of a force = 5.6 Nm
1.
A 6N causes an object to spin. The force is 0.5m away from the fulcrum.
Calculate the moment of the force.
Moment of a force (Nm) = Force (N) x Distance from fulcrum (m)
Moment of a force = 6N x 0.5m
Moment of a force = 3 Nm
Levers
Lever: A rigid body that can turn about a fixed point called a fulcrum.
Fulcrum: point where the
lever turns
Effort: Force being
applied by the student
Load: The large rock
Everyday examples of Levers
Everyday Examples of Levers
Scissors
• Load: The paper being cut
• Effort: Force applied by the fingers
• Fulcrum: Located between the effort and the load
Wheelbarrow
• Load: The item being carried
• Effort: Force applied by the person
• Fulcrum: axle of the wheel at the front
Door
• Load: door being moved
• Effort: push or pull applied to the handle
• Fulcrum: hinges on the door
Law of the Lever
Law of the lever: when a lever is balanced the sum of the clockwise
moment equals of the sum of the anticlockwise moment.
You know how to calculate the moment of a force
Moment of a force (Nm) = Force (N) x Distance from fulcrum
(m)
Law of the Lever
Anticlockwise Moment
10N x 2m = 20Nm
Clockwise Moment
20N x 1m = 20Nm
Use your knowledge of the Law of the lever
to find missing value of the force
Anticlockwise moment
20N x 12m = 240Nm
Clockwise moment
F x 8m
Anticlockwise moment = Clockwise moment
240Nm = 8m x F
240Nm / 8m = F
30N = F
What is Pressure?
Pressure is the amount of force acting on a certain area.
Pressure = Force
Area
Unit of pressure: N/m2 or Pascal (Pa)
The unit you use will depend on the information given in the question
Pressure Calculations
F – Force
P – Pressure
A – Area
F
P
x
A
Pressure Calculations
F
P
Force (F)= Pressure (P) x Area (A)
Pressure (P) = Force (F) / Area (A)
Area (A) = Force (F) / Pressure (P)
x
A
Question 1
A force of 200N acts on an area of 25m2. Calculate the pressure.
Pressure = Force / Area
Pressure = 200N / 25m2
Pressure = 8 N/m2
Question 2
A 100kg block rests on a table. The area of contact between the block
and the table is 4m2. Calculate the pressure the block exerts on the
table.
Force = Mass x 10
Force = 100kg x 10
Force = 1000N
Pressure = Force / Area
Pressure = 1000N / 4m2
Pressure = 250N/m2
Question 3
Calculate the pressure on the ground when a person weighing 500N
balances on one heel of a stiletto shoe. The area of the heel is 1cm2.
Pressure = Force / Area
Pressure = 500N / 1cm2
Pressure = 500N/cm2
Question 4
Calculate the pressure on the ground when an elephant weighing
50000N balances on one foot. The area of the elephant’s foot is
400cm2.
Pressure = Force / Area
Pressure = 50000N / 400cm2
Pressure = 125N/cm2
Increasing Pressure
To increase pressure you have two options:
1. Increase the force
2. Decrease the value of the area
Example:
A block has a weight of 1000N.
The block has a length of 20cm,
width of 10cm and a height of
5cm. What way do you place
the block on the table to get
maximum pressure?
Pressure varies with the depth of a liquid
Pressure varies with the depth of a liquid
Pressure varies with the depth of a liquid
Air has mass and occupies space
Atmospheric pressure
Earth’s atmosphere: layer of gases around the earth’s surface.
These gases exert force on the earth due to gravity
• Atmospheric pressure
Why don’t we feel the pressure?
The outside atmospheric pressure pushing in
is balanced by the inside pressure pushing
out
Atmospheric Pressure
Atmospheric pressure at the top of a high mountain is less that the
atmospheric pressure on a valley.
Measuring Pressure
Pressure gauge
• Used to measure tyre pressures on cars
Barometer
• Measures atmospheric pressure
• Used in forecasting weather
Measuring Pressure
Altimeter
• Measures the atmospheric pressure and converts it to height
• Used in aeroplanes
Weather and Pressure
Atmospheric pressure has a huge influence on the weather.
• Low atmospheric pressure: when the air is warm and contains moisture
• Brings wind and rain
• High atmospheric pressure: when air is cold and dry
• Brings settled, dry weather
Weather and Pressure
Work
• Work is done if a force causes an object to move.
The trolley is moving so Work IS
being done
The house is not moving so Work
IS NOT being done
How to calculate Work
• The Unit of Work is Joules (J)
Work (J) = Force (N) x Distance (m)
The cat is using a force of 15N to move the trolley a distance of 2 metres.
Calculate the work being done.
Work (J) = Force (N) x Distance (m)
Work (J) = 15N x 2m
Work (J) = 30 J
Complete the following calculations to find out the amount of work
being done.
Calculate the work done when:
1. A force of 60N is used to
move an object 3 metres.
2.
A force of 150N is used to
move an object 1.5 metres.
3.
A force of 3N is used to
move an object 5 metres.
4.
A force of 20N is used to move
an object 4.2 metres.
5.
A force of 15N is used to move
an object 8 metres.
More difficult calculations
• Calculate the work done when:
1. A force of 2.5N is used to move an object 200cm.
2.
A force of 5N is used to move and object 60cm
3.
A weightlifter lifts 120kg to a height of 2m