1. CAMBRIDGE PRIMARY SCIENCE:
Physics Year 7
Unit 9: Forces and Motion
Isini Sehansa Amarathunga
Bsc(Hons) in Biomedical Science
9.1 Seeing forces
2. 9.1 Seeing forces
Must understand that forces can’t be seen
Should be able to explain the effects of force
Could be able to label force arrows
Learning objectives
10. 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)
11. 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?
12. What forces are acting on this accelerating boat?
What forces act on a moving boat?
thrust
drag weight
upthrust
air
resistance
Air resistance is friction caused by movement through air.
13. 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.
50N 30N
Resultant force on the crate = 50 N – 30 N
= 20 N to the left
14. 9.2 Forces big and small
Must know that Newton is the unit of force
Should understand that force can be measured
Could know how to use force meter to measure force
Learning objectives
15. Force
A force can be a push or a pull.
For example, when you push open a door you
have to apply a force to the door.
You also have to apply a force to pull open a
drawer.
16. Measuring forces
Forces can be measured using a force meter.
Force meters contain a spring connected to a metal
hook. The spring stretches when a force is applied to
the hook. The bigger the force applied, the longer the
spring stretches and the bigger the reading
The unit of force is called the newton, and it has the
symbol N. So 100 N is a bigger force than 5 N.
18. 9.3 Mass and weight
Must know that weight is a force
Should distinguish between mass measured in kilograms
(kg) and weight measured in Newtons.
Could recognise and use units of mass and weight and
identify the direction in which forces act.
Learning objectives
20. People often confuse mass and weight.
Remember that,
Weight is a force, and is measured in newtons, N.
Mass is measured in kilograms, kg.
Weight, mass and gravity
21. Mass
The mass of an object is the amount of matter or "stuff" it contains.
The more matter an object contains, the greater its mass.
An elephant contains more matter than a mouse, so it has a greater mass.
Mass is measured in kilograms, kg.
Remember an object's mass stays the same wherever it is
22. All objects have a force that attracts them towards each other. This is called gravity.
Gravitational force increases when:
1. the masses are bigger
2. the objects are closer
We are pulled down towards the ground because of gravity.
The gravitational force pulls in the direction towards the centre of the Earth.
Gravity
23. Weight
Weight is a force caused by gravity.
The weight of an object is the gravitational force between the object and
the Earth.
The more mass the object has the greater
its weight will be.
On the surface of the Earth an object with a
mass of 1 kg has a weight of about 10 N.
24. Mass and weight
The mass of an object stays the same wherever it is, but its weight can
change.
This happens if the object goes somewhere where gravity is stronger, or
weaker, such as the Moon.
The Moon has less mass than the Earth, so its gravity is less than the
Earth's gravity.
This means that objects weigh less on the Moon than they do on the Earth.
Remember that mass is measured
in kilograms, kg, and weight is measured
in newtons, N.
25. Mass and weight
The Moon's gravity is one sixth of the Earth's gravity.
A 120 kg astronaut weighs 1200 N on Earth.
On the Moon they would weigh only 200 N.
The astronaut's mass is 120kg wherever they are.
26. 9.4-Friction -an important force
• Friction is a force that appear
when two objects are in contact
with each other.
27.
28. Steps
•Set up: Attach a force meter to a block and place it
on a surface.
•Static Friction: Pull the force meter slowly until the
block begins to move, record that force.
•Friction (Optional): Keep pulling at a constant
speed, record the new force reading.
•Analyze: Write down the friction forces (static &
kinetic if measured) for calculations.
