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# KS4 Physics Forces.

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KS4 Physics Forces

Forces Types of forces Mass and weight Forces and motion Friction
Contents Forces Types of forces Mass and weight Forces and motion Friction Summary activities

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

What type of force?

Different types of forces

Contact and non-contact forces
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 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: gravity electrostatic magnetic Non-contact forces act over a distance. Are these non-contact forces attractive, repulsive or both?

Forces Types of forces Mass and weight Forces and motion Friction
Contents Forces Types of forces Mass and weight Forces and motion Friction Summary activities

Gravitational attraction
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.

Gravity and weight 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.

Mass and weight 1 kg What is the difference between mass and weight?
mass = 1 kg 1 kg weight = 10 N 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. 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 mass are kilograms (kg). The units of weight are newtons (N).

Mass and weight – true or false?

Forces Types of forces Mass and weight Forces and motion Friction
Contents Forces Types of forces Mass and weight Forces and motion Friction Summary activities

Force diagrams air resistance weight
A force diagram uses arrows to show the forces acting on an object. 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. air resistance What is the force diagram for this falling object when it first starts to fall? weight

Forces on still objects
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 reaction force. weight reaction force These forces are balanced so the computer does not move. What forces are acting on Mel as she works at her computer?

Forces on a swimming fish
What forces are acting on this fish as it swims? weight thrust friction upthrust Upthrust is the upwards force on the fish caused by the water around the fish. Sometimes this is called buoyancy. Thrust is the forwards force acting on the fish.

weight air resistance thrust friction upthrust
Forces on a moving boat What forces are acting this moving boat? weight air resistance thrust friction upthrust

Changing forces on a ferry
An empty ferry arrives at port and is loaded with travellers. What happens to the weight of the ship? What happens to the upthrust on the ship? Which force is now largest – weight or upthrust? It increases. It increases. The increase in upthrust equals the increase in weight, so both forces remain equal and balanced.

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.

Unbalanced forces and motion
If an object is stationary and unbalanced forces act on it, what will happen to the object? The object will start to move – its speed and direction have changed. If an object is moving and unbalanced forces act on it, what can happen to the object? The speed of the object can change. It might speed up or slow down. The direction of the object can change.

Balanced and unbalanced forces
How many pairs of forces can you spot? Are they balanced or unbalanced? Teacher Notes Items of note for the pairs of forces in this seaside scene include: On the promenade, there are people walking, with one child jumping off the railings. There is also a family eating an ice cream and the woman’s ice cream is dripping and has just started falling to the ground. There is a man selling helium balloons on the promenade. There is a a roller coaster at the end of the promenade. In the sea there are a couple of rowing boats – one being rowed and one anchored. Also, in the distance, there is a large ship. In the sky there is a parachutist, a hot air balloon, seagulls and an aeroplane.

Balanced or unbalanced forces?

Forces Types of forces Mass and weight Forces and motion Friction
Contents Forces Types of forces Mass and weight Forces and motion Friction Summary activities

What is 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? It is called friction. 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.

In which direction does friction act?
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? friction friction friction friction

Friction and movement

Effects of friction 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.

Friction – useful or a problem?

Forces Types of forces Mass and weight Forces and motion Friction
Contents Forces Types of forces Mass and weight Forces and motion Friction Summary activities

upthrust – An upwards force acting on an object in a liquid or a gas.
Glossary air resistance – A frictional force that acts against an object moving through air. contact force – A force that has an effect when two objects are touching. friction – A contact force between an object and another material. This force opposes the motion of the object. gravity – The force of attraction between any two objects that have mass. mass – The amount of matter that makes up an object, measured in grams (g) or kilograms (kg). unbalanced forces – Forces acting on an object that 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).

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