KS4 Physics. Forces. 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?.
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A force is a push or a pull. A force cannot be seen but you can see how a force affects an object.
When two objects or materials need to be touching for a force to have an effect, it is a contactforce.
When two objects or materials do not need to be touching for a force to have an effect, it is a non-contactforce.
Non-contact forces act over a distance.
Are these non-contact forces attractive, repulsive or both?
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.
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 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.
What is the difference between mass and weight?
mass = 1kg
weight = 10 N
Mass is the amount of matter that makes up
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).
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?
What forces are acting on Mel’s computer?
The computer is pulled downwards by the force
of gravity and causes
it to have weight.
Thetable exerts an
equal and opposite
force pushing upwards
on the computer. This is called the reaction force.
These forces are balanced so the computer does not move.
What forces are acting on Mel as she works at her computer?
What forces are acting on this fish as it swims?
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.
What forces are acting this moving boat?
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?
The increase in upthrust equals the increase in weight,
so both forces remain equal and balanced.
at the same speed and in the same direction.
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?
It might speed up or slow down.
How many pairs of forces can you spot?
Are they balanced or unbalanced?
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 calledfriction.
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.
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 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.
on a mass, measured in newtons (N).