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## Ignore words

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Force
A push, pull or twist.
Air resistance
A force on objects moving through air.
Contact forces
A force where there needs to be contact between objects before the force can have an effect (e.g. friction).
Friction
A force between two objects that are touching. It usually acts to slow things down or prevent movement.
Gram(g)
A unit for measuring mass.
Gravity
The force of attraction between any two objects. The Earth is very big and so has strong gravity that pulls everything down towards it.
Kilogram(kg)
A unit for measuring mass. There are 1000 g in 1 kg.
Magnetism
A force that attracts objects made of iron or other magnetic materials. Two magnets can also repel each other.
Mass
The amount of matter that something is made from. Mass is measured in grams (g) and kilograms (kg). Your mass does not change if you go into space or to another planet.
Newton(N)
The unit of force.
Non-contact force
A force that can affect something from a distance (e.g. gravity).
Static electricity
A force that can attract or repel things. It is caused when certain materials rub together.
Upthrust
A force that pushes things up in liquids and gases.
Water resistance
A force on objects moving through water.
Weight
The amount of force with which gravity pulls things. It is measured in newtons (N). Your weight would change if you went into space or to another planet.
Compress
To squash something, or make it smaller.
Elastic
An elastic material changes shape when there is a force on it but returns to its original shape when the force is removed.
Elastic limit
If you stretch a spring beyond its elastic limit it will be permanently stretched. It is no longer elastic.
Extension
The amount by which a spring or other stretchy material has stretched. It is worked out from the stretched length minus the original length.
Force meter
Piece of equipment containing a spring, used to measure forces.
Hooke’s Law
The law that says that the extension of a spring is proportional to the force on it.
Limit of proportionality
The extension of a spring is proportional to the force on it, up to a certain point called the limit of proportionality. If you apply more force the extension is no longer proportional to the force.
Plastic
A plastic material changes shape when there is a force on it, but does not return to its original shape when the force is removed.
Proportional
A relationship between two variables where one doubles if the other doubles. A graph of the two variables would be a straight line through the origin.
Spring
A coil of wire that can be stretched or compressed.
Stretch
To pull something to make it longer.
Lubricant
A substance (usually a liquid) used to reduce friction.
Lubrication