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Concept of Force is taught in the note for Force. Read it below and look for the section on this topic.

Force

9 min read

What is a force?

A force is a push or a pull. It is measured in newtons (N) with a spring balance. A force can change the speed, the direction or the shape of an object.

Some types of force
ForceWhat causes it
Weightthe pull of the Earth (gravity) on an object
Frictionrubbing between surfaces; it opposes motion
Upthrusta fluid pushing up on an object in it
Tensiona stretched string or rope pulling
Magnetic and electric forcesmagnets and charges acting at a distance

Mass and weight

Weight = mass × gravitational field strength. On Earth g ≈ 9.8 N/kg (often rounded to 10).
  • weight (N)
  • mass (kg)
  • gravitational field strength (N/kg)
Mass and weightThe same 70 kilogram mass on Earth and on the Moon. Its mass is 70 kilograms in both places, but its weight is 700 newtons on Earth and only 112 newtons on the Moon.70 kgW = 700 NOn Earthg = 10 N/kg70 kgW = 112 NOn the Moong = 1.6 N/kg
The mass is always 10 kg, but the weight changes with the planet: about 16 N on the Moon, 37 N on Mars, 98 N on Earth and 248 N on Jupiter.

Combining forces

When several forces act on an object, we can replace them by one force called the resultant. Forces in the same direction add. Forces in opposite directions subtract. Forces at an angle are added as arrows (vectors).

Combining forcesThree examples of a resultant force: two forces in the same direction add, two forces in opposite directions subtract, and two forces at right angles combine like the sides of a triangle.5 N3 N8 NSame direction: addresultant 8 N →5 N3 N2 NOpposite: subtractresultant 2 N →4 N3 NAt an angle: arrowsresultant 5 N (Pythagoras)
Try itTwo forces, one resultant

Pull an object with two forces. Change their sizes and the angle between them. The green arrow is the resultant.

Worked exampleWeight on the Moon

An astronaut of mass 70 kg stands on the Moon where N/kg. What is the astronaut's weight?

  1. Use W = mg.

Answer: The weight on the Moon is 112 N (about 700 N on Earth), but the mass is still 70 kg.

Check yourself A force of 8 N pushes right and a force of 3 N pushes left on the same object. What is the resultant?
Check yourself Which quantity changes when you travel to the Moon?

Key terms

Newton (N)
The unit of force.
Weight
The force of gravity on a mass.
Resultant
The single force that has the same effect as several forces.
Friction
A force that opposes motion between surfaces.

Summary

  • A force is a push or pull measured in newtons.
  • Forces can change speed, direction or shape.
  • Weight W = mg; mass stays the same everywhere, weight does not.
  • Resultant: add forces in the same direction, subtract opposite ones.
Types of Forces