1.1.4 Types of Force

What is a force?

• A force is a vector quantity, it has both magnitude and direction
• A force is a push or a pull on an object
• A force is measured in Newtons
• In A level mathematical models, forces act at a single point called a particle which occupies a single point in space

What types of force are used in mechanics?

• Weight is the effect of mass and gravity, it always acts downwards
• Tension is a pulling force, it always acts away from an object
• Thrust is a pushing force, it always acts towards an object
• Friction is a resistive force, it acts to oppose the motion of an object
• Every surface will produce a reaction force, it will always act perpendicular to the surface

Mass, Gravity and Weight

• Mass is a measure of the amount of matter in an object
  • Mass is a scalar quantity, measured in kilograms, kg
  • Mass is universal, it does not change based on location
• Gravity is the force by which a body (usually a planet or a star) pulls objects towards its centre
  • g is the acceleration due to gravity, measured in  m s^-2
  • On Earth, g is approximately 8 m s^-2 , although its exact value varies with location
  • g is different elsewhere in the universe
• Weight is the product of mass and gravity, it is a force measured in Newtons
  • Weight is a vector quantity
  • Weight varies with location
  • Weight always acts vertically downwards towards the ground

Worked Example

The diagram below shows a car towing a trailer by a light inextensible string.

Forces D, R, F and T N act on the car and caravan as shown.

Write a brief description of which each force represents.

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