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
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.
- There are many phrases/words that can be used implying the types of force
- Braking – suggests thrust
- Air resistance – suggests a resistive force
- Always sketch a diagram and mark the forces on the diagram clearly
- It will help you understand the problem
- Add more things to the diagram as you progress through the question
- You may not even need all the forces from your diagram
- The value used for g in questions tells you how you should round any answers
- If g = 9.8 m s-2, use two significant figures
- If g = 10 m s-2, use one significant figure