Forces and Motion
Forces and Newton's Laws
Pearson Edexcel International GCSE Physics
Forces and their effects
- A force can change the speed, the direction of motion or the shape of an object.
- A resultant force in the opposite direction to the velocity: the speed will decrease.
| Force | Direction |
|---|---|
| weight | vertically downwards |
| air resistance, or drag in a liquid | opposite to the velocity |
| friction | opposite to the motion, between two surfaces |
| upthrust | upwards, on an object in a liquid or a gas |
| tension | along a string or cable: upwards on a hanging object |
| thrust, driving force | forwards, pushing a vehicle |
| lift | upwards, on a wing |
| magnetic force, electrostatic force | between magnets, or between charges |
- Label the downward force on any object with mass weight.
- Draw each force as an arrow starting on the object, in the direction of the force, its length showing its size. Two balanced forces: arrows of equal length.
Scalars and vectors
- A scalar has magnitude only. A vector has magnitude and direction.
- Both have magnitude: only vectors have a direction. Force is a vector.
| Scalars | Vectors |
|---|---|
| speed | velocity |
| distance | displacement |
| mass | acceleration |
| time | momentum |
| energy, power, temperature | force, weight |
Resultant force
- The resultant force is the single force with the same effect as all the forces acting on an object.
- Forces along one line: add forces in the same direction, subtract forces in opposite directions.
- Give the resultant as a magnitude in N and a direction: left, right, up or down.
- Balanced forces: the resultant force is zero, so there is no acceleration. The object stays at rest or moves at constant velocity.
- A vehicle at constant velocity: the driving force = friction + air resistance.
- Friction and air resistance act in the opposite direction to the velocity.
Force, mass and acceleration
force = mass × accelerationF = m × a
- F is the resultant force: subtract opposing forces before using the equation.
- Rearranged: a = F / m, m = F / a.
Weight
weight = mass × gravitational field strengthW = m × g
- Rearranged: m = W / g, g = W / m.
- On Earth, g = 10 N/kg.
- g is the gravitational field strength.
| Quantity | Symbol | Unit |
|---|---|---|
| force | F | N |
| mass | m | kg |
| acceleration | a | m/s2 |
| weight | W | N |
| gravitational field strength | g | N/kg |
Newton's third law
- Every force has an equal and opposite reaction: when body A exerts a force on body B, body B exerts a force on body A.
- The two forces: both forces are the same size and the forces are in opposite directions.
- The two forces act on two different bodies and are the same type of force.
- Two balanced forces on one object are not a third-law pair: the forces are acting on the same body and the forces are not the same type.
- A third-law pair: the weight of block and the gravitational force of block on Earth.
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