Mechanics
Forces and Newton's Laws
Pearson Edexcel International A Level Physics
Free-body force diagrams
- Show only the forces acting on the one body, each as an arrow that starts on the dot or the body, with a label.
- Weight: an arrow vertically downwards labelled weight, drawn from the centre of gravity.
- Force labels: weight, normal reaction, friction, tension, air resistance, upthrust, or the force from a named object.
- Balanced forces at constant velocity: opposite arrows are the same length, for example the arrow lengths of weight and air resistance same length.
- Draw each force once, whole. Never draw components of forces on a free-body diagram.
Resultant force and F = ma
- Resultant force: the vector sum of all forces acting on an object.
- The acceleration is in the direction of the resultant force; the mass is constant.
g = 9.81 N kg−1 at the surface of the Earth.
Newton's first law and terminal velocity
- Newton's first law: a body stays at rest or moves at constant velocity unless a resultant force acts on it.
- Resultant force zero: a = 0, so the body is at rest or travels at constant velocity.
Falling to terminal velocity
- Initially there is a resultant force downwards: the object accelerates.
- As the velocity increases the drag increases, so the resultant force and the acceleration decrease.
- This continues until the resultant force becomes 0, when weight = drag.
- Terminal velocity is the velocity when acceleration = 0.
Opening a parachute
- The air resistance is greater than weight.
- The resultant force is upwards and the skydiver decelerates.
- As the velocity decreases the air resistance decreases.
- Then resultant force = 0 and acceleration = 0: a lower terminal velocity.
A vehicle reaches its top speed when the air resistance, increasing with velocity, makes the resistive forces equal the driving force: no resultant force, no acceleration.
Newton's third law
- If body A exerts a force on body B, then B exerts an equal and opposite force on A.
- A third law pair: forces of equal magnitude that act in opposite directions, the same type of force, acting on different bodies.
| Situation | Third law pair |
|---|---|
| Rocket | The rocket motor exerts a force on the gases, so the gases exert a force on the rocket motor. |
| Athlete pushing off | By Newton's third law, the ground exerts a force equal and opposite on the athlete, so there is a resultant force on the athlete. |
| Body resting on the ground | The normal reaction R and the weight W act on the same body: R and W are different types of force, so they are not a N3 pair of forces. |
Name the type of force and both bodies: the pair to a weight is gravitational: one force is gravitational, the gravitational force of the Earth on the body and the gravitational force of the body on the Earth. Never write equal and opposite reaction for a Newton's third law pair and never write weight for the gravitational force in a third law pair, because neither names the type of force or the two bodies it acts on.
Quantities and units
| Quantity | Symbol | Unit |
|---|---|---|
| Resultant force | ΣF | N |
| Mass | m | kg |
| Acceleration | a | m s−2 |
| Weight | W | N |
| Gravitational field strength | g | N kg−1 |
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