| 1.1.3 Forces and Newton’s Laws of Motion |
|---|
Forces acting on an object are best analysed using a free-body diagram, in which each external force is drawn as an arrow from its point of application.
For an extended rigid body, weight is drawn as a single arrow acting from the centre of gravity.
Newton’s second law, ΣF = ma, links the resultant force on a body of constant mass to the acceleration it produces; the acceleration is directly proportional to the resultant force and acts in the same direction.
ΣF = ma
Newton’s first law
When the resultant force is zero, the acceleration is zero and the body remains at rest or continues to move at constant velocity; this is Newton’s first law.
Terminal velocity
Terminal velocity is the special case of a falling body in which the drag force has grown to equal the weight, so the resultant force is zero and the acceleration is zero, and the body falls at constant velocity.
Gravitational field strength
Gravitational field strength g is defined as the gravitational force per unit mass, and the weight of a body is found from W = mg.
W = mg
Near the Earth’s surface g ≈ 9.81 N kg⁻¹, equal numerically to the free-fall acceleration measured in Core Practical 1.
Core Practical 1
- The practical uses an electromagnet to hold and release a steel ball, together with a trapdoor and an electronic timer to measure the fall time and so remove reaction-time errors.
- Because s = ½gt², a graph of fall distance s against ½t² is a straight line whose gradient gives g directly.
Newton’s third law
Newton’s third law states that an interaction always involves two equal and opposite forces of the same type acting on two different bodies.
These third-law pairs must not be confused with the balanced forces acting on a single body in equilibrium.
Key Definition Gravitational field strength g is defined as the gravitational force per unit mass, and weight is found from W = mg.
Key Definition Terminal velocity is the special case in which drag has grown to equal the weight, giving zero resultant force and zero acceleration.