Forces, Movement & Changing ShapeEdexcel IGCSE Physics: Revision notes

A force acting on a body can cause changes in its:

• speed (speeding up or slowing down)
• shape (stretching, compressing, bending)
• direction (changing the path of motion)

Forces arise from different origins, including gravitational forces (e.g. weight, attraction between masses) and electrostatic forces (e.g. attraction/repulsion between charges).

Force is a vector quantity — it has both magnitude (size) and direction. This differs from a scalar quantity, which has magnitude only (e.g. speed, mass, energy).

When several forces act along the same line on an object, they combine to give a single resultant force — the overall net effect of all the forces.

• Forces acting in the same direction are added together
• Forces acting in opposite directions are subtracted

Friction is a force that always opposes motion, acting in the direction opposite to an object's movement (or attempted movement).

The relationship between unbalanced (resultant) force, mass and acceleration is:

F = m × a

where F is the resultant force (N), m is mass (kg), and a is acceleration (m/s²).

Weight is the force of gravity acting on an object's mass, related by:

W = m × g

where W is weight (N), m is mass (kg), and g is gravitational field strength (N/kg).

Stopping distance = thinking distance + braking distance

• Thinking distance — the distance travelled during the driver's reaction time, before the brakes are applied
• Braking distance — the distance travelled once the brakes are applied, until the vehicle stops

Factors affecting stopping distance include:

• Speed — higher speed increases both thinking distance (more distance covered in the same reaction time) and braking distance
• Mass — a greater mass increases braking distance (more kinetic energy to dissipate)
• Road conditions — wet, icy or poorly maintained roads reduce grip, increasing braking distance
• Reaction time — tiredness, alcohol, drugs or distraction increase reaction time, increasing thinking distance

As an object falls, it accelerates due to its weight acting downward. As speed increases, air resistance (drag) acting upward also increases.

• Initially, weight is greater than air resistance, so there is a resultant force downward and the object accelerates
• As speed increases, air resistance increases, reducing the resultant force and acceleration
• Eventually, air resistance becomes equal to weight — the resultant force is zero, so acceleration is zero, and the object falls at a constant maximum speed called terminal velocity

Applying a force to a spring, wire or rubber band can stretch or compress it. Investigating how extension varies with applied force shows:

• Initially, extension is directly proportional to the applied force — this is the linear region described by Hooke's law
• Beyond a certain point (the limit of proportionality), the extension is no longer proportional to force, and the graph curves

Elastic behaviour is the ability of a material to return to its original shape once the deforming force is removed. If a material is stretched beyond its elastic limit, it will not fully return to its original shape (permanent/plastic deformation).