What Is Forces, Movement, Shape And Momentum? A Visual Guide for Edexcel IGCSE Physics Students

Forces, movement, shape and momentum is the largest and most commonly examined topic in Edexcel IGCSE Physics

If there is one area of the 4PH1 specification that you cannot afford to leave under-revised, it is this one. Forces, movement, shape and momentum explained here covers everything from resultant forces and Newton's laws through to Hooke's law, momentum conservation and moments. The edexcel igcse forces, movement, shape and momentum content stretches across both Paper 1 and Paper 2, and the extended (P) material on momentum appears exclusively on Paper 2.

Key Facts

Forces and their effects

A force can change an object's speed, direction or shape. The edexcel igcse physics definition of a resultant force is the single force that has the same effect as all the individual forces acting on an object combined. When forces act along a line, you add those in the same direction and subtract those in the opposite direction.

If the resultant force on an object is zero, the object either stays still or continues at constant velocity. If the resultant force is not zero, the object accelerates in the direction of the resultant force. This is Newton's second law, expressed as F = m x a.

Worked example: A box of mass 8 kg sits on a frictionless surface. A horizontal force of 24 N is applied. Calculate the acceleration.

• F = m x a
• a = F / m = 24 / 8 = 3 m/s²

Weight and gravitational field strength

Weight is the force of gravity acting on a mass. On Earth, the gravitational field strength g = 9.8 N/kg (often rounded to 10 N/kg in exam questions). The relationship is W = m x g.

Gravitational field strength varies: on the Moon it is approximately 1.6 N/kg, so an object with a mass of 60 kg has a weight of 588 N on Earth but only 96 N on the Moon. The mass stays the same; the weight changes because g is different.

Stopping distance

Stopping distance = thinking distance + braking distance. This is one of the edexcel igcse physics notes topics that appears in both written and applied contexts.

• Thinking distance depends on the driver's reaction time, which increases with tiredness, alcohol, drugs or distractions.
• Braking distance depends on the speed of the vehicle, the mass of the vehicle, the road surface condition (wet, icy, gravel) and the condition of the brakes and tyres.

Higher speed increases both thinking distance and braking distance, but braking distance increases more steeply because kinetic energy is proportional to speed squared.

Terminal velocity

When an object falls through a fluid (air or liquid), it accelerates initially because weight is greater than the drag force. As speed increases, the drag force increases until it equals the weight. The resultant force becomes zero, and the object falls at a constant speed called terminal velocity.

Hooke's law and elastic behaviour

The initial linear region of a force-extension graph shows that extension is directly proportional to the applied force. This is Hooke's law. Beyond the limit of proportionality, the relationship is no longer linear, and the material may deform permanently.

Elastic behaviour means the material returns to its original shape when the deforming force is removed. If the material does not return to its original shape, it has been plastically deformed.

Worked example: A spring extends by 0.04 m when a force of 10 N is applied. Calculate the spring constant.

• F = k x e
• k = F / e = 10 / 0.04 = 250 N/m

Momentum

Momentum is the product of mass and velocity: p = m x v. It is a vector quantity, so direction matters. What is forces, movement, shape and momentum igcse without a solid grasp of the conservation of momentum? In any collision or explosion where no external forces act, the total momentum before the event equals the total momentum after.

Worked example: A 2 kg trolley moving at 3 m/s collides with a stationary 1 kg trolley. They stick together. Find their velocity after the collision.

• Total momentum before = (2 x 3) + (1 x 0) = 6 kg m/s
• Total momentum after = (2 + 1) x v = 3v
• 3v = 6, so v = 2 m/s

The force-momentum relationship F = (mv - mu) / t explains why safety features work. Crumple zones, seat belts and airbags all increase the time over which the change in momentum occurs, which reduces the force on the occupant.

Newton's third law

For every action, there is an equal and opposite reaction. The two forces in a Newton's third law pair act on different objects, are equal in size, opposite in direction and of the same type. A book resting on a table pushes down on the table with its weight; the table pushes up on the book with a normal contact force of the same magnitude.

Moments and the principle of moments

The moment of a force about a pivot is calculated using Moment = F x d, where d is the perpendicular distance from the line of action of the force to the pivot. The unit is newton metres (Nm).

The principle of moments states that for a system in equilibrium, the sum of clockwise moments about any point equals the sum of anticlockwise moments about that same point.

Worked example: A uniform beam is balanced on a pivot at its centre. A 40 N weight is placed 0.3 m from the pivot on the left. What force must be placed 0.6 m from the pivot on the right to balance the beam?

• Clockwise moment = anticlockwise moment
• 40 x 0.3 = F x 0.6
• 12 = 0.6F
• F = 20 N

The weight of a body acts through its centre of gravity. For a uniform object, the centre of gravity is at its geometric centre.

Vector and scalar quantities in force problems

The specification requires you to understand how vector quantities differ from scalar quantities. Force is a vector: it has both magnitude and direction. When two forces act on an object along the same line, you must account for their directions. Two 10 N forces acting in the same direction give a resultant of 20 N. Two 10 N forces acting in opposite directions give a resultant of 0 N. This distinction is essential for solving equilibrium problems and for understanding why an object can have forces acting on it and yet not accelerate.

Mass is a scalar quantity. It has magnitude (measured in kg) but no direction. Weight, however, is a vector because it acts downwards towards the centre of the Earth. Every edexcel igcse physics explained resource emphasises this distinction because confusing scalar and vector quantities leads to errors in force diagrams, momentum calculations and resultant force problems.

Friction and its role in motion

Friction is a force that opposes motion. It acts in the opposite direction to the movement of an object (or the direction it would move if it could). On a rough surface, friction between the object and the surface converts kinetic energy to thermal energy. This is why brakes get hot and why sliding across a carpet causes a burn.

In the exam, friction questions often appear alongside Newton's second law. If a 500 N driving force acts on a car and friction provides 200 N of resistance, the resultant force is 500 - 200 = 300 N forward. The acceleration is then a = F / m.

Worked example: A cyclist applies a forward force of 150 N. Air resistance and friction provide a total opposing force of 90 N. The combined mass of cyclist and bicycle is 80 kg. Calculate the acceleration.

• Resultant force = 150 - 90 = 60 N
• a = F / m = 60 / 80 = 0.75 m/s²

Self-check questions

1. A 1500 kg car experiences a resultant forward force of 4500 N. Calculate its acceleration.
2. A spring has a spring constant of 200 N/m. How much does it extend when a 15 N force is applied?
3. A 0.5 kg ball moving at 4 m/s hits a wall and bounces back at 3 m/s. Calculate the change in momentum.
4. A 60 N force acts 0.25 m from a pivot. Calculate the moment.
5. Explain why a parachutist eventually reaches terminal velocity.

Use the Green Bridge CBT platform to practise Edexcel IGCSE Physics questions on forces, movement, shape and momentum, track your progress, and identify which sub-topics need the most attention before your exam.