Understanding Forces and motion: Edexcel IGCSE Physics Explained

Forces and motion is the backbone of the Edexcel IGCSE Physics specification, and getting it right sets you up for half the exam. Here is everything you need, explained straight.

If there is one section of the edexcel igcse physics forces and motion content that you need to be rock-solid on, it is this one. Forces and motion covers speed, velocity, acceleration, Newton's laws, momentum, stopping distances, Hooke's law and moments. It is the most heavily examined area of the course, and the calculations you learn here appear in nearly every sitting of the igcse 4ph1 forces and motion exam.

These edexcel igcse physics revision notes walk you through the core content, show you worked examples, flag the mistakes that cost marks, and give you practice questions to test yourself. No fluff, just what you need for the exam.

Units you must know

Before you touch a single equation, lock these units into memory. The forces and motion edexcel igcse specification uses:

• Mass: kilogram (kg)
• Distance: metre (m)
• Speed/velocity: metre per second (m/s)
• Acceleration: metre per second squared (m/s²)
• Force: newton (N)
• Time: second (s)
• Gravitational field strength: newton per kilogram (N/kg)
• Momentum: kilogram metre per second (kg m/s)
• Moment: newton metre (Nm)

Getting units wrong is the fastest way to throw marks away. If a question gives you mass in grams, convert to kilograms before substituting into any equation.

Movement and position

Speed, distance and time

The relationship is simple: average speed = distance moved / time taken. Rearrange as needed: distance = speed x time, or time = distance / speed.

Distance-time graphs

A flat line means the object is stationary. A straight diagonal line means constant speed. A curved line means acceleration (if curving upward) or deceleration (if curving to flatten). The gradient of a distance-time graph gives the speed: steeper gradient means faster speed.

Acceleration

Acceleration is the rate of change of velocity: a = (v - u) / t, where v is final velocity, u is initial velocity, and t is time taken. The unit is m/s².

Velocity-time graphs

A flat line means constant velocity. An upward slope means acceleration. A downward slope means deceleration. The gradient gives the acceleration, and the area under the graph gives the distance travelled. For a triangle, area = 0.5 x base x height. For a rectangle, area = base x height.

The suvat equation

You also need: v² = u² + 2as, where s is the distance moved. This is useful when you are not given time.

Forces, movement, shape and momentum

Types of force and resultant force

Forces can be gravitational, electrostatic, magnetic, frictional, or applied. Force is a vector quantity, meaning it has both magnitude and direction. Scalar quantities (like mass or speed) have magnitude only.

To find the resultant force on an object, add forces acting in the same direction and subtract forces acting in opposite directions. If the resultant is zero, the forces are balanced and the object stays at rest or continues at constant velocity. If the resultant is non-zero, the object accelerates in the direction of the resultant.

Newton's second law: F = ma

The relationship between force, mass and acceleration is: force = mass x acceleration, F = m x a. Force is in newtons (N), mass in kilograms (kg), acceleration in m/s².

Weight, mass and gravity

Weight is the force of gravity on an object: W = m x g, where g is gravitational field strength (9.8 N/kg on Earth, often rounded to 10 N/kg in calculations). Mass is a measure of how much matter an object contains; weight is the gravitational force acting on that mass. Mass stays the same everywhere. Weight changes depending on the gravitational field.

Stopping distance

Stopping distance = thinking distance + braking distance. Thinking distance depends on the driver's reaction time, which is affected by tiredness, alcohol, drugs, distractions and speed. Braking distance depends on speed, mass of the vehicle, road conditions (wet, icy, gravel) and the condition of brakes and tyres.

Terminal velocity

When an object falls, gravity pulls it downward while air resistance pushes upward. As the object accelerates, air resistance increases until it equals the weight. At that point, the resultant force is zero, acceleration is zero, and the object falls at a constant speed called terminal velocity.

Hooke's law and elasticity

The initial linear region of a force-extension graph follows Hooke's law: extension is proportional to the applied force. Beyond the limit of proportionality, the graph curves and the material may not return to its original length when the force is removed. Elastic behaviour means the material recovers its original shape after deformation.

Momentum

Momentum = mass x velocity, p = m x v. The unit is kg m/s. Momentum is a vector quantity. In any collision or explosion where no external forces act, total momentum before = total momentum after. This is the conservation of momentum.

Safety features like crumple zones, seat belts and airbags work by increasing the time over which momentum changes, which reduces the force on the occupant. The relationship is F = (mv - mu) / t. A longer time t means a smaller force F.

Newton's third law

For every action, there is an equal and opposite reaction. If you push a wall with 50 N, the wall pushes back on you with 50 N. The two forces act on different objects, are equal in magnitude, opposite in direction, and of the same type.

Moments

The moment of a force is: moment = force x perpendicular distance from the pivot. The unit is newton metres (Nm). The weight of a body acts through its centre of gravity. For a system in equilibrium, the sum of clockwise moments equals the sum of anticlockwise moments (the principle of moments).

Common mistakes in forces and motion

1. Confusing mass and weight. Mass is measured in kg and does not change with location. Weight is a force measured in N and depends on gravitational field strength. Writing "the mass is 50 N" will lose you the mark.
2. Forgetting to convert units. If speed is given in km/h and distance in metres, you need to convert before using the equations. 1 km/h = 1000/3600 m/s.
3. Getting velocity-time graph area wrong. Students sometimes calculate the gradient when the question asks for distance, or vice versa. Gradient = acceleration. Area = distance.
4. Ignoring direction in momentum. Momentum is a vector. If two objects move in opposite directions, one momentum is positive and the other is negative. You must assign signs and keep them consistent.
5. Missing the perpendicular distance in moments. The distance must be measured perpendicular to the line of action of the force, not along the beam. If the force is at an angle, you need to resolve it or find the perpendicular component.

Self-check: edexcel igcse physics practice questions

Test yourself on these. Write out full solutions before checking your answers.

1. A car accelerates from 5 m/s to 25 m/s in 10 s. Calculate the acceleration.
2. A 70 kg person stands on the Moon where g = 1.6 N/kg. Calculate their weight on the Moon.
3. A 2 kg trolley moving at 3 m/s collides with a stationary 4 kg trolley. They stick together. Calculate the velocity after the collision.
4. A force of 80 N acts 0.25 m from a pivot. What force, acting 0.4 m from the same pivot on the opposite side, would balance the system?
5. A car has a mass of 1500 kg and decelerates at 5 m/s². Calculate the braking force.

Answers: (1) 2 m/s²; (2) 112 N; (3) 1 m/s; (4) 50 N; (5) 7500 N.

These edexcel igcse physics notes cover the essential content for forces and motion. The edexcel igcse physics explained approach here strips away unnecessary detail and focuses on what actually earns marks. Go through each worked example, attempt every practice question, and you will be well prepared for this section of the exam.