Forces — GCSE Physics Revision
Everything you need to revise forces for GCSE Physics: clear notes, the key facts and terms to learn, the mistakes that cost students marks, and practice questions with answers.
Revision notes
Introduction to Forces
Forces are pushes or pulls acting on an object. They are vector quantities, meaning they have both magnitude (size) and direction. Common forces include gravity, normal force, friction, and tension. Forces can cause an object to accelerate (change speed or direction), deform (change shape), or rotate. The overall effect of multiple forces acting on an object is determined by the resultant force, which is the single force that has the same effect as all the individual forces combined. If the resultant force is zero, the object remains at rest or continues at a constant velocity.
Contact and Non-Contact Forces
Forces can be categorised into two main types: contact forces and non-contact forces. Contact forces require two objects to be physically touching for the force to act, such as friction, air resistance, tension, and normal force. Non-contact forces can act on an object from a distance without physical contact. Examples include gravitational force (between any two objects with mass), electrostatic force (between charged particles), and magnetic force (between magnets or magnetic materials). Understanding these categories helps to analyse how different interactions affect objects.
Newton's Laws of Motion
Newton's First Law states that an object will remain at rest or continue at a constant velocity unless acted upon by a resultant force. Newton's Second Law describes the relationship between force, mass, and acceleration: resultant force (N) = mass (kg) × acceleration (m/s²), or F=ma. This law shows that a larger force produces a larger acceleration for a given mass, and a larger mass requires a larger force for the same acceleration. Newton's Third Law states that for every action, there is an equal and opposite reaction force.
Work Done and Energy Transfer
Work is done when a force causes an object to move through a distance in the direction of the force. The amount of work done is calculated using the formula: Work done (J) = Force (N) × distance (m), or W=Fd. Work done is a measure of energy transferred. For example, lifting an object transfers energy from the lifter to the object's gravitational potential energy store. If a force is applied but no movement occurs, no work is done. Power is the rate at which work is done or energy is transferred, measured in watts (W).
Key facts
- Forces are vector quantities (magnitude and direction).|Resultant force determines an object's acceleration.|F=ma is Newton's Second Law.|W=Fd calculates work done (energy transferred).|Weight is mass × gravitational field strength (W=mg).|For every action, there is an equal and opposite reaction.
Key terms
- Force::A push or pull acting on an object; a vector quantity.|Resultant Force::The single force that has the same effect as all the individual forces acting on an object.|Newton's First Law::An object at rest stays at rest, and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.|Newton's Second Law::The acceleration of an object is directly proportional to the resultant force acting on it and inversely proportional to its mass (F=ma).|Newton's Third Law::For every action, there is an equal and opposite reaction force.|Work Done::The energy transferred when a force causes an object to move through a distance in the direction of the force (W=Fd).
Common mistakes
- Confusing mass with weight; mass is a measure of matter, weight is a force due to gravity.|Forgetting that forces are vectors and not considering direction when calculating resultant forces.|Applying F=ma incorrectly when the force involved is not the resultant force.|Thinking work is done simply by applying a force, rather than requiring movement in the direction of the force.
Exam tips
- Always state units in your answers, e.g., Newtons (N) for force, joules (J) for work.|When asked about forces, consider if they are contact or non-contact forces.|For F=ma calculations, ensure force is in Newtons, mass in kilograms, and acceleration in m/s².|Draw free-body diagrams for complex force problems to visualise all forces and their directions.
Quick quiz
1. Which of the following is a non-contact force?
- Friction
- Air resistance
- Gravitational force
- Tension
Show answer
Gravitational force — Gravitational force acts over a distance without the need for physical contact between objects. Friction, air resistance, and tension are all contact forces.
2. An object of mass 5 kg is accelerated at 3 m/s². What is the resultant force acting on the object?
- 1.67 N
- 8 N
- 15 N
- 25 N
Show answer
15 N — Using Newton's Second Law, F = m × a. So, F = 5 kg × 3 m/s² = 15 N.
3. Which statement correctly describes Newton's First Law of Motion?
- Force equals mass times acceleration.
- For every action, there is an equal and opposite reaction.
- An object at rest stays at rest and an object in motion stays in motion with constant velocity unless acted on by a resultant force.
- The acceleration of an object is proportional to the net force and inversely proportional to its mass.
Show answer
An object at rest stays at rest and an object in motion stays in motion with constant velocity unless acted on by a resultant force. — Newton's First Law describes inertia, stating that an object's state of motion only changes if a resultant force acts on it. The other options describe Newton's Second and Third Laws.
4. A force of 20 N is applied to push a box 5 metres horizontally. How much work is done?
- 4 J
- 15 J
- 25 J
- 100 J
Show answer
100 J — Work done = Force × distance. So, Work done = 20 N × 5 m = 100 J.
5. What is the unit of power?
- Joule
- Newton
- Watt
- Metre
Show answer
Watt — The Watt (W) is the unit of power, representing joules per second. Joule (J) is for energy/work, Newton (N) for force, and Metre (m) for distance.
Exam-style questions
Describe what is meant by 'work done' in physics, and state its unit. [3 marks]
Show mark scheme
- Work done is the energy transferred (1 mark)
- when a force causes an object to move (1 mark)
- through a distance in the direction of the force. Its unit is the Joule (J). (1 mark)
State Newton's Third Law of Motion and provide an example of a force pair. [3 marks]
Show mark scheme
- Newton's Third Law states that for every action, there is an equal and opposite reaction force. (1 mark)
- Example: A rocket pushing exhaust gases downwards (action force). (1 mark)
- The exhaust gases push the rocket upwards (reaction force). (1 mark)
A car of mass 1200 kg accelerates from rest to 20 m/s in 5 seconds. Calculate the resultant force acting on the car. [4 marks]
Show mark scheme
- Calculate acceleration: a = (v-u)/t (1 mark)
- a = (20 - 0) / 5 = 4 m/s² (1 mark)
- Apply F = ma (1 mark)
- F = 1200 kg × 4 m/s² = 4800 N (1 mark)
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