Forces
Forces are pushes or pulls between objects that can change an object's speed, direction or shape, and are classed as either contact forces, such as friction, or non-contact forces, such as gravity. GCSE physics covers resultant forces, Newton's three laws of motion, weight, work done, and the relationship between force and acceleration.
Method
- Identify every force acting on the object and whether each is a contact force, such as friction or air resistance, or a non-contact force, such as gravity or a magnetic force.
- Combine forces acting in the same line to find the resultant force, adding forces acting in the same direction and subtracting forces acting in opposite directions.
- Use Newton's Second Law, resultant force = mass x acceleration, to link force, mass and acceleration, rearranging the equation if you need mass or acceleration.
- For weight, use weight = mass x gravitational field strength; remember mass stays constant but weight changes with gravitational field strength.
- For work done, use work done = force x distance moved in the direction of the force, which is the same as the energy transferred.
- State clearly whether the resultant force is zero, so the object stays at rest or moves at constant velocity, or non-zero, so the object accelerates in the direction of the resultant force.
Worked example
A trolley of mass 4.0 kg accelerates at 2.5 m/s^2 along a bench. Calculate the resultant force needed to produce this acceleration.
- Write down Newton's Second Law: resultant force = mass x acceleration.
- Substitute the values: resultant force = 4.0 x 2.5.
- Calculate: 4.0 x 2.5 = 10.
- State the final answer with the correct unit: resultant force = 10 N.
Practice questions
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Q1State whether friction is a contact force or a non-contact force.Show answer
Answer: Contact force
Q2A cyclist rides 300 m north then 300 m back south to the start. State the total distance travelled and total displacement.Show answer
Answer: Distance = 600 m; displacement = 0 m
Q3Calculate the weight of an object of mass 8.0 kg on Earth, where g = 9.8 N/kg.Show answer
Answer: 78.4 N (8.0 x 9.8)
Q4A box has a driving force of 250 N forward and friction of 180 N backward. Calculate the resultant force, including its direction.Show answer
Answer: 70 N forward (250 - 180)
Q5A gardener pushes a mower with a force of 60 N through a distance of 8.0 m. Calculate the work done. Use W = F x s.Show answer
Answer: 480 J (60 x 8.0)
Q6A resultant force of 45 N acts on an object of mass 9.0 kg. Calculate its acceleration. Use F = m x a.Show answer
Answer: 5.0 m/s^2 (45/9.0)
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
A skydiver falls through the air before opening their parachute. State the two forces acting on the skydiver during this fall.
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A student carries out the required practical investigation into the extension of a spring. Weights are added one at a time and the extension is measured. The results are: Force (N): 0, 1.0, 2.0, 3.0, 4.0; Extension (mm): 0, 5, 10, 15, 26. (a) Identify the anomalous result in the table. (1 mark) (b) Use the equation F = k x e, and the result for a force of 2.0 N, to calculate the spring constant k, in N/m. (3 marks) (c) State one hazard in this experiment and a way of reducing the risk it presents. (1 mark)
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A car travels at 18 m/s. The driver's reaction time is 0.70 s. (a) Define 'thinking distance'. (1 mark) (b) Calculate the thinking distance for this car. Use: distance = speed x time. (2 marks) (c) State one factor that could increase the driver's reaction time. (1 mark)
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Free printable worksheet
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This topic is chapter 4 of GCSE Physics Workbook, the whole course as one free printable PDF.
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