Forces and Motion
Forces and motion is the study of pushes and pulls between objects, and how these forces affect an object's speed, direction and shape. Forces can be contact forces, such as friction, or non-contact forces, such as gravity, and the resultant of all forces on an object determines whether it speeds up, slows down or changes direction. This KS3 topic covers weight, resultant force, speed, pressure and moments.
Before you start
No specific prerequisites - this is a good place to start.
Method
- Identify whether each force acting is a contact force (e.g. friction, air resistance, the normal contact force) or a non-contact force (e.g. gravity, magnetism, static electricity).
- Find the resultant force by combining all forces acting on an object along the same line: add forces acting in the same direction, subtract forces acting in opposite directions.
- Use weight = mass x gravitational field strength (W = m g) to calculate weight, remembering mass stays constant but weight changes with gravitational field strength.
- Use speed = distance / time to calculate speeds from distance-time data, and describe motion from the shape of a distance-time graph (flat means stationary, a steady slope means constant speed, a curve getting steeper means accelerating).
- Use pressure = force / area to explain why spreading a force over a larger area reduces pressure, and moment = force x distance to solve balancing (seesaw-style) problems.
- When answering stopping distance questions, split the answer into thinking distance (affected by the driver, e.g. tiredness or distraction) and braking distance (affected by the vehicle and road, e.g. worn tyres or wet roads).
Worked example
A delivery van has a mass of 1200 kg. Use weight = mass x gravitational field strength (W = m g) with g = 10 N/kg. Calculate the weight of the van on Earth.
- Write down the equation: weight = mass x gravitational field strength.
- Substitute the values: W = 1200 x 10.
- Calculate: W = 12000.
- Final answer: 12000 N.
Practice questions
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Q1State whether friction between a sledge and the snow is a contact or non-contact force.Show answer
Answer: Contact force
Q2A crate has a mass of 8 kg. Calculate its weight on Earth (g = 10 N/kg).Show answer
Answer: 80 N (8 x 10)
Q3A cyclist travels 60 m in 12 s at a steady speed. Calculate the cyclist's speed.Show answer
Answer: 5 m/s (60 divided by 12)
Q4Two teams in a tug of war pull with forces of 450 N and 380 N in opposite directions. Calculate the resultant force, including its direction.Show answer
Answer: 70 N in the direction of the stronger team's pull (450 - 380)
Q5A box exerts a force of 240 N on the floor through a base area of 0.4 m^2. Calculate the pressure on the floor.Show answer
Answer: 600 Pa (240 divided by 0.4)
Q6A 300 N weight sits 2 m from the pivot of a balanced seesaw. Calculate the distance from the pivot at which a 400 N weight must sit on the other side to balance it.Show answer
Answer: 1.5 m (moment = 300 x 2 = 600, then 600 divided by 400)
Exam-style questions
Written in the style of a KS3 Science exam paper, with a full mark scheme.
A remote-control car has a mass of 2.5 kg. Use weight = mass x gravitational field strength, with g = 10 N/kg. (a) Calculate the car's weight on Earth. (2) (b) The car is then taken to the Moon, where the gravitational field strength is much lower than on Earth. State what happens to the car's mass and what happens to its weight, giving a reason for each. (1)
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A farmer's tractor has a total weight of 18000 N. Its wide tyres give a total contact area with the ground of 3 m^2. Use pressure = force / area. (a) Calculate the pressure the tractor exerts on the ground. (2) (b) A smaller tractor with the same weight has a contact area of only 1.5 m^2. Calculate the pressure this tractor exerts on the ground. (2)
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A cycling club is updating its safety advice for members who ride in winter conditions. Explain what determines a cyclist's overall stopping distance, and use this to justify the club's advice that cyclists should leave extra space behind the rider in front when the roads are wet. Refer to thinking distance, braking distance and the forces involved in your answer.
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Free printable worksheet
Want more practice on paper? Download the forces and motion worksheet pack - 16 pages of exam-style questions with a full mark scheme. One email opens every download in this browser for 14 days - no account, no card. Print it for personal and classroom use.
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