Edexcel GCSE Physics Topic 9: Forces and Their Effects Exam Questions

About Edexcel GCSE Physics Topic 9: Forces and Their Effects

Topic 9 on Paper 2 looks at how forces interact and the turning effects they produce. You learn to identify interaction pairs, draw free body diagrams, find resultant forces and, on the higher tier, use scale vector diagrams to resolve a force into two components at right angles. Separate physics content then covers moments, the principle of moments, and how levers and gears transmit rotational effects of forces. The topic builds on Newton's laws from Topic 2 and links to Topic 8, since forces doing work transfer energy. Examiners commonly set moment balance calculations, vector diagram drawing and explanations of how gears change speed and turning effect.

What you need to know

• Interaction pairs When two objects interact, they exert equal and opposite forces on each other. Forces can be contact forces (friction, normal contact, air resistance) or non-contact forces (gravity, magnetic, electrostatic).
• Free body diagrams Draw an object as a point or box with arrows showing every force acting on it, with arrow length showing size. Use these to find the resultant force.
• Resultant forces Add forces acting in the same direction and subtract those in opposite directions. Two forces at right angles combine using a scale drawing or Pythagoras.
• Resolving forces Higher tier: a single force can be split into two perpendicular components using a scale drawing. An object is in equilibrium when the vector diagram forms a closed shape.
• Moments Moment of a force = force x perpendicular distance from the pivot (M = F x d), measured in N m.
• Principle of moments For a balanced object, the sum of clockwise moments equals the sum of anticlockwise moments about the pivot.
• Levers and gears Levers increase the turning effect, so a small force can lift a large load. When a small gear drives a larger gear, the larger gear turns more slowly but with a bigger moment.

Common mistakes in Topic 9: Forces and Their Effects

• Using the distance along the lever instead of the perpendicular distance from the pivot to the line of the force.
• Drawing both forces of an interaction pair on the same free body diagram. A free body diagram shows only the forces acting on one object.
• Giving moments in N/m or N. The unit of moment is the newton metre, N m.
• Adding forces at right angles as if they were in a straight line, e.g. 3 N and 4 N giving 7 N. Use Pythagoras or a scale drawing to get 5 N.
• Saying a larger driven gear turns faster. It turns more slowly but with a larger moment.

Try three Topic 9: Forces and Their Effects exam-style questions