Eduqas GCSE Physics 3 Forces Exam Questions

About Eduqas GCSE Physics 3 Forces

Forces in Eduqas GCSE Physics has three parts: forces and their interactions, pressure and pressure differences in fluids, and moments, levers and gears. You learn scalar and vector quantities, contact and non-contact forces, weight and gravitational field strength, resultant forces and free body diagrams, and the elastic behaviour of springs. Pressure covers atmospheric pressure, pressure in liquids and why objects float or sink. Moments explain levers, gears and balanced objects. The topic leads straight into forces and motion in topic 4. Examiners often test Hooke's law graphs (SP3.1), weight and moment calculations, and explanations of upthrust using pressure differences.

What you need to know

• 3.1 Types of force Vectors have size and direction (force, velocity, displacement); scalars only have size. Contact forces include friction, air resistance, tension and normal contact force; non-contact forces include gravitational, electrostatic and magnetic forces.
• 3.1 Weight and resultant force Weight W = mg, where g is gravitational field strength in N/kg. The resultant force is the single force with the same effect as all the forces acting; free body diagrams show all forces on one object.
• 3.1 Elasticity F = kx: extension is proportional to force up to the limit of proportionality. Elastic deformation is reversed when the force is removed; inelastic deformation is not. SP3.1: investigate the force-extension graph for a spring.
• 3.2 Pressure in fluids Pressure = force / area, in pascals. Pressure in a liquid increases with depth and density: p = hρg (given). Atmospheric pressure decreases with height because there is less air above.
• 3.2 Upthrust Pressure on the bottom of a submerged object is greater than on the top, causing an upward resultant force called upthrust. An object floats if its weight equals the upthrust before it is fully submerged, which depends on its density compared with the fluid.
• 3.3 Moments, levers and gears Moment M = Fd, where d is the perpendicular distance from the pivot. For balance, total clockwise moments = total anticlockwise moments. Levers and gears transmit the rotational effect of forces and can increase force or speed.

Common mistakes in 3 Forces

• Measuring the length of the spring instead of the extension. Extension = stretched length - original length.
• Using mass instead of weight in force calculations. Convert mass to weight with W = mg.
• Using the length of the lever instead of the perpendicular distance from the pivot. Measure from the pivot to the line of action of the force.
• Saying objects float because they are light. An object floats if it is less dense than the fluid, so the upthrust can equal its weight.
• Saying the spring obeys F = kx right up to the elastic limit. It obeys Hooke's law up to the limit of proportionality.

Try three 3 Forces exam-style questions