Forces: GCSE questions with answers

Answer: A Magnetic force

A force is a push or a pull caused by an interaction between objects. Contact forces act when objects touch: friction, air resistance, tension and normal contact force. Non-contact forces act at a distance: gravitational, electrostatic and magnetic forces.

Checked against AQA GCSE Physics 8463 specification, 4.5.1.2 Contact and non-contact forces

Answer: D The apple pulls the Earth upwards with an equal force

Forces come from interactions between pairs of objects, and each object feels a force. The Earth pulls the apple and the apple pulls the Earth, with equal forces in opposite directions. The Earth has a huge mass, so its movement is far too small to notice.

Checked against AQA GCSE Physics 8463 specification, 4.5.1.2 Contact and non-contact forces

Answer: A The size of the force

Force is a vector: it has a size (magnitude) and a direction. It is drawn as an arrow, where the length of the arrow shows the size of the force and the arrow points in its direction.

Checked against AQA GCSE Physics 8463 specification, 4.5.1.1 Scalar and vector quantities

Answer: D Normal contact force

When objects touch, they push on each other. A surface pushes at right angles to itself on anything resting on it: the normal contact force. Friction acts along the surfaces, against the direction of sliding.

Checked against AQA GCSE Physics 8463 specification, 4.5.1.2 Contact and non-contact forces

Answer: D The force of gravity on an object

Weight is the force acting on an object because of gravity, so it is measured in newtons. Mass is the amount of matter in the object, measured in kilograms. They are different quantities.

Checked against AQA GCSE Physics 8463 specification, 4.5.1.3 Gravity

Answer: C A newtonmeter (spring balance)

Weight is a force, so it is measured with a calibrated spring balance, called a newtonmeter. The object hangs from the spring, and the stretch of the spring shows the force in newtons.

Checked against AQA GCSE Physics 8463 specification, 4.5.1.3 Gravity

Answer: A Weight = mass × gravitational field strength

Weight = mass × gravitational field strength. Gravitational field strength is measured in newtons per kilogram (N/kg). In one place, weight is directly proportional to mass, so twice the mass has twice the weight.

Checked against AQA GCSE Physics 8463 specification, 4.5.1.3 Gravity

Answer: B Her weight falls but her mass stays the same

The weight of an object depends on the gravitational field strength where it is. Mass is the amount of matter, which does not change from place to place. Where the field is weaker, the weight is smaller.

Checked against AQA GCSE Physics 8463 specification, 4.5.1.3 Gravity

Answer: B One force with the same effect as all the forces together

Several forces on an object can be replaced by a single resultant force that has the same effect. A free body diagram shows one object with an arrow for every force acting on it, so the resultant can be worked out.

Checked against AQA GCSE Physics 8463 specification, 4.5.1.4 Resultant forces

Answer: D Zero

When the forces on an object balance, the resultant force is zero and the object is either still or moving at a steady velocity. A falling skydiver has her weight acting down and air resistance acting up; upthrust from the air is tiny.

Checked against AQA GCSE Physics 8463 specification, 4.5.1.4 Resultant forces

Answer: C At least two forces pulling in opposite directions

One force on its own would just move an object, not change its shape. To stretch, bend or squash a still object you need more than one force, pushing or pulling against each other, such as a hook holding one end of a spring while a mass pulls the other.

Checked against AQA GCSE Physics 8463 specification, 4.5.3 Forces and elasticity

Answer: B Elastic deformation

In elastic deformation, an object returns to its original shape and length when the forces are removed. In inelastic (plastic) deformation, it stays permanently changed in shape. “Plastic” here describes the change, not the material.

Checked against AQA GCSE Physics 8463 specification, 4.5.3 Forces and elasticity

Answer: B Extension is directly proportional to force

Extension is how much longer the spring has become, not its total length. Up to a limit, the extension is directly proportional to the force: double the force, double the extension.

Checked against AQA GCSE Physics 8463 specification, 4.5.3 Forces and elasticity

Answer: C Force and extension are directly proportional

A linear relationship gives a straight-line graph; if it also passes through the origin, force and extension are directly proportional. Stretch a spring too far (past its limit of proportionality) and the graph curves: the relationship has become non-linear. A rubber band gives a curved graph too.

Checked against AQA GCSE Physics 8463 specification, 4.5.3 Forces and elasticity (spring constant)

Answer: D Force = spring constant × extension

For linear stretching, force = spring constant × extension. The spring constant, in newtons per metre (N/m), shows how stiff a spring is: a stiffer spring has a larger spring constant and stretches less for the same force.

Checked against AQA GCSE Physics 8463 specification, 4.5.3 Forces and elasticity (spring constant)

Answer: B They are equal

The force that stretches a spring does work, and that energy is stored in the spring’s elastic potential store. If the spring is not inelastically deformed, the two are equal. Energy stored = 0.5 × spring constant × extension², so doubling the extension makes it four times as big.

Checked against AQA GCSE Physics 8463 specification, 4.5.3 Forces and elasticity (spring constant)

Answer: D The energy transferred by the force

Work is done when a force makes an object move. Work done = force × distance moved along the line of action of the force. One joule of work is done when a force of one newton moves something one metre, so 1 J = 1 N m.

Checked against AQA GCSE Physics 8463 specification, 4.5.2 Work done and energy transfer

Answer: D None, as the bag does not move

Work is done on an object only when a force moves it some distance along the line of the force. A bag held still does not move, so no work is done on it, however tired your arm gets.

Checked against AQA GCSE Physics 8463 specification, 4.5.2 Work done and energy transfer

Answer: B It warms the box and the floor

Work done is energy transferred. Work done against friction raises the temperature of the surfaces, filling thermal stores. Work done lifting an object at a steady speed fills its gravitational potential store.

Checked against AQA GCSE Physics 8463 specification, 4.5.2 Work done and energy transfer

Answer: A At right angles to the surface

Liquids and gases are both fluids. The pressure in a fluid pushes on every surface it touches, with a force at right angles (normal) to that surface, whichever way the surface faces.

Checked against AQA GCSE Physics 8463 specification, 4.5.5.1 Pressure in a fluid

Answer: C Force normal to a surface ÷ area of the surface

Pressure = force normal to a surface ÷ area of that surface, measured in pascals (Pa). The same force spread over a bigger area gives a smaller pressure, which is why snowshoes and wide tyres stop things sinking.

Checked against AQA GCSE Physics 8463 specification, 4.5.5.1 Pressure in a fluid

Answer: A Air molecules colliding with the surface

The atmosphere is a thin layer of air around the Earth. Its molecules are always moving and colliding with surfaces, and these collisions push on the surfaces in every direction. That push per unit area is atmospheric pressure.

Checked against AQA GCSE Physics 8463 specification, 4.5.5.2 Atmospheric pressure

Answer: D There is less air above it

The higher you go, the less air (and so the less weight of air) there is above you. So atmospheric pressure falls as height increases.

Checked against AQA GCSE Physics 8463 specification, 4.5.5.2 Atmospheric pressure

Answer: D There is a taller column of water above

The pressure at a point in a liquid comes from the weight of liquid above it. It increases with the height of the column of liquid above the point and with the density of the liquid.

Checked against AQA GCSE Physics 8463 specification, 4.5.5.1.2 Pressure in a fluid 2

Answer: D The pressure on its bottom is greater than on its top

The bottom of a submerged object is deeper than its top, so the water pressure on its bottom surface is greater. This gives a resultant force upwards on the object, called upthrust.

Checked against AQA GCSE Physics 8463 specification, 4.5.5.1.2 Pressure in a fluid 2

Answer: A They are equal

An object floats when the upthrust on it can equal its weight. An object less dense than the liquid floats; one denser than the liquid sinks, whatever its size. Light objects do not always float.

Checked against AQA GCSE Physics 8463 specification, 4.5.5.1.2 Pressure in a fluid 2

Answer: A Its turning effect

A force can make an object turn about a pivot. Its turning effect is the moment: moment = force × perpendicular distance from the pivot to the line of action of the force, measured in newton-metres (N m).

Checked against AQA GCSE Physics 8463 specification, 4.5.4 Moments, levers and gears

Answer: B The same force gives a bigger moment

The further from the pivot a force acts, the bigger its moment. So pushing a door far from its hinges, or using a long spanner, gives a large turning effect from a small force.

Checked against AQA GCSE Physics 8463 specification, 4.5.4 Moments, levers and gears

Answer: B Total clockwise moment equals total anticlockwise moment

When an object is balanced, the total clockwise moment about the pivot equals the total anticlockwise moment (the principle of moments). A lighter child must sit further out to make a moment as big as the heavier child’s.

Checked against AQA GCSE Physics 8463 specification, 4.5.4 Moments, levers and gears

Answer: C A small force far from the pivot gives a large moment

A lever turns about a pivot. A small force a long way from the pivot makes the same moment as a large force close to it. So with the pivot near the load, a small effort can move a heavy load: the lever is a force multiplier.

Checked against AQA GCSE Physics 8463 specification, 4.5.4 Moments, levers and gears

Answer: B Turns more slowly with a bigger moment

Gears pass a turning effect from one shaft to another. When a small gear drives a bigger one, the bigger gear turns more slowly but with a larger moment, like a lever. Two meshing gears always turn in opposite directions.

Checked against AQA GCSE Physics 8463 specification, 4.5.4 Moments, levers and gears