Past paper revision

"How do you calculate a spring constant from a force and an extension?

Use k = force ÷ extension (rearranged from F = ke). Convert the extension from cm into metres first by dividing by 100."

"A spring extends 20 cm under a force of 10 N. Calculate the spring constant in N/m.

Convert 20 cm to 0.20 m first. k = F ÷ e = 10 ÷ 0.20 = 50 N/m. (Using 20 cm gives 0.5, which is wrong.)"

"What is the most common mistake when calculating a spring constant in N/m?

Forgetting to convert the extension from centimetres to metres. Always divide cm by 100 before dividing force by extension."

"How do you find the frequency of a wave from a trace that shows several waves over a time period?

Count the number of complete waves, then frequency = number of complete waves ÷ time. Do NOT treat the whole time as one period."

"A seismometer trace shows 4 complete waves in 10 seconds. What is the frequency?

f = number of waves ÷ time = 4 ÷ 10 = 0.4 Hz."

"Why is it wrong to use f = 1 ÷ 10 for a 10-second trace showing several waves?

Because 10 s is the total time for all the waves, not the time for one wave. Count the complete waves and divide: f = waves ÷ time."

"How do you estimate the uncertainty in a set of repeated readings?

Uncertainty = range ÷ 2 = (highest reading − lowest reading) ÷ 2."

"Repeated readings are 56, 49, 53 and 46. Estimate the uncertainty.

(highest − lowest) ÷ 2 = (56 − 46) ÷ 2 = 10 ÷ 2 = ±5."

"What is the difference between resolution uncertainty and range uncertainty?

Resolution = half the smallest scale division (e.g. ±0.5). Range uncertainty (for repeats) = (highest − lowest) ÷ 2. For repeated readings, use the range method."

"In a circular orbit, does the satellite's speed change?

No – in a circular orbit the speed stays constant. Only the direction changes. (Speed changing with distance is for elliptical orbits, not circular.)"

"Explain why the velocity of a satellite changes as it moves in a circular orbit.

Gravity provides a force (and acceleration) toward the Earth. This constantly changes the satellite's direction. Because velocity is a vector (speed AND direction), changing direction changes the velocity – even though the speed is constant."

"Why does a change in direction count as a change in velocity?

Because velocity is a vector – it has both size and direction. Changing the direction changes the velocity even if the speed stays the same."

"What provides the force that keeps a satellite in orbit, and which way does it act?

Gravity, acting toward the centre of the Earth. It makes the satellite accelerate toward Earth, constantly changing its direction."

"When a question says 'explain how the data in the graph/table shows…', what must your answer include?

Specific values read from the data (actual numbers), not just a general description. Quote and compare real figures from the graph or table."

"A dropped ball bounces. How does a velocity–time graph show the ball lost energy in the bounce?

The velocity just after the bounce is less than just before (e.g. ~5 m/s vs ~6.8 m/s), so the ball has less kinetic energy – it lost energy. (Total energy of ball + Earth stays constant.)"

"A velocity–time graph shows a straight line sloping up from the origin. How do you describe this motion?

Uniform (constant) acceleration – the velocity is increasing at a constant rate. Say 'uniform/constant acceleration', not just 'speeding up'."

"Why must you not write 'the acceleration is increasing' for a straight sloping velocity–time line?

A straight line means the acceleration is constant, not increasing. 'Acceleration increasing' is wrong here and scores zero."

"What does it mean for two quantities to be directly proportional?

As one increases, the other increases at the same rate – doubling one doubles the other. On a graph this is a straight line that passes through the origin (0,0)."

"How does a force–extension graph show that a spring obeys Hooke's law?

The line is straight AND passes through the origin (showing direct proportionality). Saying 'both increase' is not enough – that is only correlation."

"Why is 'when force increases, extension increases' not enough to prove direct proportionality?

Because that only shows they both increase (correlation). Direct proportionality also needs a constant ratio – a straight line through the origin."

"In a spring experiment, why measure the extension for several different forces rather than just one?

To identify any anomalous results / reduce the effect of random error (and draw a reliable line of best fit). Not 'to see the pattern' or 'to draw a graph'."

"Explain why pressure increases as a swimmer dives deeper below the surface.

The deeper you go, the greater the height (and weight) of water above you. This greater weight of water pushes down, so the pressure increases."

"Which two ideas are needed to explain why pressure increases with depth in a liquid?

(1) Greater depth means a greater height/amount of water above. (2) This greater weight/force of water acts on you, increasing the pressure."

"A swimmer goes from 0.50 m to 1.70 m deep. Density = 1030 kg/m³, g = 9.8 N/kg. Find the increase in pressure. (reinforce – you can do this)

Use p = hρg with the CHANGE in depth: h = 1.70 − 0.50 = 1.20 m. p = 1.20 × 1030 × 9.8 = 12112.8 Pa."

"What is the equation for the pressure due to a column of liquid, and what does each symbol mean?

p = hρg. p = pressure (Pa), h = height of the column (m), ρ = density (kg/m³), g = gravitational field strength (N/kg)."

"Why does moving a magnet into a coil induce a potential difference?

The movement causes a changing magnetic field through the coil (the field lines are cut), which induces a potential difference. This is the generator effect."

"Why is the potential difference induced when a magnet oscillates in a coil an alternating pd?

Because the magnet changes direction (moves in, then out). This reverses the direction of the field change, so the induced pd reverses – it alternates."

"A student gets a range of values for the angle of reflection. What type of error causes this?

Random error. (Just 'random error' – 'random human error' is not enough.)"

"Suggest a practical reason a student got a range of values for the angle of reflection.

Misjudging the centre of the reflected ray, or not replacing the mirror/ray box in exactly the same position each time. (Not 'measuring the angle wrong'.)"

"What extra evidence could be collected to support a reflection investigation's conclusion?

Take readings for angles of incidence not yet measured (e.g. 10° or 60°). Calculating a mean is NOT extra evidence – that just processes existing data."

"How would you change the apparatus to investigate diffuse reflection using the same method?

Replace the plane mirror with an irregular (rough) reflecting surface. A glass block is NOT accepted."

"On a graph of secondary pd against number of turns, how can you tell a transformer has become step-up?

When the secondary pd rises above the primary pd (e.g. goes above 2 V). In a step-up transformer Vs > Vp."

"How do you tell a step-up from a step-down transformer?

Step-up: secondary pd greater than primary (Vs > Vp), more turns on the secondary. Step-down: secondary pd less than primary (Vs < Vp), fewer turns on the secondary."

"A 100% efficient transformer has 640 primary turns, 4 secondary turns, secondary pd 1.75 V and power output 336 W. Find the input current. (reinforce – you can do this)

First find Vp: 640/4 = Vp/1.75, so Vp = 640 × 1.75 ÷ 4 = 280 V. Then power in = power out: 280 × Ip = 336, so Ip = 1.2 A."

"Explain why the coil in a motor rotates continuously when there is a current.

The two sides of the coil experience forces in opposite directions; these forces create moments that turn the coil; each half-turn the split-ring commutator swaps the brushes, reversing the current, which keeps the forces (and rotation) going the same way."

"What is the job of the split-ring commutator in a motor?

Every half-turn its two halves swap contact between the brushes. This reverses the current in the coil, keeping the forces acting the same way so the coil keeps spinning in one direction."

"How do you draw the magnetic field around a straight current-carrying wire?

As at least three concentric circles around the wire, with arrows for the direction. For current into the page the field is clockwise (right-hand grip rule)."

"Why do scientists sometimes use more than one model (e.g. the wave and particle models of light)?

Because different models are useful in different situations – no single model can explain all the observations."