OCR GCSE Physics B (Twenty First Century)
OCR GCSE Physics B P4 Explaining Motion Exam Questions
Free OCR GCSE Physics B revision for P4 Explaining Motion: what you need to know, common mistakes and exam-style questions with answers. Covers forces, Newton's laws, speed, velocity, acceleration, motion graphs, momentum, resultant force, work done, kinetic and gravitational potential energy and power.
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About OCR GCSE Physics B P4 Explaining Motion
Chapter P4 explains how and why things move. On OCR Physics B you describe forces as interactions between objects, use vectors and scalars, and analyse motion with distance-time and velocity-time graphs. You apply Newton's laws, F = ma and momentum to explain changes in motion, then describe motion through energy transfers using work done, kinetic energy, gravitational potential energy and power. Separate physics adds moments, levers and gears. P4 links to energy in P2 and to particle models in P6. It is equation heavy, so examiners test calculations, graph gradients and areas, and explanations of safety features using momentum.
What you need to know
- P4.1 Forces Forces arise from interactions between pairs of objects; Newton's third law says the two forces are equal in size and opposite in direction. Weight (N) = mass (kg) x gravitational field strength (N/kg).
- P4.2 Describing motion Speed is a scalar and velocity is a vector. Average speed = distance / time and acceleration = change in speed / time. On a distance-time graph the gradient is speed; on a velocity-time graph the gradient is acceleration and the area under it is distance.
- P4.2 Equations of motion (final speed)^2 - (initial speed)^2 = 2 x acceleration x distance. Objects in free fall near the Earth accelerate at about 10 m/s2.
- P4.3 Forces and motion A resultant force changes an object's motion: F = ma. Momentum = mass x velocity, and change in momentum = resultant force x time, so increasing collision time (crumple zones, air bags) reduces the force.
- P4.4 Energy and motion Work done = force x distance. Kinetic energy = 0.5 x mass x speed^2 and gravitational potential energy = mass x g x height. Power = energy transferred / time.
- P4.3 Moments Separate physics only: moment = force x perpendicular distance from the pivot. Levers and gears transmit the rotational effect of forces.
Common mistakes in P4 Explaining Motion
- Saying an object needs a resultant force to keep moving. With no resultant force it moves at constant velocity.
- Confusing mass and weight. Mass is in kg and does not change; weight is a force in N and depends on gravitational field strength.
- Forgetting to square the speed in kinetic energy. Calculate speed squared before multiplying by 0.5 x mass.
- Reading the gradient of a velocity-time graph as distance. The gradient gives acceleration; the area gives distance travelled.
- Explaining crumple zones as absorbing the force. They increase the time over which momentum changes, which reduces the force.
Try three P4 Explaining Motion exam-style questions
1. A car of mass 1200 kg accelerates from rest to 24 m/s in 8.0 s. Calculate its acceleration and the resultant force. (3 marks)
Show the mark scheme answer
a = 24 / 8.0 = 3.0 m/s2; F = ma = 1200 x 3.0; = 3600 N.
2. Calculate the kinetic energy of a 70 kg runner moving at 8.0 m/s. (2 marks)
Show the mark scheme answer
KE = 0.5 x 70 x 8.0^2 = 0.5 x 70 x 64; = 2240 J.
3. A car travelling at 20 m/s brakes with a deceleration of 5.0 m/s2. Calculate its braking distance. (3 marks)
Show the mark scheme answer
0^2 - 20^2 = 2 x (-5.0) x d; -400 = -10 x d; d = 40 m.
P4 Explaining Motion FAQs
What does OCR Physics B P4 Explaining Motion cover?
It covers forces and Newton's laws, speed, velocity and acceleration, motion graphs, momentum, work done, kinetic and gravitational potential energy and power. Separate physics adds moments, levers and gears.
Which paper is it examined in?
OCR Physics B has two papers at each tier, Breadth in physics and Depth in physics, and both can assess content from every chapter, so P4 can appear on either paper.
What equations do I need for P4?
Learn W = mg, speed = distance / time, a = change in speed / time, F = ma, momentum = mv, work = Fd, KE = 0.5mv^2, GPE = mgh and P = E / t, and practise using v^2 - u^2 = 2as.
