Medium
Physics
Forces, Energy & Light
Motion, energy and light, described without the equations.
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The speed of light in a vacuum is closest to which value?
Motion, Mass and Momentum
Nothing here needs an equation. Every question is about something you have watched happen, from a skydiver levelling off to a skater spinning faster with their arms tucked in.
About this topic
Mechanics is the oldest part of physics and still the most useful, because it describes the world at the scale you live in. Push something and it speeds up. Stop pushing and it keeps going until something else slows it down. Almost everything else in this quiz follows from those two observations. The questions cover forces, acceleration, momentum and the difference between mass and weight, which trips up more people than anything else on the list. Each explanation adds the detail that makes the rule stick rather than just stating it again.
Questions & answers
Motion, Mass and Momentum: all 10 questions and answers
Every question in this quiz is listed below with its correct answer and the reasoning behind it. Play first if you would rather not see the answers — or read through as a study sheet.
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1Newton's third law says forces come in pairs. What is true of the two forces in a pair?
- A They are equal in size and act on the same object
- B They are opposite in direction but rarely equal in size
- C They act on the same object and cancel each other out
- D They are equal in size, opposite in direction, and act on different objects Correct
Answer: D. They are equal in size, opposite in direction, and act on different objects
Why: The pair always acts on two different bodies, which is why they never cancel out. When you push on a wall the wall pushes back on you with exactly the same force, and it is that push that lets you walk.
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2Acceleration is defined as the rate of change of what?
- A Mass
- B Force
- C Velocity Correct
- D Distance travelled
Answer: C. Velocity
Why: Acceleration measures how quickly velocity changes, in size or in direction. That is why a car going round a roundabout at a steady speed is still accelerating.
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3Multiplying an object's mass by its velocity gives which quantity?
- A Acceleration
- B Momentum Correct
- C Kinetic energy
- D Force
Answer: B. Momentum
Why: Momentum is mass times velocity, and in any collision the total momentum of everything involved stays the same. Crumple zones work by stretching a crash out in time so the same change in momentum needs a smaller force.
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4A skydiver in free fall eventually stops accelerating and falls at a steady speed. Why?
- A Air resistance has grown until it balances their weight Correct
- B Gravity weakens as they get closer to the ground
- C They run out of the energy that was making them fall
- D The air becomes too dense for them to fall through
Answer: A. Air resistance has grown until it balances their weight
Why: Drag rises with speed, so it climbs until it matches weight and the net force reaches zero. A belly-down skydiver levels off at roughly 200 kilometres per hour, and far more head-down.
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5A hammer and a feather are dropped together inside a vacuum chamber. What happens?
- A The hammer lands first because it is heavier
- B The feather lands first because it has more surface area
- C Neither one falls, because there is no air to fall through
- D They hit the floor at the same moment Correct
Answer: D. They hit the floor at the same moment
Why: Without air resistance every object gains speed at the same rate regardless of mass. Commander David Scott tested it on the Moon in 1971 with a hammer and a falcon feather, on camera.
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6An astronaut travels from Earth to the Moon. What happens to their mass?
- A It rises slightly because of the journey
- B It drops to zero while they are in orbit
- C It stays exactly the same Correct
- D It falls to about a sixth of its Earth value
Answer: C. It stays exactly the same
Why: Mass is the amount of matter in something and does not depend on where it sits. Weight is a force and does change, which is why an astronaut who weighs less on the Moon is still just as hard to shove sideways.
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7What keeps a ball on the end of a string moving in a circle?
- A A force acting along its direction of travel
- B A force pulling it towards the centre of the circle Correct
- C A force pushing it outwards from the centre
- D No force at all, once it is already moving
Answer: B. A force pulling it towards the centre of the circle
Why: Circular motion needs a constant inward pull, supplied here by the string. Cut the string and the ball flies off along a straight tangent rather than curving outwards.
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8Near the Earth's surface, a falling object gains speed at roughly what rate each second?
- A 10 metres per second Correct
- B 1 metre per second
- C 50 metres per second
- D 100 metres per second
Answer: A. 10 metres per second
Why: The standard figure is 9.8 metres per second squared, usually rounded to 10 for mental arithmetic. It varies very slightly with latitude and altitude, being marginally weaker at the equator.
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9A spinning ice skater pulls their arms in tight and speeds up. Why?
- A Pulling their arms in adds energy to the spin
- B The ice offers less friction when their arms are close to their body
- C Their mass drops as their arms move inwards
- D Angular momentum is conserved, so reducing their spread increases their spin rate Correct
Answer: D. Angular momentum is conserved, so reducing their spread increases their spin rate
Why: Bringing mass closer to the axis lowers the skater's moment of inertia, and the spin rate rises to keep angular momentum constant. Divers use exactly the same trick to fit extra somersaults into a fall.
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10A rocket engine works in the vacuum of space. What does the rocket push against?
- A The Earth's magnetic field
- B Nothing, so it cannot accelerate in a vacuum
- C Its own exhaust gas, which it throws backwards Correct
- D The outer edge of the atmosphere
Answer: C. Its own exhaust gas, which it throws backwards
Why: Throwing mass one way pushes the rocket the other way, and no surrounding medium is needed. Rockets actually work better in a vacuum, since there is no air pressure fighting the exhaust.
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