GCSE Physics Forces and Motion 2 — Questions and Answers
Question 1: A car's stopping distance is made up of which two parts?
- Thinking distance and braking distance (Correct answer)
- Reaction time and speed
- Friction distance and gravity distance
- Acceleration distance and braking distance
Correct answer: Thinking distance and braking distance
Stopping distance = thinking distance + braking distance.
The stopping distance of a vehicle is the sum of the thinking distance (the distance travelled during the driver's reaction time) and the braking distance (the distance travelled while the brakes are applied). Thinking distance is increased by tiredness, alcohol, drugs and distractions. Braking distance is increased by wet or icy roads, worn brakes or tyres, and higher speed.
Question 2: A 1200 kg car is travelling at 15 m/s. What is its momentum?
- 80 kg m/s
- 1215 kg m/s
- 18 000 kg m/s (Correct answer)
- 1800 kg m/s
Correct answer: 18 000 kg m/s
Momentum = mass × velocity = 1200 × 15 = 18 000 kg m/s.
Momentum is calculated using momentum = mass × velocity (p = mv) and is measured in kilogram metres per second (kg m/s). Substituting gives 1200 kg × 15 m/s = 18 000 kg m/s. Momentum is a vector quantity, so it has the same direction as the velocity.
Question 3: According to Newton's first law, what happens to a moving object if the resultant force on it is zero?
- It gradually slows down and stops
- It continues to move at the same speed in the same direction (Correct answer)
- It accelerates
- It changes direction
Correct answer: It continues to move at the same speed in the same direction
With zero resultant force, a moving object keeps a constant velocity.
Newton's first law states that if the resultant force on an object is zero, a stationary object remains stationary and a moving object continues at the same speed in the same direction (constant velocity). A resultant force is needed to change an object's velocity, which means speeding up, slowing down or changing direction. In everyday life objects slow down because friction and air resistance provide a resultant force.
Question 4: A spring extends by 0.2 m when a force of 8 N is applied. What is the spring constant?
- 1.6 N/m
- 0.025 N/m
- 8.2 N/m
- 40 N/m (Correct answer)
Correct answer: 40 N/m
F = ke, so k = F ÷ e = 8 ÷ 0.2 = 40 N/m.
Hooke's law states that the extension of an elastic object is directly proportional to the force applied, provided the limit of proportionality is not exceeded: F = ke. Rearranging gives k = F ÷ e = 8 N ÷ 0.2 m = 40 N/m. The spring constant tells you how stiff the spring is; a larger value means a stiffer spring.
Question 5: What does the area under a velocity-time graph represent?
- Acceleration
- Speed
- Distance travelled (Correct answer)
- Resultant force
Correct answer: Distance travelled
The area under a velocity-time graph equals the distance (displacement) travelled.
On a velocity-time graph, the gradient represents acceleration and the area under the line represents the distance travelled. This is because distance = velocity × time, which is what the area of the shape between the line and the time axis calculates. For a straight-line section you can use the areas of rectangles and triangles to find the total distance.
Question 6: A skydiver reaches terminal velocity. Which statement is correct?
- The skydiver's weight has decreased to zero
- Air resistance is greater than the skydiver's weight
- The skydiver is still accelerating downwards
- Air resistance is equal to the skydiver's weight, so the resultant force is zero (Correct answer)
Correct answer: Air resistance is equal to the skydiver's weight, so the resultant force is zero
At terminal velocity the forces balance, so there is no acceleration.
When a skydiver first jumps, weight is greater than air resistance, so they accelerate. As speed increases, air resistance increases until it equals the weight. At this point the resultant force is zero, so the skydiver stops accelerating and falls at a constant speed called terminal velocity. Opening the parachute increases air resistance, causing deceleration to a new, lower terminal velocity.
GCSE Physics Forces and Motion 2
A car's stopping distance is made up of which two parts?