5.15 Newton's first law and terminal velocity
9 exam questions worth 22 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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According to Newton's first law, what happens to a moving object when the resultant force on it is zero?
Choose one answer.
Correct Not quite: the answer is It moves at a constant speed in a straight line
Mark scheme
C: It moves at a constant speed in a straight line
Model answer
It moves at a constant speed in a straight line (C).
State Newton's first law of motion.
Mark scheme
an object at rest stays at rest and a moving object keeps moving at constant velocity (1); unless acted on by a resultant force / if the resultant force is zero (1)
Model answer
If the resultant force on an object is zero, a stationary object stays at rest and a moving object keeps moving at a constant speed in a straight line.
A passenger in a car is thrown forwards when the car brakes suddenly. Explain this using Newton's first law.
Mark scheme
the car slows down because of the braking force (1); there is no resultant force acting on the passenger / the passenger keeps moving at the same velocity (1); so the passenger continues forwards, relative to the car, until a force (seat belt) acts (1)
Model answer
The braking force slows the car, but no resultant force acts on the passenger, so the passenger carries on moving forwards at the same velocity until a force, such as the seat belt, acts on them.
A skydiver reaches terminal velocity. State the size of the resultant force on the skydiver, and compare the weight with the air resistance.
Mark scheme
the resultant force is zero (1); the weight equals the air resistance (1)
Model answer
The resultant force is zero, because the weight equals the air resistance.
Explain why the acceleration of a skydiver decreases after they jump from the aircraft.
Mark scheme
as the speed increases, the air resistance increases (1); so the resultant force (weight − air resistance) decreases (1); so the acceleration decreases (1)
Model answer
As the skydiver speeds up, the air resistance increases. The resultant force (weight minus air resistance) decreases, so the acceleration decreases.
A skydiver opens the parachute. Explain, in terms of forces, why the skydiver slows down and why they reach a new, lower, terminal velocity.
Mark scheme
the parachute increases the surface area, so air resistance suddenly increases (1); air resistance is greater than weight, so the resultant force is upwards and the skydiver decelerates (1); as the speed falls, air resistance falls (1); until it equals the weight again, so the velocity is constant at a lower value (1)
Model answer
The parachute has a big area, so the air resistance suddenly becomes larger than the weight, giving a resultant force upwards and a deceleration. As the speed falls, the air resistance decreases until it equals the weight again, and the skydiver falls at a lower terminal velocity.
A feather and a hammer are dropped together on the Moon. Describe what happens and explain why.
Mark scheme
they hit the ground at the same time (1); there is no atmosphere / no air resistance so both have the same acceleration (1)
Model answer
They land at the same time. The Moon has no atmosphere, so there is no air resistance and both accelerate at the same rate.
A car is travelling at its top speed on a level road. Describe the forces on the car and explain why it cannot go any faster.
Mark scheme
the driving force equals the air resistance (and friction) (1); so the resultant force is zero (1); so the car cannot accelerate further / moves at constant velocity (1)
Model answer
The driving force is equal to the drag and friction, so the resultant force is zero. The car cannot accelerate any more, so it moves at a constant top speed.
Look at the velocity-time graph for the skydiver. Estimate the terminal velocity before the parachute opens, and describe how you can tell from the graph that it is a terminal velocity.
Mark scheme
about 55 m/s (1); the graph is horizontal / velocity is constant (1)
Model answer
About 55 m/s. The line is horizontal, so the velocity is constant and the acceleration is zero.
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