Success TutoringWorksheets

Speed, Velocity, Acceleration and Newton's Laws — Answers

Answer sheet with model answers and success criteria for the Year 11 forces & motion worksheet.

Download answers PDFPrint answersBack to worksheet
Success TutoringWorksheetsAnswer sheetYear 11 · Science · Forces & Motion

Speed, Velocity, Acceleration and Newton's Laws — Answers

Part A: Speed and velocity

Show the formula, your working and the units in every answer.

Speed is how fast an object moves: speed = distance ÷ time. The SI unit is metres per second (m/s); kilometres per hour (km/h) is also common.

Velocity is speed in a given direction, such as 20 m/s north. Two objects can have the same speed but different velocities.

Acceleration is the rate of change of velocity: a = (v − u) ÷ t, where u is the starting velocity, v is the final velocity and t is the time taken. The unit is metres per second squared (m/s2). A negative acceleration means the object is slowing down (decelerating).

1.A cyclist travels 150 m in 30 s. What is her average speed?

Answer: 5 m/s

  1. speed = distance ÷ time
  2. = 150 m ÷ 30 s = 5 m/s
2.A car travels at a steady 25 m/s for 8 s. How far does it go?

Answer: 200 m

  1. distance = speed × time
  2. = 25 m/s × 8 s = 200 m
3.A train travels 120 km in 1.5 hours. What is its average speed in km/h?

Answer: 80 km/h

  1. speed = 120 km ÷ 1.5 h = 80 km/h
4.Convert 72 km/h into m/s.

Answer: 20 m/s

  1. 1 km = 1000 m and 1 h = 3600 s
  2. 72 km/h = 72 × 1000 m ÷ 3600 s = 72 000 ÷ 3600 = 20 m/s
  3. Shortcut: divide km/h by 3.6 (72 ÷ 3.6 = 20)
5.Two cars both travel at 60 km/h, one heading north and one heading south. Which statement is correct?
  1. AThey have the same speed but different velocities
  2. BThey have the same speed and the same velocity
  3. CThey have different speeds but the same velocity
  4. DThey have different speeds and different velocities

Answer: They have the same speed but different velocities

Speed has size only. Velocity is speed in a particular direction, so opposite directions mean different velocities.

Part B: Acceleration

Use a = (v − u) ÷ t. Show your working and units.

6.A sprinter goes from rest to 8 m/s in 4 s. What is her acceleration?

Answer: 2 m/s2

  1. a = (v − u) ÷ t
  2. = (8 − 0) ÷ 4 = 2 m/s[sup 2]
7.A car slows down from 20 m/s to 5 m/s in 5 s. What is its acceleration?

Answer: −3 m/s2 (a deceleration of 3 m/s2)

  1. a = (v − u) ÷ t
  2. = (5 − 20) ÷ 5 = −15 ÷ 5 = −3 m/s[sup 2]
  3. The negative sign shows the car is slowing down.
8.A skateboarder accelerates at 1.5 m/s2 for 6 s, starting at 2 m/s. What is her final velocity?

Answer: 11 m/s

  1. v = u + at
  2. = 2 + 1.5 × 6 = 2 + 9 = 11 m/s

Part C: Newton's second law: F = ma

Show the formula, your working and the units.

Newton's second law: the acceleration of an object depends on the net (unbalanced) force on it and on its mass: F = ma. Force is in newtons (N), mass in kilograms (kg) and acceleration in m/s2. One newton is the force that gives a 1 kg mass an acceleration of 1 m/s2 (1 N = 1 kg·m/s2).

Rearranged: a = F ÷ m and m = F ÷ a. If several forces act, first find the net force (forces in opposite directions are subtracted).

Weight is the force of gravity on a mass: W = mg. On Earth, g ≈ 9.8 N/kg. Mass is measured in kilograms and stays the same everywhere; weight is a force, measured in newtons.

9.A 1200 kg car accelerates at 2.5 m/s2. What net force acts on it?

Answer: 3000 N

  1. F = ma
  2. = 1200 kg × 2.5 m/s2 = 3000 N
10.A net force of 50 N acts on a 10 kg box on a smooth floor. What is its acceleration?

Answer: 5 m/s2

  1. a = F ÷ m
  2. = 50 N ÷ 10 kg = 5 m/s[sup 2]
11.A net force of 600 N gives an object an acceleration of 1.5 m/s2. What is its mass?

Answer: 400 kg

  1. m = F ÷ a
  2. = 600 N ÷ 1.5 m/s2 = 400 kg
12.A student pushes a 20 kg crate with a force of 100 N. Friction acts against the motion with a force of 40 N. What is the net force, and what is the crate's acceleration?

Answer: Net force 60 N; acceleration 3 m/s2

  1. Net force = 100 N − 40 N = 60 N (in the direction of the push)
  2. a = F ÷ m = 60 N ÷ 20 kg = 3 m/s[sup 2]
13.A student has a mass of 60 kg. What is her weight on Earth (g = 9.8 N/kg)? What are her mass and her weight on the Moon (g = 1.6 N/kg)?

Answer: On Earth: 588 N. On the Moon: mass 60 kg, weight 96 N.

  1. Weight W = mg
  2. Earth: 60 kg × 9.8 N/kg = 588 N
  3. Mass does not change from place to place: still 60 kg on the Moon
  4. Moon: 60 kg × 1.6 N/kg = 96 N

Part D: Newton's laws in action

Circle the law that best explains each situation, then answer true or false.

Newton's first law: an object stays at rest, or keeps moving at a constant velocity, unless an unbalanced (net) force acts on it. This tendency to resist changes in motion is called inertia.

Newton's second law: F = ma. A bigger net force gives a bigger acceleration; a bigger mass gives a smaller acceleration for the same force.

Newton's third law: when object A exerts a force on object B, object B exerts a force on object A that is equal in size and opposite in direction.

14.When a bus brakes suddenly, the standing passengers lurch forwards.
  1. ANewton's first law
  2. BNewton's second law
  3. CNewton's third law

Answer: Newton's first law

The passengers' bodies keep moving forwards (inertia) until a force, such as gripping a rail, acts to stop them.

15.A swimmer pushes backwards on the wall of the pool, and the wall pushes the swimmer forwards.
  1. ANewton's first law
  2. BNewton's second law
  3. CNewton's third law

Answer: Newton's third law

The swimmer and the wall exert equal and opposite forces on each other.

16.The same push makes an empty shopping cart speed up much more than a full one.
  1. ANewton's first law
  2. BNewton's second law
  3. CNewton's third law

Answer: Newton's second law

For the same force, a larger mass gives a smaller acceleration (a = F ÷ m).

17.As you walk, your foot pushes backwards on the ground, and the ground pushes you forwards.
  1. ANewton's first law
  2. BNewton's second law
  3. CNewton's third law

Answer: Newton's third law

Action and reaction forces: equal in size, opposite in direction, acting on different objects.

18.A hockey puck slides across smooth ice and keeps going at almost the same speed.
  1. ANewton's first law
  2. BNewton's second law
  3. CNewton's third law

Answer: Newton's first law

With almost no friction there is almost no unbalanced force, so the puck keeps moving at a nearly constant velocity.

19.A car travelling at a constant 60 km/h along a straight, flat road has a net force of zero acting on it.

Answer: True

Constant velocity means zero acceleration, so the forces are balanced: the driving force equals the friction and air resistance acting against the car.

20.In Newton's third law, the two forces in a pair act on the same object and cancel out.

Answer: False

The two forces act on different objects, so they do not cancel each other out.

21.An object that is moving must have an unbalanced force acting on it.

Answer: False

An object can move at a constant velocity with balanced forces (Newton's first law). An unbalanced force is needed only to change its speed or direction.

Part E: Explain

22.Use Newton's first and second laws to explain (a) why wearing a seatbelt protects a passenger in a crash, and (b) why a loaded truck needs a much larger braking force than a small car to stop from the same speed in the same time.[4 marks]

Model answer: (a) In a crash the car stops very quickly, but by Newton's first law the passenger's body keeps moving forwards at the same velocity until a force acts on it. Without a seatbelt that force comes from hitting the dashboard or the seat in front. The seatbelt supplies the force instead, spreading it across the strong parts of the body and stretching slightly so the passenger slows down over a slightly longer time. (b) Stopping from the same speed in the same time means both vehicles need the same deceleration, since a = (v − u) ÷ t. By Newton's second law, F = ma, so the truck, which has a much larger mass, needs a much larger braking force to produce the same deceleration.

Success criteria:

  • Uses inertia (first law) to explain why the passenger keeps moving
  • Explains that the seatbelt supplies the stopping force
  • States that both vehicles need the same deceleration
  • Uses F = ma to show a larger mass needs a larger force
Free worksheets at worksheets.successtutoring.com · © Success Tutoring · Free for personal and classroom use

Marking tips

  • Give credit for correct method even when the final answer slips — the working shows where the thinking went right.
  • For open-ended tasks, use the success criteria as a checklist rather than looking for one "right" answer.
  • Celebrate what went well first, then pick one thing to work on next.

Great work. Here's what to try next: