Moments

marking cupcake holders

The Lever

February assessment

Physics section

1.       Dawn will travel 4,800,000,000 km in 8 years.

speed = distance ÷ time

          Dawn’s average speed in km per year will be

A           384 000 000
B           600 000 000
C          38 400 000 000
D          60 000 000 000

(Total 1 mark)

  2.     Julia and Alec are investigating how the motion of an object changes with different size forces.

          They produce this velocity-time graph for the motion of a motorcycle.

                                           

          What is the acceleration of the motorcycle?

          A            4 m/s2   a = (v-u)/t = (12-4)/2 = 8/2 = 4m/s²
B                        6 m/s2
C            8 m/s2
D            12 m/s2

(Total 1 mark)

3.      

force = mass × acceleration

          The space probe has a mass of 1200 kg.
Out in space, it accelerates at 0.00008 m/s2.
The force producing this acceleration is (I know force is measure in N so A and C are wrong. I also know that 1200 x 0.00008 will be less than 9600 so D is wrong)

          A            0.096 kg
B            0.096 N                       F=ma = 1200 x 0.00008 = 0.096N
C      9600 kg
D      9600 N

(Total 1 mark)

  4.     (a)     Stewart is a keen racing cyclist. Charlie uses a touring bike to cycle to school.
Their bikes are shown in the diagram.

          Stewart and Charlie have the same mass, and produce the same driving forces on their bikes.

  (i)    Which cyclist is able to produce the greater acceleration when they start moving?
Suggest why.

Cyclist ....Stewart

Reason the racing bike has less mass

(2)

  (ii)   Stewart is able to reach a higher maximum speed on his bike.
Suggest two reasons why. (notice the diagram, without the lines I have drawn the profile to the air resistance is not obvious)

1 Stewart is able to offer a lower profile so there is less drag

2 .The tyres on the racing bike are thinner so there is less friction between them and the road

                                                (Note you cannot use less mass again)                                                                                           (2)

(b)     One day Stewart and Charlie decided to have a bike race. Stewart started some time after Charlie, because he felt his bike was better suited to racing than Charlie’s bike.

          The graph shows how the velocity changed with time for both cyclists.

  (i)    How many seconds after Charlie did Stewart start?

time = ..............2.............................................. s

(1)

  (ii)   How many seconds did it take Stewart to reach the same speed as Charlie?

time = .......................10 – 2 = 8..................................... s

(1)

  (iii)  Use the graph to calculate Charlie’s acceleration.

a = (v-u)/t = (4-0)/10 = 0.4 m/s^-2

acceleration = ......................................... m/s2

(2)

  (iv)  What is the difference between velocity and speed?  Make sure you use the words scalar and vector in your answer.

Velocity is the rate of change of displacement and has magnitude and direction. It is a vector

Speed is the distance travelled in 1 second. It has magnitude only. Speed is scalar

 (2)

(Total 10 marks)

  5.     The diagram shows the forces on a car travelling along a level road.

(a)     How can you tell that the car is accelerating forwards?

The forward driving force is greater than the backward resistive force. There is a resultant forward force. When a force acts on an object it accelerates in the direction of the force.

(1)

  (b)   The total mass of the car is 850 kg.

          Calculate the acceleration of the car.

F = ma.           Resultant force = 950N – 550N = 400N

Rearranging a = F/m = 400/850 = 0.47 m/s²

 (3)

  (c)   Explain how the horizontal forces on the car change when the driver takes her foot off the accelerator and applies the brake.

When the driver takes her foot off the accelerator the driving force drops to zero. As air resistance is proportional to the velocity the resistive force will fall as the velocity falls. The backward forces increase when the driver applies the brakes.

(note – the question only asks you to comment on the forces not the acceleration or the velocity)

 (3)

(Total 7 marks)

Momentum and car safety

Momentum

 

Momentum = mass x velocity                                          kgms^-1 = kg x m/s

1.       A bowling ball has a mass of 10kg and a velocity of 15m/s. Calculate its momentum. 150 kgm/s

2.       A speed skater has a velocity of 15m/s and a mass of 65 kg. Calculate her momentum. 975 kgm/s

3.       A bullet of mass 0.068 kg traveling horizontally at a speed of 150 m/s. Calculate its momentum. 10.2 kgm/s

4.       The world record for bowling at cricket is 80 m/s. A cricket ball has a mass of 0.17kg. Calculate its momentum. 13.6 kgm/s

5.       A jumbo jet has a mass of 30 000 kg. Calculate its momentum at cruising speed of 300ms^-1

2 x 10 ⁴ kgm/s

6.       The Earth has a mass of 5.9742 × 10²⁴ kg. Its orbital velocity is 29.8 km/s. Calculate its momentum. 1.73 x10²⁹ kgm/s Jupiter has a mass of 1.8986×10²⁷ kg and an orbital velocity of 13.06 km/s. Calculate its momentum. 2.48 x10³¹ kgm/s

7.       N₂ has an atomic mass of 4.65173 x 10^-26 kg.  Calculate its momentum if it has a velocity 500m/s 2.33 x 10^-23 kgm/s

8.       A truck of mass 2kg travels at 8 m/s towards a stationary truck of mass 6kg. After the collision they stuck together and move off in the same direction. What is their common velocity?

16kgm/s = (2+6 kg)v so 8v = 16 and v =2 m/s

9.       A car of mass 2000kg, traveling at 10m/s, has a head on collision with a small sports car of mass 500kg. If both cars stop dead on colliding, what was the velocity of the sports car before the collision? Momentum after = 0 so (2000 x 10) + (500 x v) so 500v = 20000 and v = 40m/s

10.    A man wearing a bullet proof vest stands on roller skates. His total mass is 80kg. A bullet of mass 20g is fired at him at a speed of 400 m/s. If the bullet is stopped by the vest and falls to the ground, what velocity will the man move at? Momentum before = (0.02kg x 400) + 0 = 8kgm/s

Momentum after = 0 + (80kg x v) = 8 kgm/s  so 80v = 8 so v = 0.1m/s after being hit

How does your answer compare with “the movies”? They aren’t real. (but then you knew that)

Stopping Distances

1 (a) (i) 8m 1

(ii) 8m 1

(iii) 8m 1

(b) (i) 16m 1

(ii) 16m 1

(c) Thinking distance does not depend on these

factors (brakes, tyres or steering). 1

Thinking distance does depend on speed. 1

Total 7

2 (a) 14m 1

(b) (i) Braking distance is longer. 1

(ii) Braking distance is longer (because of

skidding). 1

Total 3

3 (a) 6m 1

(b) Braking distance more than doubles. 1

Total 2

4 (a) Stopping distance = 9 + 15 = 24m 1

(b) 24/4 = 6 car lengths. 1

Total 2

5(a) Axes labelled and at least half the paper used. 1

Points plotted correctly to give y = x2 by eye. 1

Smooth curve of best fit. (NOT dot-to-dot.) 1

(b) (i) 8 ±2m 1

(ii) 30 ±2m 1

(c) Braking distance approximately quadruples. 1

Total 6

6 (a) The empty van. 1

(b) Because it has less mass. 1

(c) The van carrying bricks. 1

Total 3

7(a) Mass does not affect thinking distance. 1

(b) Mass increases braking distance. 1

(c) Stopping distance increases because braking

distance increases. 1

Total 3