Sunday, July 10, 2016

Physics 9702 Doubts | Help Page 240

  • Physics 9702 Doubts | Help Page 240



Question 1115: [Work, Energy and Power]
The kinetic energy of a vehicle of mass 1000 kg is 4.5 × 105 J. It is stopped by applying a constant braking force of 6000 N.
What is its stopping distance?
A 37 m                        B 75 m                        C 150 m                      D 300 m

Reference: Past Exam Paper – November 2010 Paper 12 Q16



Solution 1115:
Answer: B.
The speed of the vehicle decreases as it stops. The kinetic energy of the car is converted into work done to stop it.

Work done in stopping vehicle = braking force × stopping distance
4.5 × 105 = 6000 × s
Stopping distance, s = (4.5 × 105) / 6000 = 75 m










Question 1116: [Kinematics]
A person, travelling on a motorway a total distance of 200 km, travels the first 90 km at an average speed of 80 km h–1.
Which average speed must be obtained for the rest of the journey if the person is to reach the destination in a total time of 2 hours 0 minutes?
A 110 km h–1               B 120 km h–1               C 122 km h–1               D 126 km h–1

Reference: Past Exam Paper – November 2013 Paper 13 Q9



Solution 1116:
Answer: D.
Average speed = total distance / total time
Time taken to travel first 90 km = 90 / 80 = 1.125 h = 1 h 7.5 min

For the rest of the journey,
Distance that needs to be travelled = 200 – 90 = 110 km
Time remaining = 2 h – 1.125 h = 0.875 h
Average speed = total distance / total time = 110 / 0.875 = 126 km h–1










Question 1117: [Vectors]
A vector has magnitude R and perpendicular components P and Q, as shown in the diagram.


Which row correctly describes the perpendicular components?
vertical component      horizontal component
A                     Q                                 R sinθ
B                     R cosθ                                     P
C                     R cosθ                                     R sinθ
D                     R sinθ                          R cosθ

Reference: Past Exam Paper – June 2012 Paper 12 Q3



Solution 1117:
Answer: D.
From the diagram,
Vertical component Q = R sinθ
Horizontal component P = R cosθ

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