Tuesday, December 12, 2017

The first column in the table gives four examples of work being done. The second column gives more detail of the action.



Question 5
The first column in the table gives four examples of work being done. The second column gives more detail of the action.

Which row is not correct?





Note: The question is the same in both papers, except that in the November 2011 one, ‘gravity’ is used instead of ‘gravitational field’ for option A.






Reference: Past Exam Paper – November 2011 Paper 13 Q16 + June 2017 Paper 13 Q14





Solution:
Answer: A.

Work Done = Force × Distance moved in direction of the force

Case A: a girl dives from a diving board into a swimming pool
Direction of force (gravitational) = downwards
Direction of motion = horizontal
Row A is incorrect. So, the solution to the question is A.



Case B: a man pushes a car along a level road
Both the frictional force and the motion are horizontal.



Case C: an electron is accelerated towards a positively charged plate
Direction of force (electric force) = towards positively charged plate
Direction of motion: electron is moving (accelerating) towards the positive charge



Case D: a piston is pushed outwards as a gas expands
Direction of force = on piston (the atmosphere exerts a force on the surface of the piston)
Direction of motion = piston moves outwards


Monday, December 11, 2017

The crane shown in the figure lifts its 500N load to the top of the building from A to B. Distances are as shown on the diagram. Calculate how much work is done by the crane.



Question 4
The crane shown in the figure lifts its 500N load to the top of the building from A to B. Distances are as shown on the diagram. Calculate how much work is done by the crane.


The dotted line shows the track of the load as it is lifted by the crane.





Reference: Cambridge International AS and A Level Physics Coursebook – 2nd edition – by Sang, D., Jones, G., Woodside, R. and Chadha, G.





Solution:
Work done = Force × Displacement in direction of the force

The load is first lifted vertically from A to a height of 40 m (the height of the building). It is moved (horizontally from that position to point B)

To lift load vertically (an upward force is required) from A to a height of 40 m,
Work done = 500 × 40 = 20 000 J

Now, to keep the load at this position, an upward force is still needed. But now, the load is moved horizontally. So, work done for this movement is ZERO since the force is vertical while the displacement is horizontal [there is no component of displacement in the direction of the force].

Work done by the crane = 20 000 J = 20 kJ
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