6 questions. Try each one, then open the answer to check yourself and read the explanation.
A satellite moves in a circular orbit of radius 4R round the earth. The acceleration of the satellite in terms of g is
Answer: C (g/16)
Gravitational acceleration falls as 1/r2. At 4R from the earth's centre it is g/42 = g/16.
In the diagram above, the direction of the force between N and S in the magnetic field is
Answer: C (MP)
The field runs from N up to S and the current flows into the paper, so by Fleming's left-hand rule the force is to the right, from M towards P. The field lines are also crowded on the M side, pushing the wire towards P.
What force has to be exerted on a mass 60kg to give it an acceleration of 10ms−2 vertically upwards? [g=10ms−2]
Answer: B (1200N)
The force must hold up the weight and also accelerate the mass: F − mg = ma, so F = m(g + a) = 60 × 20 = 1200 N.
A satellite revolving around the earth is kept on its orbit by
Answer: B (centripetal forces only)
The earth's gravitational pull acts towards the centre of the orbit and supplies the centripetal force that keeps the satellite going round. There is no friction in space.
The plates of a 0.5μF uncharged capacitor are connected to a 10V battery. The charge on the capacitor after a long time is
Answer: D (5.0μC.)
When fully charged, Q = CV = 0.5 μF × 10 V = 5.0 μC.
If a document is to be produced using a duplicating camera, where should the document be placed in order to obtain an exact copy?
Answer: A (At the point which is twice the focal length of the lens of the camera.)
An object at 2F of a converging lens gives a real image at 2F on the other side, the same size as the object, so the copy is exact.