9702/Oct Nov/11/2015/Q14

Two blocks X and Y are falling through a vacuum in a uniform gravitational field, as shown.

Block X has weight 2w.
Block Y has weight w.
The blocks do not move apart.
Which value best represents the force exerted by block X on block Y?

A 0
B w
C 1.5w
D 2w

Solution:
Answer: A

The figure below explain the free fall object (without air resistance) is not affected by mass or shape. Both blocks are fall at the same rate in vacuum. Thus, no force exerted by block X on block Y.

Reference: PYQ - Oct/Nov 2015 Paper 11 Q14

9702/Oct Nov/11/2015/Q4

A calibration graph is shown for an ammeter whose scale is inaccurate.

Two readings taken on the meter at different times during an experiment are 0.13 mA and
0.47 mA.

By how much did the current really increase between taking the two readings?

A 0.30 mA
B 0.35 mA
C 0.40 mA
D 0.44 mA

Solution:
Answer: A

When ammeter reading = 0.13mA, the current = 0.14mA
When ammeter reading = 0.47mA, the current = 0.44mA
The current increase between taking the two reading is 0.44mA - 0.14mA = 0.3mA

Reference: PYQ - Oct/Nov 2015 Paper 11 Q4

9702/Oct Nov/11/2015/Q6

A light-meter measures the intensity I of the light incident on it. Theory suggests that I varies
inversely as the square of the distance d.

Which graph of the results supports this theory?

Solution:
Answer: C

Theory suggests that the intensity I varies as the inverse square of the distance d.

I α 1 / d²

I = k (1 / d²) = k / d²    where k is a constant

Consider the graph of I (on y-axis) against 1/d² (on x-axis).

Equation of a line: y = mx + x

Compare I = k (1 / d²) = k / d² with the equation y = mx + c:

y = I

x = 1 / d²

m = k

c = 0

So, if a graph of I against 1/d² is plotted, it should be a straight line with a positive gradient (the constant k is positive) and starting at the origin (y-intercept, c = 0 – that is, it intercepts the y-axis at y=0). [C is correct]

Options A and B both have the graph cutting an axis, which would not be the case for an inverse square law graph.

A graph of I against d² would be similar to a graph of y = 1 / x where y = I and x = d². Such a graph is a curve with a negative gradient and not touching the axes.

Type: graph y=1/x

at Google search to see to shape of the graph.

Reference: PYQ - Oct/Nov 2015 Paper 11 Q6

9702/Oct Nov/11/2015/Q7

One of the equations of uniformly accelerated motion is shown.

s = ut + ½ at²

Apparatus is arranged to record the time t taken for a marble to fall between two light gates

connected to timers. The marble touches the stop before it is released. The vertical distance s

between the light gates is measured.

Which graph does not show a correct relationship when light gate 2 moves up to light gate 1
which is fixed?

Solution:
Answer: C

Choice A: from the formula s = ut + ½ at² , we know s ∝ t²  , so choice A is correct.

Choice B: we can derive the formula of s/t = u + ½ at. s/t ∝ t , so choice B is correct.

Choice D: The gravitational acceleration is always constant. So choice D is correct.

Choice C:
We can derive the formula of
ut = s – ½ at²

u = s/t – ½ at

So u = 0 at all the time is doesn't make sense.

Reference: PYQ - Oct/Nov 2015 Paper 11 Q7