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/May Jun/12/2015/Q7

In an experiment to determine the acceleration of free fall g, a ball bearing is held by an
electromagnet. When the current to the electromagnet is switched off, a clock starts and the ball
bearing falls. After falling a distance h, the ball bearing strikes a switch to stop the clock which
measures the time t of the fall.

If systematic errors cause t and h to be measured incorrectly, which error must cause g to
appear greater than 9.81 m s–2?

A. h measured as being smaller than it actually is and t is measured correctly
B. h measured as being smaller than it actually is and t measured as being larger than it actually is
C. h measured as being larger than it actually is and t measured as being larger than it actually is
D. h is measured correctly and t measured as being smaller than it actually is

Solution:
Answer: D.

Gravitational acceleration (symbolized g) is an expression used in physics to indicate the intensity of a gravitational field. It is expressed in meters per second squared (m/s 2 ).
g = m/s ²

Let h = m, t = s .

Consider choice A:
m decrease , t maintain
the gravitational acceleration appear lesser than 9.81 m/s ²

Consider choice B:
m decrease , t increase
the gravitational acceleration appear lesser than 9.81 m/s ²

Consider choice C:
m increase , t increase
the gravitational acceleration appear lesser than 9.81 m/s ² due to s is power 2.

Consider choice D:
m maintain, t decrease
the gravitational acceleration appear greater than 9.81 m/s ².

Reference: PYQ - May/Jun 2015 Paper 12 Q7

9702/May Jun/12/2015/Q30

A charged oil drop of mass m, with n excess electrons, is held stationary in the uniform electric
field between two horizontal plates separated by a distance d.

The voltage between the plates is V, the elementary charge is e and the acceleration of free fall
is g.

What is the value of n ?

Solution:
Answer: B.

Electric field intensity = F/ne = V/d

F = mg

mg/ne = V/d

mgd = neV

n = mgd /eV

Reference: PYQ - May/Jun 2015 Paper 12 Q30