Archive for 2014

Atmospheric pressure and Barometer: (May - June 2003 Paper 1 Question No. 11)


The diagram shows a simple mercury barometer.

Which height is a measure of the atmospheric pressure?




Correct Answer: C

Explanation: A mercury barometer is a traditional instrument widely used to measure air pressure. The simple barometer above shows that the mercury level is up to point C. In fact, air gives pressure on the surface of the mercury which pushes the mercury to go up through the tube. The following figure will illustrate the atmospheric pressure:


So, it’s clear from the above figure that height of the mercury column which is C in the question is the correct answer.



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Work, Energy, and Power: (May - June 2003 Paper 1 Question No. 10)

A boy, who weighs 50 N, runs up a flight of stairs 6.5 m high in 7 seconds.

How much power does he develop?

A   6.5 / (50 x 7) W
B   (7 x 6.5) / 50 W
  50 / (7 x 6.5) W
D   (50 x 6.5) / 7 W

Correct Answer: D

Explanation: Here, Weight of the boy, w = 50 N
                        Height of the stairs, h = 6.5 m
                        Time taken, t = 7 s
                        Power developed, p = (work done / time taken)
                                                       = [(weight x height) / time taken]
                                                       = [(50 N x 6.5 m) / 7 s]
                                                       = (50 x 6.5) / 7 W
                        Hence the correct answer is  D.

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Transformation of energy: (May - June 2003 Paper 1 Question No. 09)




 Which energy changes take place when a pedalling cyclist uses a generator (dynamo) to light his bicycle lamp?

A  chemical kinetic electrical light
B  electrical chemical kinetic light
C  kinetic chemical light electrical
D   light electrical kinetic chemical


Correct Answer: A

Explanation: We know from the law of conservation of energy: Energy cannot be produced or destroyed rather we can just transform energy from one form to another. Here, chemical energy of the cyclist’s body produces kinetic energy through pedaling the cycle. This kinetic energy is then transformed into electrical energy by the dynamo of the cycle and then in the lamp electrical energy is changed into light energy. Hence the correct answer is A.

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Scalars and Vectors: (May - June 2003 Paper 1 Question No. 08)

Which of the following groups of physical quantities consists only of scalars?

A acceleration, force, velocity
B acceleration, mass, speed
C force, time, velocity
D mass, speed, time

Correct Answer: D


Explanation:

Scalars are the physical quantities which have only a magnitude (value) but no direction is associated with them. On the other hand, Vectors are the quantities which have a magnitude as well as a direction. Another basic rule about vector and scalar is if we multiply a vector with a scalar it will be a vector, if we multiply a vector with another vector then the result might be a scalar or a vector which can be found by cross multiplication of the vectors (cross multiplication is beyond the O level syllabus and hence its avoided here) and if we multiply a scalar with another scalar it will be a scalar but if we multiply a scalar with a vector then it will be a vector. Briefly we can say:

Scalar x Scalar = Scalar
Scalar x Vector = Vector
Vector x Scalar = Vector
Vector x Vector = Scalar or Vector

So, according to the definition above we can see:
            A: acceleration is a vector as it is the rate of change of velocity which itself is a vector. Velocity has a direction and a magnitude. Then force is a vector which is defined as, Force = (mass x acceleration). Here acceleration is a vector. Velocity is also a vector as it’s the speed of an object toward a particular direction.
            B: Acceleration → vector, mass → scalar (it has no direction), speed → scalar (no direction)
            C: Force → vector, time → scalar, velocity → vector
            D: mass → scalar, speed → scalar, time → scalar.
So, correct answer is D where all the quantities are scalars.

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