Shi Qi 2 Oct 2017

Question

A metal nugget floats in between some water and mercury. The densities of the metal nugget, mercury and water are 7900 kg m^-3, 13600 kg m^-3 and 1000 kg m^-3 respectively.

What is the ratio of the volume of the nugget submerged in water to that of mercury?

A) 0.541
B) 0.826
C) 0.924
D) 1.21

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Answer

Since the nugget is suspended in the liquids, the weight of the nugget is equal to the upthrust.

The nugget displaces both water and mercury, hence, the upthrust is due to the both the weight of the water displaced and the weight of the mercury displaced.

The volume of the nugget is the sum of the volume of water displaced and the volume of mercury displaced.

The answer is B.

Ashley 02 Aug 2016 - 3

Question

Which statement about an object moving in a straight line through air is correct?

A When it accelerates, the resultant force acting on it is zero.
B When it moves at a steady speed, the air resistance acting on it is zero.
C When it moves at a steady speed, the resultant force acting on it is zero.
D When it moves, there is a resultant force acting on it.

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Answer

According to Newton’s First Law of Motion, when the resultant force is zero, a stationary object remains at rest and a moving object remains moving with constant velocity.

Hence, the answer is C.

Ashley 02 Aug 2016 - 1

Question

A parachutist is falling at terminal velocity, without her parachute open.

She now opens her parachute.

What is the direction of her motion, and what is the direction of her acceleration, immediately after she opens her parachute?

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Answer

When the parachutist is falling at terminal velocity, the upwards air resistance is equal to the weight of the parachutist. The resultant force is zero and the velocity is downwards.

When the parachute opens, the air resistance increases and is now greater than the weight.

The resultant force is now upwards and the acceleration is upwards. The velocity is still downwards.

Answer is B.