This set of Engineering Physics Interview Questions and Answers for Experienced people focuses on “Friction and Forces III”.
1. There is some water in a beaker placed on the pan of a spring balance. If we dip our finger in this water without touching the bottom of the beaker,
a) The reading will be the same
b) The reading will increase
c) The reading drops to zero
d) The beaker falls down
Explanation: The water exerts upthrust on the finger and the finger will exert an equal force of reaction on the water in the downward direction. Thus the reading will increase.
2. When a person walks on a rough surface, the frictional force exerted by the surface on the person is opposite to the direction of his motion.
Explanation: When a person walks, he pushes the ground backward with his foot. The tendency of the foot when it is in the contact of the earth is to move backward. Hence the force of friction acts in the forward direction.
3. Friction is a non-conservative force.
Explanation: When the direction of motion of a body reverses, the direction of friction is also reversed. Work has to be done against both during forward and return journey. So friction is non-conservative force.
4. A stone tied at the end of a string is whirled in a circle. When the string breaks, the stone
a) Falls down vertically
b) Flies upwards
c) Takes a non-uniform path and falls to ground
d) Flies away tangentially
Explanation: The instantaneous velocity of the stone moving round the circle is along the tangent to the circular path. When the string breaks, the centripetal force vanishes. Due to the inertia of motion, the stone flies away tangentially.
5. A Bucket containing water is going in a vertical circle. The water
a) Falls drop by drop
b) Falls all at once
c) Does not fall
d) Only upper half of the water is lost
Explanation: For its revolution in a vertical cycle, water in the bucket needs centripetal force. The weight of the water due to which water can fall is used up in providing the necessary centripetal force and the water does not fall.
6. It is easier to catch a tennis ball than a cricket ball even when both are moving with the same velocity.
Explanation: Due to the small mass of tennis ball, the momentum of the table tennis is much smaller than that of the cricket ball of same velocity. Less force is required to stop the table tennis ball than the cricket ball. Hence it is easy to catch the table tennis ball than the cricket ball.
7. The speed of driving a car safely depends on
a) Capacity of the car
b) Fuel in the tank
c) Inflation of tyre
d) Headlight range
Explanation: The driver has to stop the car before it reaches the obstacle. The retarding force acting on the car is constant. If, on applying brakes, the retardation in the car is a, then to stop the car within distance s, the speed v of the cars should be lesser than√2as. Thus the speed of the car depends on the range of the headlight.
8. What is the magnitude of the net force acting a kite skilfully held stationary in the air?
d) Cannot be predicted
Explanation: As the kite is held stationary, the net force acting on it is zero. The force exerted by the air on the kite is balanced by the tension produced in the string.
9. The driver of a three wheeler with a speed of 36km/h sees a child in the middle of the road and brings his vehicle to rest in 4s just in time to save the child. What is the average retarding force on the vehicle? The mass of the three-wheeler is 400kg and the mass of the driver is 65kg.
a) 116.25 N
b) 2122 N
c) 1162.5 N
d) 212.2 N
Explanation: u = 36km/h=10m/s, v = 0,
t = 4s, m = 400+65kg = 465kg
v=u+at; a =-2.5m/s2
Magnitude of the retarding force on the vehicle is
F = ma = 465×2.5 = 1162.5N.
Sanfoundry Global Education & Learning Series – Engineering Physics.
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