Physics Questions and Answers – Motion in a Straight Line – Relative Velocity

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This set of Physics Multiple Choice Questions & Answers (MCQs) focuses on “Motion in a Straight Line – Relative Velocity”.

1. A body is moving with respect to a stationary frame, its motion can be called _____
a) Absolute
b) Relative
c) Circular
d) Parabolic
View Answer

Answer: a
Explanation: The motion with respect to a stationary frame is called absolute motion. Relative motion is with respect to a moving frame of reference.
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2. A small block is placed over another block which is moving with a velocity of 5m/s. What is the absolute velocity of the small block?
a) 5m/s
b) 10m/s
c) 0m/s
d) 14m/s
View Answer

Answer: a
Explanation: The absolute velocity of the small block is the same as that of the block moving with 5m/s. This is because absolute velocity is studied with respect to the ground frame. The relative velocity of the small block with respect to the other block is zero.

3. What is the correct formula for relative velocity of a body A with respect to B?
a) Vector VR = Vector VA – Vector VB
b) Vector VR = Vector VA + Vector VB
c) Vector VR = Vector VA x Vector VB
d) Vector VR = Vector VB – Vector VA
View Answer

Answer: a
Explanation: The relative velocity of one body with respect to another body can be found out by subtracting their respective velocities in respective order. This rule applies not only to velocity, to all other vector quantities.
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4. The relative velocity of a body A with respect to a body B is 5 m/s. The absolute velocity of body B is 10 m/s. Both the bodies are moving in the same direction. What is the absolute velocity of body A?
a) 10m/s
b) 15m/s
c) -5m/s
d) 0m/s
View Answer

Answer: b
Explanation: Here we will use the formula for relative velocity, Vector VR = Vector VA – Vector VB. Since both the bodies are moving in the same direction, the velocity vectors are of the same sign. VR = 5, VB = 10, therefore, VA = 15m/s.

5. If two bodies are moving in opposite directions with non-zero velocities, which of the following statements is true?
a) Relative velocity > Absolute velocity
b) Relative velocity < Absolute velocity
c) Relative velocity = Absolute velocity
d) Relative velocity <= Absolute velocity
View Answer

Answer: a
Explanation: The formula for relative velocity is, Vector VR = Vector VA – Vector VB. When both the velocities are opposite in direction, the equation becomes VR = VA – (-VB). Hence the magnitudes add up making the relative velocity greater than the absolute velocity of any of the two bodies.
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6. A car is moving with 20m/s velocity, another car is moving with a velocity of 50 m/s. What is the relative velocity of first car with respect to the second?
a) 30 m/s
b) -30 m/s
c) 20 m/s
d) 25 m/s
View Answer

Answer: b
Explanation: The formula for relative velocity is VR = Vector VA – Vector VB. Assuming the cars move in the same direction, the relative velocity = 20-50 = -30 m/s. The relative velocity of the second with respect to the first car is 30 m/s.

7. A truck is moving with 40 m/s velocity, a train is moving with a velocity of 80 m/s. How fast is the rain moving with respect to the truck?
a) 40 m/s faster
b) -40 m/s faster
c) 40 m/s slower
d) 60 m/s slower
View Answer

Answer: a
Explanation: Here we need to find the relative velocity of the train with respect to the truck is VR = VA-VB, VB = 40, VA = 80. On solving we get, VR = 40 m/s. Hence the train will move faster by 40 m/s.
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8. A point A is placed at a distance of 7 m from the origin, another point B is placed at a distance of 10 m from the origin. What is the relative position of B with respect to A?
a) 3 m from A
b) 4 m from A
c) -3 m from A
d) 5 m from A
View Answer

Answer: a
Explanation: Relative displacement of B with respect to A = Displacement of B -Displacement of A. On solving we get, relative displacement = 3 m. Hence B is placed at a distance of 3 m from A.

9. An observer is sitting on a car moving with some constant velocity. The observer sees things around him, in the ____
a) Relative frame of reference
b) Absolute frame of reference
c) Valid frame of reference
d) Ground frame of reference
View Answer

Answer: a
Explanation: The observer is not stationary with respect to ground. The observer is stationary with respect to the frame of the moving car, i.e., to the relative frame of reference. The observer will see everything around him/her with respect to the relative frame of reference.
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10. A body A is moving in North direction, while another body B is moving towards South. Velocity of A is greater than that of B. If North is taken as positive, which of the following relative velocities is positive?
a) Velocity of A with respect to B
b) Velocity of B with respect to A
c) Velocity of A with respect to ground
d) Velocity of B with respect to ground
View Answer

Answer: a
Explanation: The formula for relative velocity is, Vector VR = Vector VA – Vector VB. The velocity of A is greater in magnitude hence the relative velocity of A will be positive. The direction of the relative velocity will be Northwards.

11. What does relative motion signify?
a) The motion of a body with respect to other body
b) Uniformly accelerated
c) Non-uniformly accelerated
d) Motion along a curve
View Answer

Answer: b
Explanation: The three equations of motion are valid for uniformly accelerated motion. The equations do not work in situations where the acceleration is non-uniform. In that case it is better to work with the differential forms of velocity and acceleration.

12. What method is used to find relative value for any vector quantity?
a) Vector sum
b) Vector difference
c) Vector multiplication
d) Vector division
View Answer

Answer: b
Explanation: To find the relative value of any quantity, vector difference is used. The relative value is defined as the subtraction of the remaining vector from the vector whose relative value is to be calculated with respect to the remaining vector.

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