Motion — Set 3
Physics · गति · Questions 21–30 of 60
Which quantity is a vector?
Correct Answer: A. Mass
The correct answer is 'Mass'. A vector has both magnitude and direction. Velocity includes direction, while speed has only magnitude. Therefore, velocity is a vector quantity.
Convert 250 cm/s into m/s.?
Correct Answer: B. 250 m/s
Since 100 cm = 1 m, divide by 100 to convert cm to m. So 250 cm/s = 250/100 = 2.5 m/s. Therefore, 2.5 m/s is correct.
Terminal velocity of a falling object is reached when?
Correct Answer: D. Acceleration becomes maximum
Terminal velocity occurs when net force becomes zero. That happens when air resistance balances the weight of the object. Then acceleration becomes zero and speed becomes constant.
A car slows down from 20 m/s to 10 m/s over 50 m. What is the acceleration?
Correct Answer: C. -2 m/s^2
Use v^2 = u^2 + 2as with v = 10, u = 20, s = 50. So 100 = 400 + 100a, giving a = -3 m/s^2. Hence, the acceleration is -3 m/s^2.
If the speed of a vehicle is doubled, the stopping distance (for the same braking force) generally becomes?
Correct Answer: B. Unchanged
The correct answer is 'Unchanged'. For constant deceleration, stopping distance s is proportional to u^2. Doubling speed makes u^2 four times larger. So stopping distance becomes four times.
A person covers 120 km in 3 hours. What is the average speed?
Correct Answer: D. 40 km/h
Average speed = total distance / total time. So average speed = 120/3 = 40 km/h. Therefore, 40 km/h is correct.
Which situation has zero acceleration but nonzero velocity?
Correct Answer: A. Uniform straight-line motion
Acceleration is zero when velocity does not change in magnitude or direction. In uniform straight-line motion, velocity remains constant. Hence acceleration is zero but velocity is nonzero.
A stone is dropped from a height of 20 m. Taking g = 10 m/s^2, how long does it take to reach the ground?
Correct Answer: B. 3 s
Use s = (1/2)gt^2 with s = 20 and g = 10. Then 20 = 5t^2, so t^2 = 4 and t = 2 s. Therefore, the time taken is 2 s.
Retardation is?
Correct Answer: D. Zero speed
The correct answer is 'Zero speed'. Retardation means acceleration opposite to the direction of motion. In one-dimensional motion, that is represented by negative acceleration. So retardation is negative acceleration.
Two cars move in the same direction with speeds 60 km/h and 40 km/h. What is the relative speed of the faster car with respect to the slower car?
Correct Answer: B. 20 km/h
For motion in the same direction, relative speed is the difference of speeds. So relative speed = 60 - 40 = 20 km/h. Therefore, 20 km/h is correct.