# Case Study Questions for Class 11 Physics Chapter 3 Motion in a Straight Line

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Average Speed and Average Velocity
When an object is in motion, its position changes with time. So, the quantity that describes how fast is the position changing w.r.t. time and in what direction is given by average velocity. It is defined as the change in position or displacement (Δx ) divided by the time interval (Δt ) in which that displacement occur. However, the quantity used to describe the rate of motion over the actual path, is average speed. It defined as the total distance travelled by the object divided by the total time taken.

(i) A 250 m long train is moving with a uniform velocity of 45 km/h. The time taken by the train to cross a bridge of length 750 m is
(a) 56 s
(b) 68 s
(c) 80 s
(d) 92 s

(ii) A truck requires 3 hr to complete a journey of 150 km. What is average speed?
(a) 50 km/h
(b) 25 km/h
(c) 15 km/h
(d) 10 km/h

(iii) Average speed of a car between points A and B is 20 m/s, between B and C is 15 m/s and between C and D is 10 m/s. What is the average speed between A and D, if the time taken in the mentioned sections is 20s, 10s and 5s, respectively?
(a) 17.14 m/s
(b) 15 m/s
(c) 10 m/s
(d) 45 m/s

(iv) A cyclist is moving on a circular track of radius 40 m completes half a revolution in 40 s. Its average velocity (in m/s) is
(a) zero
(b) 2
(c) 4π
(d) 8π