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Motion Worksheet-7

Motion Worksheet-7

 

  1. Which one of the following is not a vector quantity.

A. Displacement                           B. Speed

C. Acceleration                             D. Velocity

 

  1. Which of the following could not be a unit of acceleration ?

A. mm/s2           B. cm s–2            C. km/s              D. m/s2

 

  1. A scooter acquires a velocity of 36 km per hour in 10 seconds just after the start. Calculate the acceleration of the scooter.

A. 1 m/s2.           B. -1 m/s2.         C. 2 m/s2.          D. -2 m/s2.

 

  1. A moving train is brought to rest within 20 seconds by applying brakes. Find the initial velocity, if the retardation due to brakes is 2 m s–2.

A. 20 m/s          B. 30 m/s          C. 10 m/s           D. 40 m/s

 

  1. A body starts to slide over a horizontal surface with an initial velocity of 0.5 m/s. Due to friction, its velocity decreases at the rate of 0.05 m/s2 (acceleration, - 0.05 m/s2). How much time will it take for the body to stop ?

A. 10 seconds                                 B. 20 seconds

C. 5 seconds                                   D. Body will not stop at all

 

Answer:

  1. B

Explanation: Speed is a scalar quantity.

 

  1. C

Explanation: Accleration = velocity/time = (displacement/ time)/ time hence the power of time is -2. Which is not there in km/s.

 

  1. A

Explanation: First of all we need to convert the given velocity into proper units, that is, we should convert the velocity of 36 kilometres per hour into metres per second.

Now,         1 km = 1000 m

So,    36 km = 36 × 1000 m

                   = 36,000 m      ……..(1)

Also, 1 hour = 60 minutes

                   = 60 × 60 seconds

                   = 3600 s            ………(2)

So,    36 km per hour

                   = 10 m/s  ……….(3)

Thus, the given velocity of 36 km per hour is equal to 10 metres per second.

Now, initial velocity, u = 0 (second starts from rest)

          Final velocity, v = 10 m/s from (3)

          Acceleration, a = ?

And, Time, t = 10 s

By putting these values in the first equation of motion :

                   v = u + at

we get :     10 = 0 + a × 10

                   10 a = 10

                   a = 10/10

                   a = 1 m/s2

Thus, the acceleration is 1 m/s2.

 

  1. D

Explanation: In this case the value of retardation is given but in formulas value of acceleration is required. We know that retardation is negative acceleration. So, if the retardation is, +2 m/s2 (as given here), then the acceleration will be, -2 m/s2 (the minus sign  indicates the negative acceleration).

Here, initial velocity, u = ?                   

Final velocity, v = 0 (As the train stops)

Acceleration, a = -2 m/s2

And, Time, t = 20 s

Now, putting these values in the formula :

                   v = u + at

We get :    0 = u + (-2) × 20

                   0 = u – 40

                   u = 40 m/s

thus, the initial velocity of the train is 40 metres per second.

 

  1. A

Explanation: Here, Initial velocity, u = 0.5 m/s

          Final velocity, v = 0 (Since the body stops)

          Acceleration, a = - 0.05 m/s2

And, Time, t = ?

Now, from the first equation of motion, we have :

          v = u + at

so,    0 = 0.5 + (-0.05) × t

or      0.05 t =  0.5

          t = (0.5/0.05)

          t = 10 s

thus, the body will take 10 seconds to stop.