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Can you find the impracticability of the following question?

The following question is from a recent question paper for class XI Physics where the teacher tried to restructure a question from NCERT Textbook in Physics for Class XI. This is an example where careless restructuring creates problematic situation. However, can you tell me what is the impracticability with  the following problem? Also tell me what assumptions can make the question right.

Q. A police van moving with a speed of 36 km/h fires a bullet at a thief’s  car  speeding away in the same direction with a speed of 180 km /h  . If the muzzle speed of bullet is 150 m/s , what is the time taken by the bullet to hit the thief’s car, if the distance between the two vehicles is 1km at the instant the bullet is fired.In a position time graph represent the motion of the police van ,  thief’s car and the bullet.

 [The unmodified question from textbook is given below, which is harmless]

A police van moving on a highway with a speed of 30 km/h fires a bullet at a thief’s car speeding away in the same direction with a speed of 192 km/h. If the muzzle speed of the bullet is 150 m/s, with what speed does the bullet hit the thief’s car ? (Note: Obtain that speed which is relevant for damaging the thief’s car).

A Question from Optics (awaiting answer)

Question: The difference between the speed of light compared to any other body on earth is negligible then if we take a car moving at 30m/s and i stand at a distance of 1m from it i am a stationary observer,if i assume the speed of light to be 3 x 10^8 then even i will observe the world at such a speed then when i look at the car say for one second then light has already reached my eye after bouncing off the car, by that time the car would have moved a very very small distance and this repeats until the car moves past me.what i cant understand is then why does the car whizz past me because at that rate it should take a lot of time to go past me and i should be able to see the cars every nanometer of motion just like in slow motion videos.i have read in a psychology book that it is because of the angle made by the retina of the eye and the objectively regardless of my human perception what could be the reason?

A few Numerical Problems from Mechanics

ninzyy posted:


1. A 12 kg block sliding at 6.5 m/s along a level floor encounters rough area where the coefficient of friction is 0.20. The rough area is 5 meters long. How fast will the block be moving as it leaves the rough area? Where did the energy go?

2. A 800 kg car traveling southward at 9.2 m/s collides with a 900 kg car traveling northward at 4.5 m/s.?
The cars become entangled. What is the speed and direction of the cars immediately following the collision?

3. A 6kg block slides down a 2.0 m long frictionless ramp which is inclined 40 degrees above the ground.?
a) find the height of the top of the ramp and then determine the speed at the bottom of the ramp.

b) If the block collides with, and sticks to a 8.2 kg block which had been at rest at the bottom of the ramp, how fast would they now move?

c) Determine how much energy the two blocks have after the collision.


1. The problem can be solved using work energy theorem

i.e; work done against friction = change in KE

Substitute the values and you will get the answer.

2. This is an example for inelastic collision.

According to the law of conservation of momentum,

total momentum before collision = total momentum after collision

Taking northwards as positive and southwards as negative,

Substitute in the formula

Substitute the values of m1, m2, v1 and v2 and you can calculate the value of v.

If v is positive, it will be towards north and if negative it will be towards south.

Flies in a car

Arvind MR Asks:

“while travelling, i have noticed some flies buzzing in front of me even if the car is going at a high speed…..

if the fly is stationary and moving freely the it can only do by running in the speed of the car. Howz that possible?


Any fly inside the car will have the same velocity as the car. Just imagine you are running and tossing a ball. You can see that the tossed ball returns to your hand though you were running.

A question from Kinetic energy

Atul asked:

A lorry and a car moving with the same KE are brought to rest having same retarding force .which one of them should come at rest at shorter distance.



According to Work Kinetic Energy theorem

Both should stop travelling the same distance

(This was a quick answer without a deep thought. If the readers have a diffrent opinion, please comment)

Numericals from Kinematics

Posted By – Abdul Hameed

  1. During the first 18 minutes of a 1.0-hour trip, a car has an average speed of 11 m/s. What must the average speed of the car be during the last 42 minutes of the trip be if the car is to have an average speed of 21 m/s for the entire trip?
  2. An elevator is moving upward with a speed of 11 m/s. Three seconds later, the elevator is still moving upward, but its speed has been reduced to 5.0 m/s. What is the average acceleration of the elevator during the 3.0 s interval?
  3. A turtle takes 3.5 minutes to walk 18 m toward the south along a deserted highway. A truck driver stops and picks up the turtle. The driver takes the turtle to a town 1.1 km to the north with an average speed of 12 m/s. What is the magnitude of the average velocity of the turtle for its entire journey?



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April 2021

Schrodinger’s Cat in Daily Life

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