## What is the momentum of an object of mass m, moving with a velocity v ?

[question] Question. What is the momentum of an object of mass m, moving with a velocity v ? (a) (mv)2 (b) mv2 (c) 1/2mv2 (d) mv [question] [solution] Solution: Correct answer is option (d). [solution]...

## An automobile vehicle has a mass of 1500 kg.

[question] Question. An automobile vehicle has a mass of 1500 kg. What must be the force between the vehicle and road if the vehicle is to be stopped with a negative acceleration of 1.7 ms–2 ? [question] [solution] Solution: Mass of the vehicle, $\mathrm{m}=1500 \mathrm{~kg} ;$ negative acceleration, $\mathrm{a}=-1.7 \mathrm{~ms}^{-2}$ $\therefore$ Force of friction between the road and vehicle $\mathrm{F}=\mathrm{ma}=1500 \times(-1.7)=-2550 \mathrm{~N}$ Negative sign means force is acting in th...

## An 8000 kg engine pulls a train of 5 wagons, each of 2000 kg, along a horizontal track.

[question] Question. An 8000 kg engine pulls a train of 5 wagons, each of 2000 kg, along a horizontal track. If the engine exerts a force of 40000 N and the track offers a frictional force of 5000 N, then calculate: (a) the net accelerating force (b) the acceleration of the train (c) the force of wagon 1 on wagon 2 [/question] [solution] Solution: (a) Force exerted by the engine $=40,000 \mathrm{~N}$ Force of friction exerted by the tracks $=5000 \mathrm{~N}$ As the force of friction always acts...

## A stone of 1 kg is thrown with a velocity

[question] Question. A stone of 1 kg is thrown with a velocity of 20 ms–1 across the frozen surface of a lake and comes to rest after travelling a distance of 50 m. What is the force of friction between the stone and the ice ? [/question] [solution] Solution: Mass of stone, $\mathrm{m}=1 \mathrm{~kg}$; initial velocity of stone, $\mathrm{u}=20 \mathrm{~ms}^{-1}$; final velocity of stone, $\mathrm{v}=0 ;$ distance covered by the stone, $\mathrm{s}=50 \mathrm{~m} ;$ acceleration of stone, $\mathrm...

## A truck starts from rest and rolls down a hill with a constant acceleration.

[question] Question. A truck starts from rest and rolls down a hill with a constant acceleration. It travels a distance of $400 \mathrm{~m}$ in $20 \mathrm{~s}$. Find its acceleration. Find the force acting on it if its mass is 7 metric tonnes. (Hint : 1 metric tonne $=1000 \mathrm{~kg}$.) [/question] [solution] Solution: Initial velocity of truck, $\mathrm{u}=0 ;$ time, $\mathrm{t}=20 \mathrm{~s} ;$ distance covered, $\mathrm{s}=400 \mathrm{~m} ;$ acceleration, $\mathrm{a}=? ;$ mass of truck, $...

## A batsman hits a cricket ball which then rolls on a level ground. After covering a short distance, the ball comes to rest.

[question] Question. A batsman hits a cricket ball which then rolls on a level ground. After covering a short distance, the ball comes to rest. The ball slows to a stop because (a) the batsman did not hit the ball hard enough. (b) velocity is proportional to the force exerted on the ball. (c) there is a force on the ball opposing the motion. (d) there is no unbalanced force on the ball, so the ball would want to come to rest. [/question] [solution] Solution: Correct answer is option (c). There i...

## Why is it advised to tie any luggage kept on the roof of a bus with a rope?

[question] Question. Why is it advised to tie any luggage kept on the roof of a bus with a rope? [/question] [solution] Solution: Luggage on the top of the bus is a loose fixture and not a compact part of the bus. Thus, when a speeding bus brakes suddenly, the luggage continues moving forward because of inertia of motion and is likely to fall off the bus. Conversely, if a stationary bus accelerates suddenly, the luggage continues in the same state because of inertia, and hence is left behind rel...

## When a carpet is beaten with a stick,

[question] Question. When a carpet is beaten with a stick, dust comes out of it. Explain. [/question] [solution] Solution: Initially, the carpet and loose dust in it are in the state of rest. When the carpet is hit with a stick, it is suddenly set into motion, but the loose dust in it remains in the state of rest because of inertia of rest. Thus, in a way, dust is left behind relative to carpet and hence comes out in air. [/solution]...