Tag: physics

Questions Related to physics

A hockey ball of mass 200 g travelling at $10\ ms^{-1}$ is struck by a hockey stick so as to return it along its original path with a velocity of $5\ ms^{-1}$. Calculate the change of momentum which occurred in the motion of the hockey ball by the force applied by the hockey stick:

  1. $-3\ Ns$

  2. $3\ Ns$

  3. $6\ Ns$

  4. $-6\ Ns$


Correct Option: A
Explanation:

Mass of ball(m)$=$200g$=$0.2kg
Initial velocity of ball(u)$=$10m${s}^{-1}$
Final velocity of ball(v)$=$5m${s}^{-1}$
Therefore, change in momentum ($\triangle$p) $=$ Final momentum - initial momentum $=m(v-u)=0.2\times(-5-10)=-3\ kgm{s}^{-1}=-3\ Ns$

The change in momentum of a vehicle weighing 2000 kg when its speed increases from 36 km/h to 72 km/h uniformly is :

  1. 15000 kg.m/s

  2. 4000 kg.m/s

  3. 20000 kg.m/s

  4. 7200 kg.m/s


Correct Option: C
Explanation:

Given :   $m = 2000$ kg            $u = 36$ km/h $ = 36 \times \dfrac{5}{18}  = 10$ m/s                $v= 72$ km/h $ = 72 \times \dfrac{5}{18}  = 20$ m/s             

Change in momentum       $\Delta P = m(v-u) = 2000\times (20-10)  =20000$  kg.m/s

A molecule of mass $m$ of an ideal gas collides with the wall of the vessel with the velocity $v$ and returns back with the same velocity. The change in the linear momentum of the molecule will be:

  1. $4mv$

  2. $8mv$

  3. $2mv$

  4. $-2mv$


Correct Option: D
Explanation:

Initial momentum  $=mv$

Final momentum  $=-mv$

Change in momentum  $=-mv-(mv)=-2mv$

A 4-kilograms ball slides over horizontal friction less surface with velocity of 1 meter per second ball strike the post and rebounds upward perpendicular to the initial direction with same speed.
Calculate the change of momentum of the ball in initial direction.

  1. -4 kg.m/s

  2. -1 kg.m/s

  3. 0 kg.m/s

  4. 1 kg.m/s

  5. 4 kg.m/s


Correct Option: A
Explanation:

Initially the velocity of the ball in horizontal direction is $1m/s$.

Finally the velocity of ball remains in vertical direction. Thus velocity in initial direction(horizontal)=$0m/s$
Thus the change in momentum=$\vec{p _2}-\vec{p _1}=m(\vec{v _2}-\vec{v _1})=4\times (0-1)kgm/s=-4kgm/s$

A cricket ball of mass 500 g is moving with speed of $36 \,km\,h^{-1}$. It is reflected back with the same speed. What is the impulse applied on it?

  1. 20 Kg m/s

  2. 10 Kg m/s

  3. 5 Kg m/s

  4. none of the above


Correct Option: B
Explanation:

Impulse $I=$ change in momentum $=mv _2-mv _1$

Here, $v _1=-v _2=36 km/h$
So, $I=(500/1000)[-36-36]=-36 kg.m/s$
The magnitude of impulse $=36 kg.m/s$

A mass of 2 kg at rest travels for 4 seconds with an acceleration of 1.5 m $s^{-2}$. Find the gain of the momentum of the body.

  1. 5 kg m $s^{-1}$

  2. 10 kg m $s^{-1}$

  3. 12 kg m $s^{-1}$

  4. 14 kg m $s^{-1}$


Correct Option: C
Explanation:

$v=u+at=1.5\times 4=6ms^{-1}$

Gain in momentum=$mv=2\times 6=12kgms^{-1}$

A force acts on a body of mass 3 kg such that its velocity changes from $4 \,m\, s^{-1}$ to $10 \,m\, s^{-1}$. The change in momentum of the body is:

  1. $42 \,kg\, m\, s^{-1}$

  2. $2 \,kg\, m\, s^{-1}$

  3. $18 \,kg\, m\, s^{-1}$

  4. $14 \,kg\, m\, s^{-1}$


Correct Option: C
Explanation:

Given, $m=3 kg$, $v _1=4 m/s$ and $v _2=10 m/s$

Change in momentum $=p _2-p _1=mv _2-mv _2=m(v _2-v _1)=3(10-4)=18 $ $kg $ $ms^{-1}$

State whether true or false.
If two bodies of different masses move with the same momentum, then the heavier body will have the greater velocity.

  1. True

  2. False


Correct Option: B
Explanation:

If two bodies of different masses move with the same momentum, then the heavier body will not have the greater velocity.

Hence,
option $B$ is correct answer.

The time, in which a force of 2 N produces a change in momentum of $0.4\, kg\, m\, s^{-1}$ in a body whose mass is 1 kg is: 

  1. 0.2 s

  2. 0.02 s

  3. 0.5 s

  4. 0.05 s


Correct Option: A
Explanation:

Impulse $I=Ft$

also $I=$change in momentum $=0.4 kg m/s$
So, $0.4=(2)t $ or $t=0.2 s$

The time, in which a force of 4 N produces a change in momentum of 0.4 kg m $s^{-1}$ in the body is:

  1. 0.1 s

  2. 0.02 s

  3. 0.5 s

  4. 0.05 s


Correct Option: A
Explanation:

Impulse $=$ change in momentum

or $Ft=0.4$
or $t=0.4/F=0.4/2=0.2$ sec