Tag: work, energy and power

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Questions Related to work, energy and power

Multiple choice collisions in one dimension collisions work, energy and power mechanics physics

A gun fires a shell of mass $1.5 $kg with velocity of $150$ m/s and recoils with a velocity of $2.5$ m/s. 

Calculate the mass of the gun.

  1. $20$ kg
  2. $30$ kg
  3. $90$ kg
  4. $60$ kg
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Mass of shell $=1.5$kg

Velocity of shell ${V _1} = 159\,m/s$
Recoil velocity $=2.5$ m/s
mass of gun $=M$
According to the conservation of momentum,
$m{v _1} = m{v _2}$
$\begin{array}{l} 1.5\times 150=M\times 2.5 \ M=\dfrac { { 1.5\times 159 } }{ { 2.5 } }  \ M=90\, kg \end{array}$

Multiple choice collisions in one dimension collisions work, energy and power mechanics physics

A particle of mass m, is attached to one end of a massless spring of force constant k, lying on a frictionless horizontal plane. The other end of the spring is fixed. The particle starts moving horizontally from its equilibrium position at time t = 0, with an initial velocity $u _0$. When the speed of the particle is $0.5\, u _0$, it collides elastically with a rigid walk. After this collision:

  1. The speed of the particle when it returns to its equilibrium position is $u _0$
  2. The time at which the particle passes through the equilibrium position for the first time is $t = \pi \sqrt{\dfrac{m}{k}}$
  3. The time at which the maximum compression of the spring occurs is $t = \dfrac{4\pi}{3}\sqrt{\dfrac{m}{k}}$
  4. The time at which the particle passes througout the equilibrium position for the second time is $t= \dfrac{4\pi}{3}\sqrt{\dfrac{m}{k}}$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

In an elastic collision with a rigid wall, the particle reverses its velocity. Since the spring potential energy is conserved and the collision is elastic, the total energy remains constant. The particle will return to the equilibrium position with the same speed u0 it had initially.

Multiple choice collisions in one dimension collisions work, energy and power mechanics physics

A ball hits the floor and rebounds after an elastic collision. In this case:

  1. the momentum of the ball just after the collision is same as that just before the collision.

  2. the kinetic energy of the ball remains same during collision.

  3. the total momentum of the ball and the earth is conserved.

  4. the total energy of the ball and the earth remains the same.
Reveal answer Fill a bubble to check yourself
C,D Correct answer
Explanation

Since the velocity before and after the collision change hence momentum of the ball will change. So (a) is not true.  


Since the collision is inelastic a part of the mechanical energy is lost hence (b) is not true.  

Taking earth and the ball as a system there is no external force on the system. Hence the total momentum of the ball and the earth is conserved. So (c) is true.  

From the conservation principle of the energy, the total energy of the ball and the earth remains the same. Hence (d) is true.

Multiple choice collisions in one dimension collisions work, energy and power mechanics physics

Two identical spheres move in opposite direction with speed $v _1$ and $v _2$ and pass behind an opaque screen, where they may either cross without touching ( Event 1) or make an elastic head-on collision ( Event 2)

  1. We can never make out which events has occured
  2. We cannot make out which event has occured only if $v _1= v _2$

  3. We can always make out which event has occured

  4. We can make out which event has occured only if
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

For identical spheres colliding elastically, they exchange velocities. If they cross without touching, they continue with their original velocities. If they collide, they also end up with the same final velocities as if they had passed through each other. Thus, the final state is indistinguishable.

Multiple choice collisions in one dimension collisions work, energy and power mechanics physics

A bullet of mass m is fired into a large block of wood of mass M with velocity v. The final velocity of the system is?

  1. $\left(\dfrac{m}{M-m}\right)v$
  2. $\left(\dfrac{m+M}{M}\right)v$
  3. $\left(\dfrac{M-m}{M}\right)v$
  4. $\left(\dfrac{m}{m+M}\right)v$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

u is initial velocity 

v is finall velocity 
m1 is m 
m2 is M 
u1 is v 
u2 is 0 
${ v } _{ 1 }={ v } _{ 2 }={ v } _{ 3 }$
$total\quad initial\quad momentum=finall\quad momentum$
${ m } _{ 1 }{ u } _{ 1 }+{ m } _{ 2 }{ u } _{ 2 }={ m } _{ 1 }{ v } _{ 1 }+{ m } _{ 2 }{ v } _{ 2 }$
$mv+M\times 0=m{ v } _{ 3 }+{ Mv } _{ 3 }$
$mv=\left( m+M \right) { v } _{ 3 }$
${ v } _{ 3 }=\dfrac { mv\quad  }{ m+M } $

Multiple choice collisions in one dimension collisions work, energy and power mechanics physics

sphere collides with another sphere of identical mass kept at rest. Mier collision, the two spheres move. The collision is perfectly inelastic, then the angle between the directions of motion of the two spheres is

  1. ${ 0 }^{ o }$
  2. ${ 45 }^{ o }$
  3. different from ${ 90 }^{ o }$
  4. ${ 90 }^{ o }$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

In a perfectly inelastic collision between identical masses, the spheres stick together and move with a common velocity after collision. Since they combine and move as one object, they travel in the same direction, making the angle between their motion directions 0°.

Multiple choice collisions in one dimension collisions work, energy and power mechanics physics

A smooth sphere A of mass m collides elastically with an identical sphere B at rest. The velocity of A before collision is $8 m/s$ in a direction making $60^{o}$ with the line joining the centres at the time of impact. Which of the following is/are possible:

  1. the sphere a comes to rest after collision
  2. the sphere B will move with a speed of $8 m/s$ after collision

  3. the directions of motion of A and B after collision are at right angles
  4. the speed of B after collision is $2 m/s$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

For an elastic collision between identical masses where one is at rest, the particles move at 90 degrees to each other after the collision, provided the collision is not head-on.

Multiple choice collisions in one dimension collisions work, energy and power mechanics physics

An object is moving towards a mirror with  a velocity v as shown in figure.if the collision between the mirror and the object is perfectly elastic, then the velocity of the image after collision with mirror in vector form is.

  1. $ - v\;\widehat j$
  2. $ - v\;\cos \;2\theta \widehat j + v\;\sin \;2\theta \;\widehat i$
  3. $ - v\widehat i$
  4. $ - v\;\cos \theta \;j - \;v\;\sin \theta \;\widehat i$
Reveal answer Fill a bubble to check yourself
A Correct answer
Multiple choice collisions in one dimension collisions work, energy and power mechanics physics

A body of mass $m$ moving with velocity $u$ collides elastically with another body of mass $m$ at rest. After collision, they moves in a plane with velocities $V _{1}$ and $V _{2}$ respectively. Then the angle beteen $\vec{V} _{1}$ and $\vec{V} _{2}$ is

  1. $zero$
  2. $60^{o}$
  3. $90^{o}$
  4. $180^{o}$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

In a head-on elastic collision between two bodies of equal mass, they exchange velocities. If the second body was at rest, the first body comes to rest and the second moves with the initial velocity of the first. The angle between them is 180 degrees if we consider the path of the first body after it stops (which is not applicable) or 0 degrees. However, in general 2D elastic collisions of equal masses, they move at 90 degrees. The option 180 degrees is likely a distractor or refers to a specific 1D case.

Multiple choice collisions in one dimension collisions work, energy and power mechanics physics

Two solid balls of rubber $A$ and $B$ whose masses are $200\ gm$ and $400\ gm$ respectively, are moving in mutually opposite directions. if the velocity of ball A is $0.3\ m/s$ and both the ball come to rest after collision, then the velocity of ball $B$ is :

  1. $0.15\ m/s$
  2. $-0.15\ m/s$
  3. $1.5\ m/s$
  4. $None\ of\ these$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation
Initial linear momentum of system 

$= [{{m} _{A}}{{\vec{v}} _{A}}+{{m} _{B}}{{\vec{v}} _{B}}]$

$= 0.2 \times  0.3 + 0.4 \times  vB$

Finally both balls come to rest \ final linear momentum = 0 

By the law of conservation of linear momenum        

$0.2 \times  0.3 + 0.4 \times  vB = 0$

$[{{v} _{B}}=-\dfrac{0.2\times 0.3}{0.4}=-0.15\ m/s]$