Tag: work, energy and power

Questions Related to work, energy and power

A ball is dropped on the floor from a height of $10m$. It rebounds to a height of $2.5\ m$. If the ball is in contact with the floor for $0.01\ s$, then the average acceleration during contact is nearly.

modelling collisions collisions momentum work, energy and power physics

  1. $500\sqrt {2}m/s^{2} upwards$

  2. $1800\sqrt {2}m/s^{2} downwards$

  3. $1500\sqrt {2}m/s^{2} upwards$

  4. $1500\sqrt {2}m/s^{2} downwards$

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Correct Option: A

Which of the following statements are correct

modelling collisions collisions momentum work, energy and power physics

  1. Collision does not require physical contact

  2. Collision between sub atomic particles is elastic

  3. Collision between macroscopic bodies is generally inelastic

  4. None of these

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Correct Option: A
Explanation:

Collision does not require physical contact because if we go by definition of collision is the meeting of particles or of bodies in which each exerts a force upon the other causing exchange of energy and momentum. In short we can say that collision is a event in which two bodies, exert force on each other for a relative short time. however mostly he study collision in terms of physical contact since force applied is mechanical force. Mostly force between two bodies if it is not physical then it would not be studied as collision more as force applied to bodies.

Choose the incorrect statement

modelling collisions collisions momentum work, energy and power physics

  1. Physical contact is not necessary for a collision

  2. Heat is a form of energy that resided in a system

  3. Kinetic energy of a system may increases due to collision

  4. Work done by internal forces may not be zero

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Correct Option: B

Which of the following is not an inelastic collision.

modelling collisions collisions momentum work, energy and power physics

  1. a man jumps on cart.

  2. a bullet imbedded in a block.

  3. collision of two glass balls.

  4. none of these.

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Correct Option: C
Explanation:

"Man jumping on cart" and "bullet imbedded in a block" are examples when two bodies have to move together after collision, therefore both of them are inelastic collision.
Where as it is not the case for "collision of two glass balls".

In a perfectly elastic collision :

modelling collisions collisions momentum work, energy and power physics

  1. both KE and momentum are conserved.

  2. only KE is covered.

  3. only momentum is covered.

  4. neither KE nor momentum are conserved.

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Correct Option: A
Explanation:

Elastic collisions occur only if there is no net conversion of kinetic energy into other forms. During the elastic collision, kinetic energy is first converted to potential energy associated with a repulsive force between the particles (when the particles move against this force, i.e. the angle between the force and the relative velocity is obtuse), then this potential energy is converted back to kinetic energy (when the particles move with this force, i.e. the angle between the force and the relative velocity is acute).
Also, the average of the momenta before and after the collision is the same. 
Hence, in a perfectly elastic collision both KE and momentum are conserved.

When two bodies collide elastically then the quantity conserved is:

modelling collisions collisions momentum work, energy and power physics

  1. kinetic energy

  2. mometum

  3. both

  4. none

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Correct Option: C
Explanation:

In elastic collision both momentum and kinetic energy are conserved.

In an elastic collision between two particle :

modelling collisions collisions momentum work, energy and power physics

  1. the total kinetic energy of the system is always conserved.

  2. the kinetic energy of the system before collision is equal to the kinetic energy of the system after collision.

  3. the linear momentum of the system is conserved.

  4. the mechanical energy of the system before collision is equal to the mechanical energy of the system after collision.

Show answer
Correct Option: B,C,D
Explanation:
Total kinetic energy of the system is not conserved always. 
Total kinetic energy of a system is conserved just before and after the collision only in case of perfectly elastic collision.

According to law of conservation of momentum, total linear momentum of the system is always conserved irrespective of elastic or inelastic collision

According conservation of energy, total mechanical energy of a system is always conserved irrespective of elastic or inelastic collision.
Hence total mechanical energy of the system before collision is equal to the mechanical energy of the system after the collision. 

A ball hits the floor and rebound after inelastic collision. In this case :

modelling collisions collisions momentum work, energy and power physics

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

  2. the mechanical energy of the ball remains the same in the collision

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

  4. the total energy of the ball and the earth is conserved

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Correct Option: C
Explanation:

Momentum of the system changes only due to external force (OR impulse).
When a ball hits the floor and gets rebound, then no external force (Impulse) acts on the ball as well as the floor, thus total momentum of the "ball + earth" system remains conserved.
 Also some of the mechanical energy of the ball is always lost in an inelastic collision due to its deformation, hence it cannot remain conserved in this collision.

A ball of mass 0.20 kg falls freely from a certain height and rebounds elastically with a speed of $40 \,ms^{-1}$. The change in momentum of the ball is :

modelling collisions collisions momentum work, energy and power physics

  1. $4\,kg \,ms^{-1}$

  2. $8\,kg \,ms^{-1}$

  3. $16\,kg \,ms^{-1}$

  4. $40\,kg \,ms^{-1}$

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Correct Option: C
Explanation:

Change in momentum, $\Delta p =- 2mv$
$= 2 \times 0.2\times 40 = 16 kg ms^{-1}$(magnitude)

A ball rebounds after colliding with the floor, then in case of inelastic collision:

modelling collisions collisions momentum work, energy and power physics

  1. The momentum of the ball before and after collision is same.

  2. The mechanical of the ball before and after collision is same.

  3. The total momentum of the earth-ball system is conserved

  4. The total kinetic energy of earth and ball is conserved

Show answer
Correct Option: C
Explanation:

Momentum of Earth ball system is conserved because no external force is acting on the system.