work and energy Question 9th class (lec 6)

 video lecture for work & energy

Question 1: What will the work done be when a bullet of mass `10 g` at rest is accelerated to a velocity of `20 ms^{-1}` in `10 s`? Calculate the power developed by the bullet during `10 s`. 

Question 2: A stone, tied to the string is whirled in a vertical

circle. Then _____.

(a)  the potential energy of the stone is maximum at

the top most position

(b) the kinetic energy of the stone is maximum at

the lowest position

(c)  the potential energy is maximum at the bottom

most position

(d) Both (a) and (b)

Question 3: The bodies of equal masses have kinetic energy in the

ratio of `4 : 9`. The ratio of their velocity is

(a) `3 : 2`  

(b) `4 : 9`

(c) `2 : 3`  

(d) `9 : 4`

Question 4: A car is accelerated on a levelled road and attains a velocity 4 times of its initial velocity. In this process the potential energy of the car

(a) does not change

(b) becomes twice to that of initial

(c) becomes 4 times that of initial

(d) becomes 16 times that of initial

Question 5: An iron sphere of mass `10 kg` has the same diameter as an aluminum sphere of mass `3.5 kg`. Both spheres are dropped simultaneously from a tower. When they are `10 m` above the ground, they have the same

(a) acceleration

(b) momenta

(c) potential energy

(d) kinetic energy

Question 6: A girl is carrying a school bag of `3 kg` mass on her back and moves `200 m` on a leveled road. The work done against the gravitational force will be (`g =10 ms^{-2}`)

(a) `6\times10^{3} J`

(b) `6 J`

(c) `0.6 J`

(d) `zero`

Question 7: The work done on an object does not depend upon the

(a) displacement

(b) force applied

(c) angle between force and displacement

(d) initial velocity of the object

Question 8: Water stored in a dam possesses

(a) no energy

(b) electrical energy

(c) kinetic energy

(d) potential energy

Question 9: A rocket is moving up with a velocity v. If the velocity of this rocket is suddenly tripled, what will be the ratio of two kinetic energies?

a) `1`

b) `1:9`

c) `3`

d) `1:3`

Question 10: A boy is moving on a straight road against a frictional force of `5N`. After traveling a distance of `1.5km`, he forgets the correct path at a roundabout (see fig.) of a radius `100m`. However, he moves on the circular path for one and a half cycle and then he moves forward up to `2.0km`. Calculate the work done by him (`π=3.14`)

a) `18500 J`

b) `20000 J`

c) `15000 J`

d) `22220 J`

Question 11: The weight of a person on planet `A` is about half that on the earth. He can jump upto `0.4 m` height on the surface of the earth. How high he can jump on the planet `A`?

a) `0.8m`

b) `0.4m`

c) `0.5m`

d) `0.2m`

Question 12: An automobile engine propels a `1000 kg` car `(A)` along a levelled road at a speed of `36 kmh^{-1}` ’. Find the power if the opposing frictional force is `100 N`. Now, suppose after travelling a distance of `200 m`, this car collides with another stationary car `(8)` of same mass and comes to rest. Let its engine also stop at the same time. Now car `(B)` starts moving on the same level road without getting its engine started. Find the speed of the car `(6)` just after the collision.

A light and a heavy object have the same momentum. Find out the ratio of their kinetic energies. Which one has a larger kinetic energy?

Question 13: A girl with having mass of `35 kg` sits on a trolley of mass `5 kg`. The trolley is given an initial velocity of `4m s^{-1}` by applying a force. The trolley comes to rest after traversing a distance of 16 m.

(a) How much work is done on the trolley?

(b) How much work is done by the girl?

Question 14: Can a body undergo displacement in the absence of any force acting on it? Sophie. Discuss this question with your friends and teachers.

Obtained 15:  A rubber ball falling from a free snake hits the ground and bounces up. Is the total energy conserved in this process? If yes then in what form it would be changing when it hits the ground?

Question 16:  What is the reason that in the early days most of the factories were located near flowing rivers or dams? 

Question 17: Where do energy loss and energy gain take place?

Question 18: How is the energy contained in an object measured? 

Question 19: A light object and another heavy object have the same kinetic energy. Which of these will have more momentum? 

Question 20:  When a force of `10 N` is applied on a toy car making an angle of `60°` the horizontally, the car is displaced `5 m` on the horizontal plane. What will be the work done on the car?

a) `25 J`

b) `50 J`

c) `12 J`

d) `24 J`

Question 21: A man drops a stone of mass `10 kg` from the top of a ladder of height `5m`. What will be the energy of the stone when it reaches the earth? What will be the speed of the stone closest to the earth?

a) `5 ms^{-1}`

b) `3 ms^{-1}`

c) `4.9 ms^{-1}`

d) `9.88 ms^{-1}`

Question 22: A car of `1000kg` mowing at `30 ms^{-1}` accelerates uniformly to a distance of `50 m` when brakes are applied. Find the force exerted on the car by the brakes and the work done.

a) `4.5\times10^{5} J`

b) `4.5\times10^{-5} J`

c) `4.5\times10^{4} J`

d) `2.5\times10^{5} J`

Question 23: A `500-`watt electric stove works for `10` hours a day in a house. Energy consumption of that house in `30` days Calculates in units (`kWh`).

a) `160` units

b) `200` units

c) `150` units

d) `250` units

Answer Key

Q1    `0.2w`  

Q2   (d),   Q3   (c),   Q4   (a) In the given process, the potential energy of the car does not change as it does not depend on velocity.

Q5   (a) Both spheres have the same acceleration that is acceleration due to gravity `g`. Momenta, potential energy and kinetic energy depend on mass which is different for the two spheres and hence the quantities will differ for the two spheres.

Q6   (d) The gravitational force on the bag, 

i.e. the weight of bag is in vertically downward direction whereas the distance moved by her is in horizontal direction. It means that the force and displacement are perpendicular to each other, therefore no work is done.

Q7    (d) The work done on an object does not depend upon the initial velocity of the object.

Q8    (d) Water stored in a dam possesses potential energy as it is stored at a certain height from the ground level.

Q9   (b),   Q10   (a),    Q11   (a),

Q12   Solution::

Here, mass of the car `A`, `mA = 1000 kg`

Initial speed of car `A`, `uA = 36 kmh^{-1}`

`= 36\times518= 10ms^{-1}`

Opposing frictional force, `F = 100 N`

∴ Power of the engine of car `A`

`= F µA = (100 N)\times(10 ms^{-1}) = 1000 W`

When car A collides with car `B` of mass `1000 kg`,

Final speed of car `A`, `vA = 0`

Initial speed of car `B`, `uB = 0`

Applying conservation of momentum, `P,= P`,

`mAvA + mBuB = mAvA + mBvB`

1000\times10 + 1000\times0 = 1000\times0 + 1000\timesvB

`∴vB = 10ms^{-1}`

So, the speed of the car `B` just after collision is `10ms^{-1}`

Q13   Solution: 

Effective mass of girl + trolley system = mass of girl + mass of trolley `= 35 kg + 5 kg = 40 kg`

Here, `u = 4 ms^{-1}`, `v – 0`, `s = 16 m`

Using `v^{2} – u^{2}= 2as`, we get

`a=\frac{v^{2}-u^{2}}{2s}=\frac{0-4^{2}}{2\times16}=\frac{-1}{2}ms^{-2}`

force`=F=ma=40\times(\frac{-1}{2})N=-20N`

(a) Work done on the trolley` = –` Work done by the trolley

`= -Fs`

`= – (-20 N)\times(16 m)`

`= 320 J`

(b) As the girl keeps sitting on the trolley, there is no displacement in her position with respect to trolley, so no work is done by the girl.

Q14   Answer: In the absence of any force acting on a body, it can be displaced. If the body is already moving with uniform velocity, then in the absence of any force, it will continue to move with the same velocity as before. So its replacement is possible.

If the body is at rest then its displacement is not possible in the absence of any force.

Q15   Answer:- According to the principle of conservation of energy, the total energy of the ball remains conserved. After hitting the ground, its energy gets converted into the following forms-

(i) heat energy and sound energy

(ii) kinetic energy

(iii) potential energy.

Q16   Answer: The machines of the factory start running due to the kinetic energy of flowing river water or water falling from the dam. Therefore, in the beginning, most of the factories were located near the flowing river or dam only to run the machines of the factory using the kinetic energy of the flowing water.

Q17 Answer: There is a loss of energy in the object which does work and there is an increase in energy in the object on which work is done. 

Q18   Answer:  In the form of its ability to work.

Q19   Answer: 

                 `p=\sqrt{2mK}`           (same for both)

                 `p ∝m`

So the heavier object will have more momentum than the lighter object.

Q20   (a),   Q21  (d),   Q22    (a),   Q23    ©,