Energy conversation 9th class (lec 4)

 video lecture for Energy conversation 

Question 1: A cricket ball, during its flight after being hit, possesses ______ energy and ______ energy.

Question 2: When a body is dropped from a height, its _____ 

energy changes to its _____ energy.

Question 3: A body with a mass of `100 g` is allowed to fall

freely from a height of `1000 m` under 

gravity. Its kinetic energy after `10` seconds is

 (take `g = 1000 m sec^{2}`)

(a) `5 J`

(b) `50 J`

(c) `500 J`

(d) `5000 J`

Question 4: A marble is dropped into a frictionless U-tube as shown

in the figure. If the tube is semicircular with a mean radius

`5 cm` and the mass of the ball is `2 g`, find its velocity

at the bottom of the tube. (Take `g = 10 ms^{-2}`).

(a) `1 ms^{-1}`  

(b) `1 cms^{-1}`

(c) zero  

(d) `0.1 ms^{-1}`

Question 5: When a body falls freely towards the earth, then its total energy

(a) increases

(b) decreases

(c) remains constant

(d) first increases and then decreases

Question 6: A body is falling from a height of h. After it has fallen a height `\frac{h}{2}`, it will possess

(a) only potential energy

(b) only kinetic energy

(c) half potential and half kinetic energy

(d) more kinetic and less potential energy

Question 7: Can any object have mechanical energy even if its momentum is zero? Explain.

Question 8: Can any object have momentum even if its mechanical energy is zero? Explain.

Question 9: A ball is dropped from a height of `10 m`. If the energy of the ball reduces by `40%` after striking the ground, how much high can the ball bounce back? (`g = 10 ms^{-2}`)

a) `1m`

b) `2m`

c) `3m`

d) `6m`

Question 10: What energy conversions take place when you ride a bicycle?

Question 11:  A body of mass `40 kg` is raised to a height of `5m` from the ground. What is the potential energy? If  the body is allowed to fall freely, the body is exactly at the halfway point. Calculate its kinetic energy at the time.(`g=10ms^{-2}`)

a) `K_{p}=2000J,  E_{k}=1000J`

b) `K_{p}=1000J,  E_{k}=2000J`

c) `K_{p}=200J,  E_{k}=100J`

d) `K_{p}=100J,  E_{k}=200J`

Question 12: What is the principle of conservation of energy?

Question 13:Two bodies of equal mass are moving with velocities `v` and `2v`. Find the ratio of energy.

a) `3 : 4`

b) `1 : 4`

c) `2 : 3`

d) `3 : 1`

Question 14: Under what circumstances does a human's ability to do physical work decrease?

Answer Key

Q1   Potential,    Q2   potential, kinetic,    Q3    (c),     Q4   (a),

Q5   (c) When a body falls freely towards the earth, its potential energy decreases which intum increases the kinetic energy. Therefore, its total energy remains constant.

Q6   (c) When a body falls from a height `\frac{h}{2}` its potential energy becomes half (as potential energy `= mgh` ). The rest half of initial potential energy gets converted into kinetic energy.

Hence, the body will possess half potential energy and half kinetic energy.

Q7    Solution:

 Yes, if momentum of an object is zero, i.e.; `p = mv =0`

which gives `v = 0` as `m ≠ 0`

Kinetic energy of the object` =\frac{1}{2}mv^{2}= 0`

Now, Mechanical energy = potential energy + kinetic energy (`= 0`) – potential energy (∴ `K. E = 0`)

So any object can have mechanical energy even if its momentum is zero.

Q8    Solution:

 No, if mechanical energy = kinetic energy + potential energy `= 0`,

So, potential energy = kinetic energy `= 0`

or `\frac{1}{2}mV^{2}= 0` or `v = 0`

So, momentum, `p = mv = 0`

Any object can not have momentum even if

its mechanical energy is zero.

Q9    (d),

Q10    On cycling, the muscular energy of the body is converted into kinetic energy. Kinetic energy is expended in doing work against friction she goes.

Q11     (a)

Q12    Answer: – Law of Conservation of Energy - Energy changes from one form to another but the total amount of energy remains unchanged. Energy can neither be created nor destroyed, it can only be changed from one form to another Could

According to the principle of conservation of energy, the work done on an object is equal to the work done by that object, but in reality this is not possible. This is because while doing work on the machine some part of the energy is converted into heat due to the friction of the different parts of the machine. Because of which the work done by the machine is less, but if we can use that thermal energy also, then the law of conservation of energy is verified.

Q13    (b)

Q14     In the conditions of disease and old age, the capacity of humans to do physical work decreases because the energy of the muscles of their body decreases.