work and energy 9th class(lec 1)

video lectures for work and energy 

 Question 1: Give two examples of renewable sources of energy.

Question 2: Give two examples of non-renewable sources of energy

Question 3: Distinguish between renewable and non-renewable sources of energy

Question 4: What is meant by energy crisis?

Question 5. A steam engine uses coal to produce steam. Then,

ultimately, the chemical energy of the coal is converted

Into

(a) mechanical energy.

(b) heat energy.

(c) control energy.

(d) sound energy.

Question 6: When do we say that work is done?

Question 7. In the case of negative work, the angle between the force and displacement is

(a) `0°`

(b) `45°`

(c) `90°`

(d) `180°`

Question 8. Is it possible that an object is in a state of accelerated motion due to external force acting on it, but no work is being done by the force. Explain it with an example.

Question 9: Four men lift a `250 kg` box to a height of 1 m and hold it without raising or lowering it.

(a) How much work is done by the men in lifting the box?

(b) How much work do they do in just holding it?

(c) Why do they get tired while holding it? (`g = 10 ms^{-2}`)

Question 10: Compare the power at which each of the following is moving upwards against the force of gravity? (given `g =10ms^{-2}`)

a butterfly of mass `1.0 g` that flies upward at a rate of `0.5 ms^{-1}`.

a 250 g squirrel climbing up on a tree at a rate of `0.5 ms^{-1}`.

Question 11: A pair of oxen exerts a force of `140 N` on a plow while plowing a field. The plowed field is `125m` long. How much work is done in plowing the length of the field?

a) `2100 J`

b) `2000 J`

c) `1900 J `

d) `2200 J`

Question 12: How much work will be done by the force of gravity on a satellite revolving around the earth? justify your Answer: make.

Question 13: A man keeps a bundle of straw on his head for `30 min` and gets tired. Did he do any work or not? Justify your Answer.

Question 14: Which two conditions are necessary for it to work?

Question 15: If a force F acting on an object displaces it to a distance of s then what will be the work done by it?

Question 16: If `F = 1 N`, `s = 1m`, then what is the work done by the force?

Question 17: What is a joule?

Question 18: If a force is applied in the direction of displacement, then what is the work done?

Question 19: What is the type of work done if the force is applied in the opposite direction to the displacement?

Question 20: Now if you lift an object upwards by applying a force, then which force acts?

Question 21: A man of mass `60 kg` climbs a flight of stairs `30` feet high in `40 s`. If each rung is `20 cm` high, then Find the work done.

a) `3600 J`

b) `3000 J`

c) `2600 J`

d) `1600 J`

Question 22: A child pulls his toy car through a distance of `10 m` on a level horizontal floor. The string tied to the toy cart makes an angle of `60°` with the horizontal plane. If the force exerted by the child is `5 N`, calculate the work done by him.

a) `10 J`

b) `25 J`

c) `12 J`

d) `24 J`

Answer Key

Q1 Answer: Renewable resources include solar energy, wind, falling water, the heat of the earth (geothermal), plant materials (biomass), waves, ocean currents, temperature differences in the oceans, and the energy of the tides.

Q2 Answer: Nonrenewable energy resources include coal, natural gas, oil, and nuclear energy.

Q3 Answer: Renewable resources cannot be depleted over time. Non-renewable resources deplete over time.

Q4 Answer: Any severe blockage in the delivery of energy resources to an economy is referred to as an energy crisis.

Q5  (a), 

Q6 Answer:: If a force is applied to an object and the object moves in the direction of the force, then we say that work is done.

Work = Force`\times` Displacement

Q7 Solution: (d) Work done is negative when a force acts opposite to the direction of displacement, 

i.e. the angle between the two directions is `180°`.

Q8 Solution: Yes, it is possible that an object is in a state of accelerated motion due to an external force acting on it, but no work is being done by the force.

Q9 Solution:

Mass of the box, `m = 250 kg`

Height upto which it is raised, `h = 1 m`

(a) Work done by the men in lifting the box

`= F.s = 250\times10\times1 J`

(Here `F = mg`, `s = h`)

`= 2500 J`

(b) In just holding the box, there is no displacement, so no work is done.

(c) In order to hold the box at a certain height, men are applying a force which is equal and opposite to the gravitational force acting on the box. While applying the force, muscular effort is involved.

So, men get tired while holding it.

Q10 Solution:

1. Mass of butterfly, `mb= 1 g = 10^{-3}kg`

Speed, `vb = 0.5 ms^{-1}`

Force of gravity on butterfly, `Fh = mbg`

`= 10^{-3}\times10 = 10^{-2}N`

Power, `Pb = Fb.vb = 10^{-2}N\times0.5 ms^{-1} = 0.005 W`

2. Mass of squirrel, `ms = 250 g = 0.25 kg`

Speed, `vs = 0.5 ms^{-1}`

Force of gravity on squirrel,

`Fs = msg = 0.25\times10 = 2.5 N`

Power, `Ps = Fs.vs= 2.5 N x 0.5 ms^{-1} = 1.25 W`

Hence, the squirrel exerts more power in climbing than a butterfly exerts in flying at the same rate

Q11  (a)

Q12 Answer:  Zero work will be done on the force of gravity on a satellite moving around the earth because the force is perpendicular to the direction of motion. 

Q13 Answer:- A man keeps a bundle of straw on his head for `30 min` and gets tired but there is no displacement in its direction. He didn't do any work.

Q14 Answer:- (i) A force must be applied to the object.

(ii) the object must be displaced.

Q15 Answer:

                          `W=FS`

Q16 Answer:- `1Nm`.

Q17 Answer:: One joule is the amount of work done on an object when a force of `1 N` displaces the object by `1` meter in the direction of the line of action of the force Do it

Q18 Answer: - Positive.

Q19 Answer:- Negative.

Q20 Answer: – Gravitational force.

Q21   (a),  Q22    (b)