Power 9th class (lec 5)

 video lecture for Power

Question 1: A body of mass `5 kg` is dropped from a height of `20 m`.

(i) What are the potential and kinetic energy of a body, when it falls through a distance of `15 m`?

(ii) Find the kinetic energy of the body at the ground level. Take `g = 10 ms^{-1}`.

Question 2. To do the same work in less time, the power should be _____.

Question 3: Two electric motors of power `1.75 hp` and `3.5 hp` pump water simultaneously. The ratio of amount of water pumped by them, in a given time, is (a) `1 : 2` (b) `2 : 1` (c) `1 : 4` (d) `4 : 1`

Question 4: An engine develops `10 kW` power. How much time will it take to lift a mass of `200 kg` to a height of `40 m`? (take `g = 10 ms^{-2}`) (a) `10 s` (b) `20 s` (c) `7 s` (d) `8 s`

Question 5: Avinash can run with a speed of `8 ms^{-1}` against the frictional force of `10 N`, and Kapil can move with a speed of `3 ms^{-1}` against the frictional force of `25 N`. Who is more powerful and why?

Question 6: The power of a motor pump is `2 kW`. How much water per. minute the pump can raise to a height of `10 m`? (Given `g = 10ms^{-2}`) a) `100` b) `150` c) `200` d) `250`

Question 7: If an electric iron of `1200 W` is used for `30` minutes everyday, find electric energy consumed in the month of April

a) `6.48\times10^{7} J`

b) `4.48\times10^{7}J`

c) `8.48\times10^{7} J`

d) `6.48\times10^{-7} J`

Question 8: What is power? How do you differentiate ’ kilowatt from kilowatt hour? The Jog Falls in Karnataka state are nearly `20 m` high. `2000` tonnes of water falls from it in a minute. Calculate the equivalent power if all this energy can be p utilized? (`g = 10 ms^{-2}`)

Question 9: How is the power related to the speed at which a body can be lifted? How many kilograms will a man working at the power of `100 W`, be able to lift at constant speed of `1 m s^{-1}` vertically?

(`g=10ms^{-2}`) ,

Question 10: Define watt. Express kilowatt in terms of joule per second. A `150 kg` car engine develops `500 W` for each kg. What force does it exert in moving the car at a speed of `20 ms^{-1}`?

Question 11: A lamp consumes `1000 J` of electrical energy in `10s`. What is its power? a) `100 W ` b) `250 W` c) `150 W` d) `50 W`

Question 12: define average power.

Question 13: in a month, `250` unit of energy were consumed in a house? How much will this energy be in joules? a) `9\times10^{8} J` b) `9\times10^{6} J` c) `3.6\times10^{8} J` d) `3.6\times10^{6} J`

Question 14: The declared power of an electric heater is `1500 W`. how much energy will it use in `10` hours? a) `20` units b) `10` units c) `15` units d) `12` units

Question 15: Four devices each of power `500 W`. can be used for `10` hours. Calculate the energy spend by them in `KWh`.

a) `50 KWh`

b) `45 KWh`

c) `20 KWh`

d) `25 KWh`

Question 16: A burner uses `110 kJ` of energy in `55` seconds. What will be the power of the burner in kilowatts? a) `2 kW` b) `1 KW` c) `0.5 KW` d) `2.5 KW`

Question 17: An object does `50` joules of work in `5` seconds. What is its power? a) `100 W` b) `50 W` c) `25 W` d) `10 W` Question 18: The weight of a man is `50 kg`. It climbs a hill vertically `10 m` in the `20s`. What his power.

(`g=9.8 ms^{-2}`) a) `250 w` b) `300 w` c) `245 w` d) `900 w`

Question 19: The consumption of electricity in homes is measured in kilowatts. If a `2 kW` immersion rod is used for `60` days for heating water in the house for one hour a day, how much will it cost at the rate of Rs `1.50` per `kW`? power of immersion rod `2kw`.

a) `360.00 ₹`

b) `240.00 ₹`

c) `120.00 ₹`

d) `310.00 ₹`

Answer Key

Q1 hence Answer in first case is `200` hence Answer in second case is `12.5`

Q2 increased/greater

Q3 (a), Q4 (d).

Q5 Solution:

Since, Power = Force`\times`Velocity ∴Power of Avinash, `PA = 10 N\times8 ms^{-1}= 80 W` Power of Kapil, `PK= 25 N\times3 ms^{-1}= 75 W` As `PA >PK` so, Avinash is more powerful than Kapil.

Q6 ©, Q7 (a),

Q8 Solution: Power is defined as the rate of doing work or the rate of transfer of energy, Kilowatt is the unit of power and kilowatt hour is the unit of energy or work as kilowatt hour = unit of power x unit of time. Energy possessed by `2000` tonnes `(= 2000\times10^{3} kg)` water at a height of `20 m`, `Ef = mgh` `= 2000\times10^{3}\times10\times20` `= 4\times10^{8} J` Power generated if all this energy can be utilized, `P=\frac{E_{p}}{t}` `=\frac{4\times10^{8}}{60s}` `=\frac{2}{3}\times10^{8} W`

Q9 Solution: Power associated in lifting a body of mass `m` upto a height h in time t is, `p=\frac{mgh}{t}` Here, `\frac{h}{t}` = speed = `v` `∴ P=mgv` Here `P = 100W, v=1ms^{-1},10 ms^{-2}` `m=pug` So, `m=100W10 ms^{-2}1 ms^{-1}=10 kg`

Q10 Solution: One watt is the power of an agent which does work at the rate of one joule per second. `1 KW = 1000 watt = 1000` joule per second ( `1 W = 1 Js^{-1}`) Mass of the car, `m = 150 kg` Power developed by engine per `kg = 500 W` Total power developed by engine, `P = 500 W\times150` `= 75000 W` Speed of car,` v = 20 ms^{-1}` Power, `P = Fv` `=>` Force exerted in moving the car, `f = Pv` `=75000W\times20ms^{-1}` `=3750 N`

Q11 (a),

Q12 Answer: the ratio of total energy used and total time is called average power.

average power=total energy used/total time

Q13 (a), Q14 ©, Q15 ©, Q16 (a), Q17 (d), Q18 ©, Q19 (a).