Efficiency of Machines

Q.

Calculate the time for which a motor pump of $10HP$ and efficiency $80\%$ must be switched on, so as to pump $20m^3$ of water through a vertical height of $20m$. [Density of water = $\raisebox{1ex}{$1000kg$}\!\left/ \!\raisebox{-1ex}{$m^3$}\right.$, $g=\raisebox{1ex}{$10m$}\!\left/ \!\raisebox{-1ex}{$s^2$}\right.$, $1HP=750W$]

Q.

The SI unit of power is a watt. It is expressed in the terms of mass, length, and time as

1. kg m²s^-3

2. kg ms^-1

3. kg² m²s^-2

4. kg ms^-2

Q.

Define Potential energy. Give two examples of potential energy.

Q.

A spring is kept compressed by a toy car with a mass of 100g On releasing the pressure the car moves out with a speed of 0.5ms^-1. Calculate the potential energy of the compressed spring?

Q.

A load of $220kg$ is vertically pulled up by a crane through a vertical height of $16m$ in $40s$.the Calculate horsepower of the engine pulling the rope.

[Take g=$9.8\raisebox{1ex}{$m$}\!\left/ \!\raisebox{-1ex}{$s^2$}\right.$ $1HP=750W$]

Q.

In a washing machine, the electrical energy is converted into_____

1. mechanical energy

2. sound energy

3. kinetic energy

4. chemical energy

Q.

A boy weighing $40kgf$climbs up a stair of $30$ steps each $20cm$ high $4min$ and a girl weighing $30kgf$ does the same in $3min$. Compare

(i) the work done by them.

(ii) the power developed by them.

Q.

A catapult throws a stone of mass $0.10kg$ with a velocity of $\raisebox{1ex}{$30m$}\!\left/ \!\raisebox{-1ex}{$s$}\right.$. If $25\%$ of the PE of the elastic band is wasted during transmission, find the magnitude of PE.

Q.

A girl of mass $50kg$ climbs a flight of $100$ stairs each measuring $0.25m$ in height in $20s$.[Take g=$10\raisebox{1ex}{$m$}\!\left/ \!\raisebox{-1ex}{$s^2$}\right.$]

Find the power in watts.

Q.

An object of mass 2 kg is thrown up at a velocity of 10 ms^-1. Find the kinetic energy of the object at the time of the throw. Also, find the potential energy of the object at the highest point. Take the value of g as 10 ms^-2.

Q.

What is the power output of the sun if $4\times {10}^{9}kg$ of matter per second is converted into energy in the sun?

Use $E=m{c}^2$, where $c=3\times {10}^{8}m/s$

Q.

A water tank on the top of a building 25 m high, contains 100 kg of water. Calculate the potential energy stored in the water. [Take acceleration due to gravity 10 ms^-2]

Q.

Explain why water stored in a dam has potential energy.

Q.

If an engine supply $100J$ of energy to a weight of $200g$. How high it can be lifted?

Q.

(a) What do you understand by the term efficiency of the machine?

(b) A machine has an efficiency of 70 %. Explain the statement as clearly as possible.

Q.

A body is acted upon by a force. State two conditions under which the work done could be zero.

Q.

The correct relationship between the velocity ratio (V.R), the distance through which effort act (d), and the distance through which load moves (D)

1. $V.R\times D=d$

2. $V.R=D\times d$

3. $V.R\times d=D$

4. None of these

Q.

Calculate the kinetic energy of a body of mass $0.1kg$ and momentum $40kgm{s}^{-1}$.

Q. Question 3
A hammer of mass 500 g, moving at $50m{s}^{-1}$, strikes a nail. The nail stops the hammer in a very short time of 0.01 s. What is the force of the nail on the hammer?

Q.

An aeroplane flying at a height of 10, 000 m, possesses _________ energy as well as ______ energy.

Q.

One joule work is said to be done when a force of one newton acts through a distance of

1. 1 cm

2. 1 mm

3. 1 m

4. 1 km

Q.

Why are ideal machines not achievable in practice?

Q.

In a machine, the output is always ……. than the input.

Q.

A stone is tied to a string and whirled in a circular path.The work done by the stone is

1. negative

2. zero

3. positive

4. none of the above

Q.

Calculate the starting acceleration of a Tesla Model S Plaid car if it can reach 60 mph from rest in 1.98 s?
[1 mph = 1.61 km/h]

1. $13.5m/s^2$
2. $30.30m/s^2$
3. $48.78m/s^2$

4. $118.8m/s^2$

Q.

One horsepower is equal to

1. 764 W

2. 746 W

3. 700 W

4. 1000 W

Q.

Water falls from a height of$800m$ from a water fall. If the initial temperature of water at top of water fall is $10°C$, what is its temperature at the bottom? Assume all the energy of falling water changes to heat energy.
[Take $g=10m{s}^{-2}$and sp. heat capacity of water$4200Jk{g}^{-1}{K}^{-1}$ ]

Q.

How much energy is gained by a box of mass $20kg$ when a man.

(i) Carrying the box waits for $5\min$ for a bus?

Q.

Write down the units and dimensional formulae of following physical quantities:
$Power=\frac{work}{time}$

Q.

The efficiency of a machine is the ratio of the work done on the ___ by the machine to the work done on the machine by the effort.