Efficiency of Machines

Q.

Define work. When is work said to be done by a force?

Q.

Define the term efficiency of a machine. Give two reasons for a machine not to be $100\%$ efficient?

Q.

State three factors, which determine the potential energy of a body.

Q.

A scooter develops a power of $1HP$ while running at $36\raisebox{1ex}{$km$}\!\left/ \!\raisebox{-1ex}{$h$}\right.$. Calculate the force generated by its engine.

Q.

State one reason why is mechanical advantage less than the velocity ratio for an actual machine.

Q.

A rocket of $3\times {10}^{6}kg$ mass takes off from a launching pad and acquires a vertical velocity of $1km{s}^{-1}$at an altitude of $25km$.Calculate

(i) $PE$

(ii) $KE$

Take $\left(g=10m/s^2\right)$

Q.

An engine of power $200W$, operates for $4s$. Find the work done by the engine. If the force developed by the engine is $100N$. Calculate the maximum displacement caused.

Q.

State the principle of machine.

Q.

Give two reasons, why a machine cannot be 100% efficient.

Q.

What is is an ideal machine and what is its efficiency?

Q.

A machine having 100% efficiency is known as an __________ machine.

Q.

Define velocity ratio of a machine. Why it has no unit?

Q.

Two men A & B perform the same amount of work. B performs the work in 1/50^th of the time as compared to A. Calculate the ratio of their powers.

1. 1:20

2. 1:30

3. 1:40

4. 1:60

5. 1:50

Q.

Why is a machine not 100% efficient?

1. Different parts of the machine are not smooth and rigid

2. String in it (if any) is not perfectly elastic

3. Moving parts are not weightless

4. All of the above

Q.

State the energy changes taking place in the case of an air gun is loaded and then fired

Q.

State the energy changes taking place in the case of a truck climbing a hill.

Q. A machine lifts up a load of $500kg$ by a height of $2m$. Calculate the efficiency of the machine if the energy input to the machine is $25000J$. (Take $g=10m{s}^{-2}$).

1. 0.4

Q. In a block and tackle system consisting of 3 pulleys, a load of 75 kgf is raised with an effort of 25 kgf, find the mechanical advantage.

Q.

Energy used by a horse to pull a cart is an example of.

1. Potential energy.

2. Sound energy.

3. Muscular energy

Q.

Check if in this case, work is done and why?

A lady cooking food.

Q.

A work of 10 J is done in moving a body through 1 m, in the direction of the applied force. Calculate the magnitude of the applied force.

Q.

State the energy changes taking place in the case of a toy car being wound and then allowed to move on the floor.

Q.

Energy possessed by a body on account of motion is called

1. mechanical energy

2. potential energy

3. kinetic energy

4. magnetic energy

Q.

The energy supplied by a machine in doing some mechanical work is called input energy.

1. True
2. False

Q.

What should be the angle between force and displacement be to get the

1 minimum work

2 maximum work

Q.

How are the units of power and work related?

Q. How much energy is gained by a box of mass $20kg$ when a man carrying the box waits for $5$ minutes for a bus? $(g=10m{s}^{-2})$

Q.

The input power to a machine is 1500 W and its efficiency is 80%. Then, the output power by the machine is :

1. 1500 W

2. 800 W

3. 1100 W

4. 1200 W

Q.

Define efficiency

Q.

The angle between force and displacement for i) Minimum Work and ii) Maximum Work should be

1. θ = 90^o ; θ = 0^o

2. θ = 180º ; θ = 90^o

3. θ = 0º ; θ = 180^o

4. θ = 0º ; θ = 90^o