Work and Energy Test

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Work and Energy Test - 26

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Weekly Quiz Competition

Question 1
1 / -0

An electric heater of power $$3 \ kW$$ is used for $$10 \ h$$. How much energy does it consume? Express your answer in joule.

A
$$1.08\times 10^5$$

B
$$1.08\times 10^8$$

C
$$1.08\times 10^7$$

D
$$1.08\times 10^3$$

Solution

We know that
$$Power = work/time$$
$$Work \ or\ energy = Power\times time$$
$$Work = 3 kW \times 10 h$$
$$Energy = 30 KWh$$.
$$Energy = 30\times 10^{3}\times 60\times 60\times =1.08\times 10^{8}\ J$$.
Question 2
1 / -0

A body is moved in a direction opposite to the direction of force acting on it. Work is done is:

A
against the force

B
zero

C
along the force

D
none of these

Solution

Here force and displacement is in opposite direction. Hence, negative work is done i.e, against the direction of force.
Work done is given by formula -
$$W = (-F )\times s $$
Question 3
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Write an expression for the work done when a force is acting on an object in the direction of its displacement.

A
$$Force \times Displacement$$

B
$$Force \times velocity$$

C
$$Force \times acceleration$$

D
$$Force \times Distance$$

Solution

Correct option: A

Explanation for correct option:
$$\textbf{Part1: Definition of work done}$$
Work is said to be done when the force applied to an object shows the object’s displacement.
$$\textbf{Part2: Expression of work done}$$

When a force F displaces a body through a distance S in the direction of applied force, then the work done W, on the body is given by the expression:

$$Work = Force \times Displacement$$

$$ \Rightarrow W = F \times S$$

Hence, the correct option is A.
Question 4
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Name the type of energy (kinetic energy $$K$$ or potential energy $$U$$) possessed in the following case.
A piece of stone placed on the roof.

A
$$U$$

B
$$K$$

C
$$U$$ and $$K$$

D
No energy

Solution

When a stone is placed at the roof it is at a certain height that is given by $$U = m\times g\times h$$

As we have a certain value for $$h$$ there would be some value for potential energy.

As the stone is at rest at the roof it will not have any kinetic energy as its velocity is zero, $$K = 1/2 (m\times v^{2})=0$$.

Hence, only potential energy will be there.
Question 5
1 / -0

A pair of bullocks exerts a force of 140 N on a plough. The field being ploughed is 15 m long. How much work is done in ploughing the length of the field ?

A
$$1200 \: J$$

B
$$4800 \: J$$

C
$$2100 \: J$$

D
$$2400 \: J$$

Solution

HINT: Use work done formula.
$$\textbf{Step1:Work done by Force}$$
If angle between force and displacement is $$\theta$$ then work done by force is
$$W={\vec F}.{\vec S}$$
$$W=FS {\cos \theta}$$
$$\textbf{Step2:Work done in ploughing the field}$$
Given force $$F = 140\;N$$
Displacement $$S = 15\;m$$
According to question angle between force and displacement is zero.
Work done $$W = F S cos0=FS$$
$$= 140 \times15$$
$$= 2100\;Nm = 2100\;J$$

Option C is correct.
Question 6
1 / -0

The energy consumed by $$100 \ W$$ electric bulb in $$5 \ hours$$ is:

A
$$0.5 \ kWh $$

B
$$1.5 \ kWh $$

C
$$5 \ kWh $$

D
$$2.5 \ kWh $$

Solution

Given, power of the electric bulb,
$$P =100 \:W=\dfrac{100}{1000} \:kW=0.1 \:kW$$ ;

time for which bulb is used, $$t =5 \:h$$

As $$\displaystyle P =\frac{W}{t}$$ or $$W =Pt$$

or $$W =0.1 \:kW \times 5 \:h =0.5 \:kWh $$
Question 7
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A man is waiting for a bus. What is the work done when he is carrying a box and waits for 5 minutes for the bus?

A
$$200 \ J$$

B
$$100 \ J$$

C
$$6000 \ J$$

D
$$0 \ J$$

Solution

$$ W = Fs\cos\theta$$
where $$W$$ - work done, $$F$$ - force, $$s$$ - displacement, $$\theta$$ - angle between force and displacement

As there is no displacement $$s = 0$$.

Hence work done is zero. $$W=0$$
Question 8
1 / -0

By stretching the rubber strings of a catapult we store .......... energy in it.

A
$$potential$$

B
$$electrical$$

C
$$heat$$

D
$$kinetic$$

Solution

Answer: Potential Energy
The energy exerted or work done by our muscles on the rubber band is consumed in changing its shape. It gets stored in the stretched rubber band as its potential energy. It is this stored energy that is used by the rubber band to move to its original state, shape, and size. When the rubber band is released, this stored potential energy gets converted into kinetic energy.
If a pebble is placed in contact with the stretched rubber band, this kinetic energy is transferred to the pebble. This kinetic energy of the pebble is enough to do some destructive work, like breaking a glass window, injuring someone, etc.
Question 9
1 / -0

A force of 7 N acts on an object. The displacement is 8 m, in the direction of the force. Consider force acting on the object through the displacement. What is the work done in this case ?

A
$$96 \: J$$

B
$$56 \: J$$

C
$$76 \: J$$

D
$$156 \: J$$

Solution

$$Given, \: force =7 \: N \;; \: displacement = 8 \: m $$
$$Work \: done = Force \times Displacement$$
$$= 7 \times 8 = 56 \: J$$
Question 10
1 / -0

An engine pulls a train 1 km over a level track. Calculate the work done by the train against friction if frictional force is $$5 \times 10^5 N$$.

A
$$5 \times10^8J$$

B
$$5 \times10^4J$$

C
$$8 \times10^8J$$

D
$$6 \times10^8J$$

Solution

Given, frictional force, $$F = 5 \times 10^5 N$$ ;
Displacement of the train, $$s =1 \:km =1000 \:m$$
Work done by the train against frictional force,
$$W =F \times s = (5 \times 10^5) \times(1000) =5 \times10^8J$$ .

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Work and Energy Test - 26

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Work and Energy Test - 26

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Question 1

$$1.08\times 10^5$$

$$1.08\times 10^8$$

$$1.08\times 10^7$$

$$1.08\times 10^3$$

Solution

Question 2

against the force

zero

along the force

none of these

Solution

Question 3

$$Force \times Displacement$$

$$Force \times velocity$$

$$Force \times acceleration$$

$$Force \times Distance$$

Solution

Question 4

$$U$$

$$K$$

$$U$$ and $$K$$

No energy

Solution

Question 5

$$1200 \: J$$

$$4800 \: J$$

$$2100 \: J$$

$$2400 \: J$$

Solution

Question 6

$$0.5 \ kWh $$

$$1.5 \ kWh $$

$$5 \ kWh $$

$$2.5 \ kWh $$

Solution

Question 7

$$200 \ J$$

$$100 \ J$$

$$6000 \ J$$

$$0 \ J$$

Solution

Question 8

$$potential$$

$$electrical$$

$$heat$$

$$kinetic$$

Solution

Question 9

$$96 \: J$$

$$56 \: J$$

$$76 \: J$$

$$156 \: J$$

Solution

Question 10

$$5 \times10^8J$$

$$5 \times10^4J$$

$$8 \times10^8J$$

$$6 \times10^8J$$

Solution

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