Energy Sources and Transfer of Energy - Physics For Class IX (Science Group) - Self Assessment Questions and Test book Exercise

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Physics For Class IX (Science Group)
UNIT 8 ENERGY SOURCES AND TRANSFER OF ENERGY
Self Assessment Questions and Test book Exercise

Self Assessment Questions

Q.1: Write down the names of any three units of work.
Ans: UNITS OF WORK:

1. Joule
2. erg
3. the horsepower-hour
4. the foot-pound
5. the kilowatt hour

Q.2: According to the definition of work in physics, Urwa did not perform any work if she made an assignment on her laptop in three hours. Why?
Ans: In Physics, work is defined as the measure of the displacement of an object or a point. In this case, Urwa made an assignment on her laptop in three hours but she did not cover any distance in the direction of force, she was sitting in one place. Therefore, according to the definition of work in physics, Urwa did not perform any work.

Q.3: At what angle between force and displacement the work done by a body will be maximum?
Ans: The work done by a body will be maximum at an angle of 0°.

Q.4: A car of mass 50 kg moving with velocity 10 ms^-1 in the direction of force. Calculate its Kinetic energy.
Ans: For Solution CLICK HERE

Q.5: A body of mass 10 kg is dropped from a height of 20 m on the ground. What will be its potential energy, if g = 9.8 m/sec²?
Ans: For Solution CLICK HERE

Q.6: Give the energy changes when a ball is dropped from a height of 7 m to the ground.
Ans: Potential Energy of an Object:
The potential energy of an object at some height with respect to gravity is:

P.E = mgh

where,

• P.E is the initial potential energy in joules (J)
• m is the mass of the object in kg-mass
• g is the acceleration due to gravity (9.8 m/s²)
• h is the height above the ground in m

When the object reaches the ground,

h = 0

and thus the final potential energy is:

P.E_f = 0

Note: In reality, there is still a gravitational force on the object at the surface of the Earth, so the object has gravitational potential energy at that point. But since the object cannot go anywhere, we say its P.E from gravity is zero.

Kinetic Energy of Falling Object:
Kinetic energy (K.E) is the energy of motion. Since the object is not moving at the initial position, the initial K.E is:

K.E_i = 0

Once the object is released, it accelerates downward. When the object reaches the ground, its kinetic energy is:

K.E_f = ¹/₂ mv_f²

where,

• K.E_f is the kinetic energy at the ground in joules (J)
• v_f is the downward velocity of the object at the ground in m/s

Total Energy for Falling Object:
The total energy of the object is:

T.E = P.E + K.E

The total energy is a constant value, provided no external forces besides gravity act on the object. Thus, the initial total energy equals the final total energy:

T.E_i = T.E_f

P. E_i + K.E_i = P.E_f + K.E_f

When the object is simply dropped,

mgh + 0 = 0 + ¹/₂ mv_f²

mgh = ¹/₂ mv_f²

Final Velocity for Failing Object:
From that equivalence, we can determine the final velocity of the dropped object. Divide by m and multiply by 2:

v_f² = 2gh

v_f = √ 2gh

Summary:
Potential energy with respect to gravity is P.E = mgh. When the object is dropped, thrown downward or projected upward, its kinetic energy becomes K.E = ¹/₂ mv², along with a factor of the initial velocity. The sum of the P.E and K.E is the total energy (T.E), which is a constant. Equating the initial total energy with the final total energy, we can determine the final velocity of the object.

Q.7: What is biomass?
Ans: Biomass:
Biomass is the organic material that comes from plants and animals. Biomass consists of stored energy from Sun, garbage, wastes, sugarcane etc. Solid biomass, such as wood, organic material and garbage, can be burned directly to produce heat. Biomass can also converted into gas called biogas and into liquid biofuels such as ethanol and biodiesel.

Q.8: Write down the name of fossil fuel?
Ans: Fossils Fuels are:

1. Coal
2. Natural gas
3. Oil
4. Charcoal

Q.9: Which type of energy is stored deep in the Earth?
Ans: Geothermal energy is stored in the Earth as its natural heat. Deep in the Earth, there is hot molten part called magma. Water close to magma changes to steam due to high temperature. This thermal energy is conducted to the surface of Earth. This energy is called geothermal energy.

Q.10: Write down the names of any three renewable energy sources?
Ans: Renewable energy sources:

1. Solar energy
2. Wind energy
3. Tidal energy and
4. Geothermal energy are renewable sources.

Q.11: Write down the names of any three nonrenewable energy sources.
Ans: Non-Renewable energy sources:

1. Coal
2. Petroleum and
3. Natural gases are nonrenewable sources.

Q.12: What is the difference between renewable and non-renewable energy sources?
Ans: Difference Between Renewable And Non-renewable Energy Sources
Following are major differences between renewable and non-renewable resources:

S.NO.	Renewable Energy Sources	Non-renewable Energy Sources
Depletion	Renewable resources cannot be depleted over time.	Non-renewable resources deplete over time.
Sources	Renewable resources include sunlight, water, wind and also geothermal sources such as hot springs and fumaroles.	Non-renewable energy includes fossil fuels such as coal and petroleum.
Environmental Impact	Most renewable resources have low carbon emissions and a low carbon footprint.	Non-renewable energy has a comparatively higher carbon footprint and carbon emissions.
Cost	The upfront cost of renewable energy is high. For instance, generating electricity using technologies running on renewable energy is costlier than generating it with fossil fuels.	Non-renewable energy has a comparatively lower upfront cost.
Infrastructure Requirements	Infrastructure for harvesting renewable energy is prohibitively expensive and not easily accessible in most countries.	Cost-effective and accessible infrastructure is available for non-renewable energy across most countries.
Area Requirements	Requires a large land/ offshore area, especially for wind farms and solar farms.	Comparatively lower area requirements.

Q.13: A man pushes a car 18 m with a force of 2 N in 4 second. Calculate the power of the man.
Ans: For Solution CLICK HERE

Q.14: Why power is a scalar quantity?
Ans: Power is define as the energy or work per unit time. Since time is not consider as a vector quantity, and neither energy nor work because the work is not directional. So work and time are scalar quantities. Therefore, power is also a scalar quantity. It has a unit magnitude but no direction.

Q.15: Name the physical quantity which gives the rate of doing work.
Ans: The quantity which gives the rate of doing work is called power.

Text Book Exercise

Section (B)

Structured Questions

Work

1. a) Define work?
Ans: WORK:
In physics work has a proper meaning i.e.
“work is done only when a force makes something to move."
Definition:
Thus work can be define as:

"The amount of work is the product of force and the distance moved in the direction of force."

Nature:
It is a scalar quantity and it is denoted by 'W'.

Units of Work:
The S.I unit of work is Joule other units of work can be Foot, Pound, Erg.

1 Joule = 1Nm

b) Derive the equation; work = Fd cosθ.
Ans: Derivation Of Equation For Work:
Suppose a constant force “F” acts on a body and motion takes place in a straight line in the direction of force then work done is equal to the product of magnitude of force “F” and the distance “d” through which the body moves.

W = Fd cosθ ................ (i)

The force “F” however may not act in the direction of motion of the body instead it makes some angle “θ” with it.
In that case, we define the work by the force as the product of the component of the force along the line of motion and the distance “d”; the body moves along that line, i.e. Suppose a constant force “F” acts on a body.
Work = (component of force)· (distance)

W = (F cosθ) d

or W = (F cosθ) d

If θ = 0 ⇒ cosθ = 1

then Work = W= Fd ....... (ii)

2. How much work is needed to move horizontally a body 20 m by a force of 30 N, the angle between the body and the horizontal surface is 60°?
Ans: Click here For Solution.

3. How much work is done, if a crate is moved at a distance of 50 m, when a force of 30 N is applied along the surface.
Ans: Click here For Solution.

4. What is the work done by Usman? If a bar of weight 100 N is brought by him from A to B, then brought back to A.
Ans: Click here For Solution.

Energy Forms

5. a) Define Kinetic energy.
Ans: KINETIC ENERGY:
Kinetic energy of a body defined as:

"Energy possessed by an object due to its motion is called kinetic energy."

OR

It is also defined as:

“The work required to accelerate a body of a given mass from rest to its stated velocity”.

A moving body maintains its kinetic energy unless its speed changes.

Unit:
The S.I unit of kinetic energy is joule.

Expression For Kinetic Energy:
Mathematically kinetic energy is given as:

K.E = ½ mv² ........ (i)

b) Derive the equation.
Ans: Derivation of the Equation, K.E = ½ mv²:
To obtain an expression for K.E we have to determine the work done by the body in motion. This work is equal to the kinetic energy of the body.
Consider a body of mass 'm' placed on a horizontal surface initially at rest. When a force 'F' is applied it covers a distance 'S' and its final velocity becomes 'v'. Then work done is:

W = F . S ...... (i)

But by the second law of motion when a force acts on a body it produces acceleration in the direction of the force.

F = ma

And by using the third equation of motion i.e.:

v_f² - v_i² = 2aS

When

v_i = 0,

v_f = V and

S = ?

therefore,

v² - 0 = 2aS

6. What will be the Kinetic energy of a boy of mass 50 kg driving a bike with velocity of 2 ms^-1.
Ans: Click here For Solution.

7. a) Define Potential Energy.
Ans: POTENTIAL ENERGY:
Definition:
Potential energy of a body is defined as:

"The energy that a body possesses by virtue of its position, shape or state of a system."

OR

It is also defined as:

"The work done stored in a body in lifting it to a height “h”."

The potential energy changes only when its position relative to ground changes; otherwise it remains same.

Unit:
S.I. unit of potential energy is Joule (J).

Expression For Potential Energy:
Mathematically potential energy is given as:

P∙E = mgh ........ (i)

Where,
'm' is the mass of an object
'g' is acceleration due to gravity
'h' is height

b) Derive the equation. PE = mgh
Ans: GRAVITATIONAL POTENTIAL ENERGY:
The potential energy possessed by a body in the gravitational field is called the gravitational potential energy.

Derivation of Gravitational Potential Energy P.E=mgh:
To derive the expression for gravitational potential energy, let us consider an object of mass “m” which is raised up through height “h” from the ground. The work done in lifting it to height “h” is stored in it as its gravitational potential energy “P∙E”, i.e.

P∙E = Work done

P∙E = W

We know that W = F.d, therefore

P∙E = F ∙ d

We also know that F = mg (weight), therefore

P.E = (mg) . d

Here d = h (height), therefore

P∙E = (mg) ∙ h

Hence, equation becomes:

P∙E = mgh ........... (i)

8. a) If LED screen of mass 10 kg is lifted up and kept it on a cupboard of height 2 m. Calculate the potential energy stored in the LED screen.
Ans: Click here For Solution.

b) Calculate the potential energy of 3 kg water raised to the tank at the roof of a home 4 m high. (assume g=10 ms^-2)
Ans: Click here For Solution.