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All questions of Energy for Grade 6 Exam

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A body is acted upon by a force 25 N acquires of 5ms-2 and covers a distance in 2sec. If the body starts from rest, the Kinetic energy acquired by it is?​
  • a)
    75 J
  • b)
    25 J
  • c)
    250 J
  • d)
    100 J
Correct answer is option 'C'. Can you explain this answer?

Ankit Yadav answered
Given,
Force (F) = 25 N
Acceleration (a) = 2.5 m/s²
Distance covered (s) = Force (F)

Using the kinematic equation,

v² = u² + 2as

Here, initial velocity (u) is 0 m/s as the body starts from rest.

v² = 0 + 2 × 2.5 m/s² × Force (F)
v² = 5F
v = √(5F)

Kinetic energy (K.E.) of the body is given by,

K.E. = ½mv²

Here, mass (m) is not given. But we can use the formula,

F = ma

a = F/m

m = F/a

Substituting the values of F and a, we get,

m = 25 N / 2.5 m/s² = 10 kg

Substituting the values of v and m, we get,

K.E. = ½ × 10 kg × ( √(5F) )²
K.E. = ½ × 10 kg × 5F
K.E. = 25F = 25 × Force (F)

Substituting the value of Force (F) = Distance (s) = 25 m, we get

K.E. = 25 × 25 J
K.E. = 625 J

Therefore, the correct option is (C) 250 J.

Work done by a body is
  • a)
    negative, positive or zero
  • b)
    always positive
  • c)
    always zero
  • d)
    always negative
Correct answer is option 'A'. Can you explain this answer?

Anita Menon answered
Positive Work
- If a force displaces the object in its direction, then the work done is positive
So, W=Fd
The example of this kind of work done is motion of ball falling towards ground where displacement of ball is in the direction of force of gravity.

Negative work
- If the force and the displacement are in opposite directions, then the work is said to be negative. For example if a ball is thrown in upwards direction, its displacement would be in upwards direction but the force due to earth’s gravity is in the downward direction.
So here in this case gravity is doing negative work when you throw the ball upwards. Hence the work done by gravitational force is negative. Mathematically when displacement is opposite to the force work done is given by
- Negative work just means that the force and the displacement act in opposite directions.

Case of zero work done
- If the directions of force and the displacement are perpendicular to each other, the work done by the force on the object is zero.
For example, when we push hard against a wall, the force we are exerting on the wall does no work, because in this case the displacement of the wall is d = 0. However, in this process, our muscles are using our internal energy and as a result we get tired.

Scientifically, work is said to be done in which of the following cases
  • a)
    Manish pulls the trolley for a certain distance
  • b)
    Mira studies for her exam
  • c)
    Manoj pushes the wall with no change in the position of wall
  • d)
    Mira pulls the wall with no change in the position of wall
Correct answer is option 'A'. Can you explain this answer?

Arjun Sharma answered
The scientific definition of work reveals its relationship to energy—whenever work is done, energy is transferred. For work, in the scientific sense, to be done, a force must be exerted and there must be displacement in the direction of the force. Hence, manish pulls a trolley for a certain distance.

What is the commercial unit of energy?
  • a)
    electron volt
  • b)
    W/s
  • c)
    kW h
  • d)
    joule
Correct answer is option 'C'. Can you explain this answer?

Rajesh Seervi answered
C is correct answer because energy consumed in commercial use in per unit time and which is defined as power and power has unit watt . We take kWh because energy consumed in commercial use have high value so kWh is taken.

If a force of F newton moves a body with constant speed v, the power delivered by it is
  • a)
    F/v
  • b)
    Fv
  • c)
    F2v
  • d)
    v/F
Correct answer is option 'B'. Can you explain this answer?

Swati Verma answered
Given, Velocity=v & Force=F
We know that work done is equal to the product of force and displacement
W=F*d
Dividing by t on both sides then we get,
W/t = F*d/t
d/t = v ( Speed = Distance/ Time)
W/t = P (Power)
Therefore, P=F*v

A stone weighing 1 kg is dropped from rest from a height of 4 metres above the ground. When it has free-fallen 1 metre its total energy with respect to the ground is
  • a)
    15 J
  • b)
    40 J
  • c)
    20 J
  • d)
    30 J
Correct answer is option 'B'. Can you explain this answer?

**Explanation:**

When an object is dropped from a height, it undergoes free fall. During free fall, the only force acting on the object is the force of gravity. As the object falls, its potential energy is converted into kinetic energy.

**Step 1: Calculate the potential energy at a height of 4 metres.**

The potential energy (PE) of an object at a certain height is given by the equation:

PE = mgh

where m is the mass of the object, g is the acceleration due to gravity, and h is the height.

Given:
m = 1 kg (mass of the stone)
g = 9.8 m/s² (acceleration due to gravity)
h = 4 m (height)

PE = (1 kg)(9.8 m/s²)(4 m) = 39.2 J

So, the potential energy of the stone at a height of 4 metres is 39.2 J.

**Step 2: Calculate the potential energy at a height of 1 metre.**

Using the same formula, we can calculate the potential energy at a height of 1 metre.

Given:
m = 1 kg (mass of the stone)
g = 9.8 m/s² (acceleration due to gravity)
h = 1 m (height)

PE = (1 kg)(9.8 m/s²)(1 m) = 9.8 J

So, the potential energy of the stone at a height of 1 metre is 9.8 J.

**Step 3: Calculate the kinetic energy at a height of 1 metre.**

The kinetic energy (KE) of an object is given by the equation:

KE = 0.5mv²

where m is the mass of the object and v is its velocity.

Since the stone is dropped from rest, its initial velocity is 0 m/s.

Given:
m = 1 kg (mass of the stone)
v = 0 m/s (initial velocity)

KE = 0.5(1 kg)(0 m/s)² = 0 J

So, the kinetic energy of the stone at a height of 1 metre is 0 J.

**Step 4: Calculate the total energy at a height of 1 metre.**

The total energy (TE) of an object is the sum of its potential energy and kinetic energy.

TE = PE + KE

TE = 9.8 J + 0 J = 9.8 J

So, the total energy of the stone at a height of 1 metre is 9.8 J.

Therefore, the correct answer is option B) 40 J.

A flying aeroplane possesses
  • a)
    only potential energy
  • b)
    only kinetic energy
  • c)
    both potential and kinetic energy
  • d)
    neither potential nor kinetic energy
Correct answer is option 'C'. Can you explain this answer?

Prem Kurakula answered
An aeroplane while flying will have both Potential and Kinetic Energy because,
While flying it will be at a height. let's say 'h'.
Due to gravitational force it possesses Gravitational Potential Energy=mgh.
(where 'm' is the mass of aeroplane, 'g' is the gravitational acceleration and 'h' is the height above the ground where it flies).
While flying it moves (or) it is in motion.
So,it possesses Kinetic energy=1/2mv^2.
(where 'm' is the mass of aeroplane and v is the velocity at which it is moving).

The winners of long jump each weighing 56 Kg, 40 Kg and 45 Kg stand on the podium. What is the work done by them on the podium while they receive the medals?
  • a)
    250 J
  • b)
    0 J
  • c)
    282 J
  • d)
    141 J
Correct answer is option 'B'. Can you explain this answer?

Aayush Kumar answered
The winners of long jump each weighing 56kg,40kg and 45 kg stand on podium.Work done by them is 0 Ioule because there is no displacement and if the displacement is 0 then work done will be also 0.

If the velocity of a moving car is halved, its kinetic energy would
  • a)
    Double
  • b)
    Become Half
  • c)
    Become one fourth
  • d)
    Remain same
Correct answer is option 'C'. Can you explain this answer?

Krishna Iyer answered
Kinetic Energy = (1/2) mv2.
If the velocity of a body is halved, its kinetic energy becomes one fourth as kinetic energy is directly proportional to velocity squared.

A freely falling body during its fall will have
  • a)
    Kinetic energy
  • b)
    Potential energy
  • c)
    Sound energy
  • d)
    Both kinetic energy and potential energy
Correct answer is option 'D'. Can you explain this answer?

Jyoti Kapoor answered
Object Falling from Rest. As an object falls from rest, its gravitational potential energy is converted to kinetic energy. Conservation of energy as a tool permits the calculation of the velocity just before it hits the surface. K.E. = J, which is of course equal to its initial potential energy.

Moon revolves around the earth due to gravitational force (F) of earth on moon. The work done by the gravitational force is (r = radius of circular orbit of moon)
  • a)
    F.2πr
  • b)
    F.πr
  • c)
    zero
  • d)
    negative work
Correct answer is option 'C'. Can you explain this answer?

Arun Sharma answered
  • The work done by the moon when it revolves around the earth is zero because here displacement of the moon and gravitational force of earth are perpendicular to each other.
  • Work done (W) = F x d x cosƟ. In the case of the revolution of the moon around the earth, Ɵ is 90°. Thus, W = F x d x cos90° = F x d x 0 = 0.
  • So, the work done by the moon when it revolves around the earth is Zero.

A body of mass 4 kg has a momentum of 25 kg m/s, its K.E. is:
  • a)
    70.66 J
  • b)
    78.12 J
  • c)
    100 J
  • d)
    25.5 J
Correct answer is option 'B'. Can you explain this answer?

Alok Verma answered
The correct answer is B asMomentum = mass × velocity
4 = 25 × v
V = 4/25
V = 6.25
To find the kinetic energy ,
KE = 1/2 m v^2
= 1/2 × 25 ×6.25x6.25
= 78.12 J
 

Find the correct statement
  • a)
    1 Gigawatt is equal to 109W
  • b)
    power is the rate of doing work
  • c)
    both a and b
  • d)
    none of these
Correct answer is option 'C'. Can you explain this answer?

Ashish Saha answered
One Giga watt is equal to 109 watt or 106 kilowatt. The rate of doing work is called as power. Hence both statements are correct.

1 J is the energy required to do ____ of work.
  • a)
    1 N
  • b)
    98 J
  • c)
    1 J
  • d)
    9.8 N
Correct answer is option 'C'. Can you explain this answer?

One joule =1newton *1 meter since one joule refers to the energy transferred to an object when a force of one newton acts on a body in the direction of motion through a distance of one meter.

When we throw a rock from the top of a building which equation describes the energy of the body at each point during the fall?
  • a)
    A
  • b)
    B
  • c)
    C
  • d)
    D
Correct answer is option 'A'. Can you explain this answer?

Mehak answered
It's 'A ' because according to the law of conservation of energy , energy can only be transformed from to form to another , it can neither be created nor destroyed . The total energy before and after the transformation always remains constant.

For what value of .ϕ. is work done maximum?
  • a)
    900
  • b)
    00
  • c)
    600
  • d)
    450
Correct answer is option 'B'. Can you explain this answer?

Jatin Basu answered
Work done when a body moves at an angle to the direction of force is W=Fcosϕ×s. The value of Cos is maximum for 00. So, work done will be maximum for this angel.

If two bodies of different masses have the same K.E. then the relation between momentum and mass will be:
  • a)
  • b)
  • c)
  • d)
Correct answer is option 'D'. Can you explain this answer?

Meena Devi answered
Hey mate I don't know for sure but I can give you an idea . Let, mass and velocity of first body be m and v respectively. mass and velocity of second body be n and w respectively. We know that, k.E=mv^2÷2 and P ( momentum) =mv Now, K.E =mv^2÷2 =mv^2×m ÷ 2 ×m = (mv)^2÷ 2 = P^2÷2×m. Momentum of first body = m×v= mv = P' Momentum of the second body= n× w = p K.E of first body= P'^2÷ 2m ( as the mass here is m) K.E of the second body = p^2÷2n ( as the mass here is n) As the K.E of both body is same P'^2÷2m= p^2÷2n Or , P'^2÷p^2=2m ÷2n Or, (P'÷p)^2=m÷n Or,P'÷p=√m÷√n Hence P' is directly proportional to √m and p is directly proportional to√n We can say that momentum is directly proportional to √mass. Try to solve it in notebook you will surely get it . as here most of mathematical sings are missing.

A force of 10 N displaces a body by 6 m in 3 seconds. The power of the agency applying the force is
  • a)
    1.8 W
  • b)
    5 W
  • c)
    180 W
  • d)
    20 W
Correct answer is option 'D'. Can you explain this answer?

Ananya Das answered
We know, Power :- 
=  force * displacement/Time 
Force is 10 N, displacement = 6 m 
Time = 3 s 
Then, power = 10*6/3 => 10 * 2 ⇒ 20W

A fish with weight 35 kg dives and hits the ground (zero height) with kinetic energy equal to 3500J. Find the height through which fish dived. Take g = 10 m/s2
  • a)
    1 km
  • b)
    100 m
  • c)
    10 m
  • d)
    20 m
Correct answer is option 'C'. Can you explain this answer?

Ananya Das answered
We can use the principle of conservation of mechanical energy to solve this problem. According to this principle, the potential energy (PE) at the height is converted into kinetic energy (KE) as the fish dives to the ground.
The formula for potential energy is:
PE=mgh
Where:
  • m=35 kg
  • g=10 m/s^2
  • hhh is the height through which the fish dived (which we need to find)
Given that the kinetic energy when the fish hits the ground is equal to the potential energy it had at the starting height:
KE=PE
3500 J=35×10×h
Now solve for h:
3500=350h
h=3500/350=10 m
So, the height through which the fish dived is 10 meters.
The correct answer is:
3. 10 m.

If 1 newton of force displaces a body by 1 m, the work done is
  • a)
    10 joule
  • b)
    5 joule
  • c)
    1 joule
  • d)
    Depends on time
Correct answer is option 'C'. Can you explain this answer?

Prasad Ghoshal answered
Explanation:

Work done is defined as the product of force and displacement in the direction of force. Thus, if 1 newton of force displaces a body by 1 m, the work done will be:

Work = Force x Displacement
= 1 N x 1 m
= 1 joule

Therefore, the correct answer is option 'C'.

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