Physics 9th Work and Energy Chapter 6 bise Rawalpindi short question, MCQs, and Problems.
Mcqs Work and Energy
Table of Contents
i) The work done will be zero when the angle between the force and the distance is
- A. 45°
- B. 60°
- C. 90°
- D. 180°
ii) If the direction of motion of the force is perpendicular to the direction of motion of the body, then the work done will be
- A. Maximum
- B. Minimum
- C. zero
- D. None of the above
iii) If the velocity of a body becomes double, then its kinetic energy will
- A. remain the same
- B. become double
- C. become four times
- D. become half
iv) The work done in lifting a brid of mass 2 kg through a height of 5 m above ground will be
- A. 2.5 J
- B. 10 J
- C. 50 J
- D. 100J
v) The kinetic energy of a body of mass 2 kg is 25 J. Its speed is
- A. 5 ms-1
- B. 12.5 ms-1
- C. 25 ms-1
- D. 50 ms-1
vi) Which one of the following converts light energy into electrical energy?
- A. electric bulb
- B. electric generator
- C. Photocell
- D. Electric cell
vii) When a body is lifted through a height h, the work done on it appears in the form of its:
- A. kinetic energy
- B. potential energy
- C. elastic potential energy
- D. geothermal energy
viii) The energy stored in coal is
- A. heat energy
- B. kinetic energy
- C. chemical energy
- D. nuclear energy
viii) The energy stored in coal is
- A. heat energy
- B. kinetic energy
- C. chemical energy
- D. nuclear energy
ix) The energy stored in a dam is
- A. electric energy
- B. potential energy
- C. kinetic energy
- D. thermal energy
x) In Einstein’s mass-energy equation, c is the
- A. speed of sound
- B. speed of light
- C. speed of the electron
- D. speed of Earth
xi) Rate of doing work is called
- A. energy
- B. torque
- C. power
- D. momentum
Questions Work and Energy Physics 9th
Define work. What is its Si unit?
WORK
When a force acts to move an object in the direction of force, we say that work is done on the object by the force.
The work is calculated by multiplying the force by the displacement covered by an object.
W = F.d
Its S.I unit is joule.
1 joule = 1 Nm
Q.3) When does a force do work? Explain.
Answer:
When a force acts on a body, the body is displaced under the action of that force. Then work is said to be done.
For example, a block is in the position “a”. When a force acts on it, it is displaced from the position “a” to “b” and covers displacement d. So we can say that work is done on the block.
Q.4) Why do we need energy?
Answer:
Energy is required to do work. Without energy, work cannot be done. Therefore if we want to do some work then we should have energy.
Q.5) Define energy, give two types of mechanical energy.
Answer:
ENERGY
Energy is the capacity of a body for doing work.
We must have the energy to perform work. It is like currency for purchasing something. Mechanical energy is the energy possessed by an object due to its motion or position.
Types of Mechanical Energy:
Kinetic energy and potential energy are types of mechanical energy.
Q.6) Define K.E. and derive its relation.
Answer:
KINETIC ENERGY
The energy possessed by a body due to its motion is called Kinetic energy.
Derivation:
Consider a body of mass m moving with velocity v. The body stops after moving through some distance S due to some opposing force such as the force of friction acting on it. The body possesses kinetic energy and is capable of doing work against opposing force F until all of its kinetic energy is used up.
K.E of the body = Work done by the body due to motion
Since motion is opposed, hence a is negative. Now using 3rd equation of motion
From Eq. (i) and (ii), we get
Q.7) Define potential energy and derive its relation.
Answer:
Potential Energy:
The energy possessed by the body due to the position is called potential energy.
Derivation:
Let a body of mass m be raised up through h from the ground. The body will acquire potential energy is equal to work done in lifting the body to height of h
So
P.E = W
Work done in lifting the body to height his;
W = F.h
⇒ P.E = F.h
If F = w =mg then
P.E = mgh
This is the potential energy of mass m at height h.
Q.8) Why are fossil fuels called non-renewable forms of energy?
Answer:
Fossil fuels are used to get energy and they are completely consumed and do not recover. That’s why they are known as non-renewable energy.
Q.9) Which form of energy is most preferred and why?
Answer:
Heat energy is preferred because it is used in engines of almost every type.
Q.10) How is energy converted from one form to another? Explain.
Answer:
Energy can be converted from one to another form.
For example, water stored at a height in dams has potential energy. Then they fall on the turbine and the turbine starts rotating, so potential energy is converted into kinetic energy. Due to the rotation of the turbine, electricity is produced. Now, kinetic energy converts into electrical energy.
Electric energy can be converted into heat, light, mechanical energy, and sound, through electric heaters, bulbs, electric motors, and buzzers, respectively. Similarly, heat energy can be converted into electricity (thermal power plants), mechanical energy (railway engines), light (gas lamps), etc.
Q.11) Name a device that converts mechanical energy into electrical energy.
Answer:
Dynamo converts mechanical energy into electrical energy.
Q.12) What is meant by the efficiency of a system?
Answer:
The ratio of the useful work done by a system to the total energy taken up by it is called efficiency.
Percentage efficiency is given by;
Q.13) What is meant by the efficiency of a system?
Answer:
The ratio of the useful work done by a system to the total energy taken up by it is called the efficiency of a system.
Q.14) How can you find the efficiency of a system?
Answer:
The efficiency of a system is calculated by the formula;
Q.15) What is meant by the term power?
Answer:
The rate of doing work with respect to time is called power.
The S.I unit of power is the watt (W).
Q.16) Define watt.
Answer:
The power of a body is one watt if it does work at the rate of 1 joule per second.
Problems
Q.1) A man has pulled a cart through 35 m applying a force of 300 N. Find the work done by the man.
Answer:
Q.2) A block weighing 20 N is lifted 6 m vertically upward. Calculate the potential energy stored in it.
Answer:
Q.3) A car weighing 12 kN has a speed of 20 ms-1. Find its kinetic energy.
Answer:
Q.4) A 500 g stone is thrown up with a velocity of 15ms-1 Find its
i) P.E. at its maximum height
ii) K.E. when it hits the ground
Answer:
The stone hits the ground with the same velocity it was thrown up. So the velocity of the stone at the impact with the ground was 15m/s.
Velocity = v = 15 ms-1
Mass of stone = m = 500 g = 0.5 kg
i) Potential energy = P.E =?
ii) Kinetic energy = K.E =?
According to the condition
Loss of K.E = Gain in P.E
The velocity with which the stone is thrown up is taken as the initial velocity which is 15 ms-1 and the velocity at the highest point is zero so final velocity vf = 0
Therefore;
Thus, the potential energy at the maximum height is 56.25 J, and hit the ground with 56.25 J kinetic energy.
Q.5) On reaching the top of a slope 6 m high from its bottom, a cyclist has a speed of 1.5 ms-1. Find the kinetic energy and the potential energy of the cyclist. The mass of the cyclist and his bicycle is 40kg.
Answer:
Q.6) A motorboat moves at a steady speed of 4 ms-1. Water-resistance acting on it is 4000 N. Calculate the power of its engine.
Answer:
Speed of the boat = v = 4 ms-1
Force = F = 4000 N
Power = P =?
As
P = F v
P = 4000 × 4
P = 16000 W
P = 16 × 1000 W
P = 16 × 103 W
P = 16 kW (103 = 1 k)
Q.7) A man pulls a block with a force of 300 N through 50 m in 60 s. Find the power used by him to pull the block.
Answer:
Q.8) A 50 kg man moved 25 steps up in 20 seconds. Find his power, if each step is 16 cm high.
Answer:
Q.9) Calculate the power of a pump that can lift 200 kg of water through a height of 6 m in 10 seconds.
Answer:
Q.10) An electric motor of 1hp is used to run the water pump. The water pump takes 10 minutes to fill an overhead tank. The tank has a capacity of 800 liters and a height of 15 m. Find the actual work done by the electric motor to fill the tank. Also, find the efficiency of the system.
Answer:
