Class 9 - foundation physics (Work and Energy)Contact Number: 9667591930 / 8527521718

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How much work is done by a force of 250 N in moving an object through a distance of 100 m in the direction of force?

A body of mass 2 kg is raised to a height of 1 m. Find the work done by the force of gravity. (Take g = 9.8 m/${s}^{2}$).

From the graph of force versus displacement of a particle, find the total work done by the force.

A force of 10 N displaces a body by 5 m, the angle between force and displacementis 60$\xb0$, then find the work done.

If a stone of mass 3 kg be thrown with a kinetic energy of 37.5 joule, find its velocity.

A bullet is fired from a gun. What will be the ratio of kinetic energy of bullet and gun?

A mass of 10 kg is released from a height of 40 m above the surface of the Earth. Calculate its mechanical energy when it is at 20 m above the surface of the Earth. (Take g = 10 $m/{s}^{2}$)

Starting at rest, the cart of figure slides frictionlessly to point P, which is 4.5 meters below the top of the hill. How fast is it going at P?

A bullet leaving the muzzle of a rifle barrel with a velocity v penetrates a plank and loses one fifth of its velocity. It then strikes seond plank, which it just penetrates through. Find the ratio of the thickness of the planks supposing average resistance to the penetration is same in both the cases.

A particle of mass 0.5 kg travels in a straight line with velocity v = a${x}^{3/2}$ where a = 5 ${m}^{-1/2}{s}^{-1}$. What is the work done by the net force during its displacement from x = 0 to x = 2m?

A small sphere tied to the string of length 0.8 m is describing a vertical circle so that the maximum and minimum tensions in the string are in the ratio 3 : 1. The fixed end of the string is at a height of 5.8 m above ground.

(a) Find the velocity of the sphere at the lowest position.

(b) If the string suddenly breaks at the lowest position, when and where will the sphere hit the ground?

(Take g = 10 $m/{s}^{2}$)

A mass is released from a height H and after hitting the ground it rebounds to a height h, find the coefficient of restitution between the groud and mass.

A neutron having mass of 1.67$\times $${10}^{-27}$ kg moving at ${10}^{8}m{s}^{-1}$ collides with a deutron at rest and sticks to it. Given that the mass of neutron is $3.34\times {10}^{-27}$ kg, calculate the speed of the combination. The composite particle is called triton.

What is the power of an engine which can lift 20 metric ton of coal per hour from a 20 metre deep mine?

One coolie takes one minute to raise a box through a height 2 metre. Another one takes 30 second for the same job and does the same amount of work. Which one of the two has greater power and which one uses greater energy?

A 100 W bulb operates for 5 hour. How much electric energy will it consume?

A child pulls a toy car though a distance of 10 meters on a horizontal floor. The string held in child's hand makes an angle of 60$\xb0$ with the horizontal surface. If the force applied by the child be 10 N, calculate the work done by the child in pulling the toy car.

A man lifts 20 boxes each of mass 15 kg to a height of 1.5 m. Find the work done by the man against gravity (Take g = 9.8 m${s}^{2}$).

A scooter is moving with a velocity of 15 m/sec. Calculate its kinetic energy if its mass along with the rider is 150 kg.

A body of mass 10 kg is kept at a height 10 m from the ground, when it is released, after sometime its kinetic energy becomes 450 joule. What wil be the potential energy of the bodyat that instant?

A force acts on a body of mass 3 kg causing its speed to increase from 4 m/s to 5 m/s. How much work has the force done?

A block of mass 2.0 kg is given an initiatal velocity of 2 m/s on a horizontal table where the coefficient of friction is 0.2. Find the distance covered by the block before it stops. Take g = 10 m/${s}^{2}$.

A student pulls a bucket of water in 1 minute from a 30 m deep well. If the mass of bucket with is 25 kg, then calculate the power of this student.

A pendulum cosisting of a small heavy bob suspended from a rigid support oscillates in a vertical plane. when the bob passes through the position of equilibriu, the rod is subjected a tension equal to twice the weight of the bob. Through what maximum angle from the vertical will the pendulum be deflected? Disregard the weight of the rod and the resistance of the air.

Two balls each of mass M moving in opposite directions with equal speeds v undergo a head-on collision. Calculate the velocity of the two balls after collision.

A 8 kg ball moving with velocity 4 m/s collides with a 2 kg ball moving with a velocity 8 m/s in opposite direction. If the collision be perfectly elastic, what are the velocities of balls after the collision.

A force of 7 N on an object. The displacement is say 8 m in the force. Let us take it that the force acts on the object through the displacement. What is the work done in this case?

When do we say that work is done?

Write an expression for the work done when a force is acting on an object in the direction of its displacement.

Define 1 joule of work?

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?

What is the kinetic energy of an object?

Write an expression for the kinetic energy of an object.

The kinetic energy of an object of mass m moving with a velocity of 5 m${s}^{-1}$ is 25 J. What will be its kinetic energy when its velocity is doubled? What will be its kinetic energy when its velocity is increased three times?

What is power?

Define 1 watt of power.

A lamp consumes 1000 J of electrical energy in 10 s. What is its power?

Define average power.

Look at the activities listed below. Reason out whether or not work is done in the light of your understanding of the term 'work'.

(i) Suma is swimming in a pond.

(ii) A donkey is carrying a load on its back.

(iii) A wind-mill is lifting water from a well.

(iv) A green plant is carrying out photosynthesis.

(v) An engine is pulling a train.

(vi) Food grains are getting dried in the Sun.

(vii) A sailboat is moving due to wind energy.

An object thrown at a certain angle to the ground moves in a curved path and falls back to the ground. The initial and the final points of the path of the object lie on the same horizontal line. What is the work done by the force of gravity on the object?

A battery lights a bulb. Describe the energy changes involved in the process.

Certain force acting on a 20 kg mass changes its velocity from 5 m${s}^{-1}$ to 2 m${s}^{-1}$. Calculate the work done by the force.

A mass of 10 kg is at a point a on a table. It is moved to a point B. If the line joining A and B is horizontal, what is the work done on the object by the gravitational force? Explain you answer.

The potential energy of a freely falling object decreases progressively. Does this violate the law of conservaiton of energy? Why?

What are the various energy transformations that occurs when you are riding a bicycle?

Does the transfer of energy take place when you push a huge rock with all your might and fail to move it? Where is the energy you spend going?

A certain houshold has consumed 250 units of energy during a month. How much energy is this in joules?

An object of mass 40 kg is raised to a height of 5 m above the ground. What is its potential energy? If the object is allowed to fall, find its kinetic energy when it is half-way down.

What is the work done by the force of gravity on a satellite moving round the Earth? Justify your answer.

Can there be displacement of an object in the absence of any force acting on it? Think. Discuss this question with your friends and teacher.

A person holds a bundle of hay over his head for 30 minutes and gets tired. Has he done some work or not? Justify your answer.

An electric heater is rated 1500 W. How much energy does it use in 10 hours?

Illustrate the law of conservation of energy by discussing the energy changes which occur when we draw a pendulum bob to one side and allow it to oscillate. Why does the bob eventually come to rest? What happens to its energy eventually? Is it a violaiton of the law of conservation of energy?

An object of mass, m is moving with a constant velocity, v. How much work should be done on the object in order to bring the object to rest?

Calculate the work required to be done to stop a car of 1500 kg moving at a velocity of 60 km/h?

In each of the following a force F is acting on an object of mass, m. The direction of displacement is from west to east shown by the longer arrow. Observe the diagrams carefully and state whether the work done by the force F is negative, positive or zero.

Soni says that the acceleration in an object could be zero even when several forces are acting on it. Do you agree with her? Why?

Find the energy in kWh consumed in 10 hours by four devices of power 500 W each.

A freely falling object eventually stops on reacting the ground. What happens to its kinetic energy?

A rocket is moving up with a velocity v. If the velocity of this rocket is suddenly tripled, what will be the ratio of two kinetic energies?

A boy is moving on a straight road against a frictional force of 5 N. After travelling a distance of 1.5 km he forgot the correct path at a round about of radius 100 m. However, he moves on the circular path for one and half cycle and then he moves forward upto 2.0 km. Calculate the work done by him.

The weight of a person on a planet A is about half that on the Earth. He can jump upto 0.4 m height on the surface of the Earth. How high he can jump on the planet A?

The velocity of a body moving in a straight line is increased by applying a constant force F, for some distance in the direction of the motion. Prove that the increase in the kinetic energy of the body is equal to the work done by the force on the body.

A ball is dropped from a height of 10 m. If the energy of the ball reduces by 40% after striking the ground, how much high can the ball bounce back? (g = 10 m${s}^{-2}$).

It is generally much more difficult to stop a heavy truck than a light car when they move at the same speed. State a case in which the moving car could require more stopping force. (consider relative times)

Why is a punch more forceful with a bare fist than with a boxing glove?

A boxer can punch a heavy bag for more than an hour without triring, but will tire quickly when boxing with an opponent for a few minutes. Why?

How is it possible that a flock of birds in flight can have a momentum of zero but not have zero kinetic energy?

If the K.E of a body is increased by 300%, by what percentage will its momentum increase?

A particle of mass ${m}_{1}$ is moving with a velocity ${v}_{1}$ and another particle of mass ${m}_{2}$ is moving with a velocity ${v}_{2}$. Both of them have the same momentum but their different kinetic energies are ${E}_{1}$ and ${E}_{2}$ respectively. If ${m}_{1}$> ${m}_{2}$ then how are ${E}_{1}$and ${E}_{2}$ related?

A body of mass m accelerates uniformly from rest to ${v}_{1}$in time ${t}_{1}$. As a function of time t, find the instantaneous power delivered to the body.

A pendulum, swinging back and forth, rises at the end of its swing to a position 15 cm higher than its lowest point. How fast is it going at the lowest point?

Does the work done in raising a box onto a platform depend on how fast is raised?

If 20000 joules of work is done in pumping water up to a height of 12 meters, how much water is pumped?

A body is thrown vertically upwards. Its velocity keeps on decreasing. What happens to its kinetic energy as its velocity becomes zero?

A man runs a distance 's' on a level road. The same man ascends up a hill with the same velocity through the same distance. When does he do more work?

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