1 watt is the power of the agent which does 1J of work in one second. In other words, if the rate of energy used is 1 Js–1 then the Power will be 1 w.

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

Question:

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

Answer:

We can obtain the average power by dividing the total energy used by the total time taken.

So, Average power = Total energy used/ Total time taken

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Note: The term average power is used when the power of agent (such as an instrument or device) changes over time. That is, if works at different rates in different time intervals.

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(All Q's of this Chapter solved)

Define average power.

Define 1 watt of power.

What is power?

The kinetic energy of an object of mass, m moving with a velocity of 5 ms–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?

Write an expression for the kinetic energy of an object.

What is the kinetic energy of an object?

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?

Define 1 J of work.

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

When do we say that work is done?

The volume of a 500 g sealed packed is 350 cm3 will the packet float or sink in water if the density of water is 1 g cm–3 ? What will be the mass of the water displaced by this packet ?

The volume of 50 g of a substance is 20 cm3. If the density of the water is 1 g cm–3 will the substance float or sink ?

Why does a block of plastic released under water come up to the surface of water ?

In what direction does the buoyant force on an object immersed in a liquid act ?

A ball thrown up vertically returns to the thrower after 6s find. (a) the velocity with which it was thrown up. (b) the maximum height it reaches and (c) it's position after 4s.

A stone is allowed to fall from the top of a tower 100m high and at the same time another stone is projected vertically when and where the two stones will meet.

Calculate the force of gravitation between the earth and the sun, given that the mass of earth = 6 × 1024 kg. and of the sun = 2 × 1030 kg. The average distance between the two is 1.5 × 1011 m.

A stone is thrown vertically upward with an initial velocity of 40 m/s. Taking g = 10 m/s2 find the maximum height reached by the stone. What is the net displacement and the total distance covered by the stone ?

A stone is released from the top of a tower of height 19.6 m calculate it's final velocity just before touching the ground.

A ball is thrown vertically upwards with a velocity of 49 m/s Calculate. (i) The maximum height to which it rises. (ii) The total time it takes to return to the surface of the earth.

Gravitational force on the surface of the moon is only 1/6 as strong gravitational force on the earth. What is the weight in newtons of a 10 kg object on the moon and on the earth ?

Why will a sheet of paper fall slower than one that is crumpled into a ball ?

Amit buys few grams of gold at the poles as per the instruction of one of his friends. He hands over the same when he meets his friend at the equator will the friend agree with the weight of gold bought ? If not, why ? [Hint : The value of g is greater at the poles than at the equator]

What do we call the gravitational force between the earth and an object.

What is the acceleration of free fall ?

What is the importance of universal law of gravitation?

What happens to the force between two objects, if (i) The mass of one object is doubled. (ii) The distance between the objects is doubled and tripled. (iii) The masses of both objects are doubled.

If the moon attracts the earth, why does the earth not move towards the moon ?

The earth and the moon are attracted to each other by gravitational force. Does the earth attract the moon with a force that is greater or smaller or the same as the force with which the moon attracts the earth ? Why ?

What is the magnitude of the gravitational force between the earth and a 1 kg object on its surface? (mass of the earth is 6 × 1024 kg and radius of the earth is 6.4 × 106m).

Gravitational force acts on all objects in proportion to their masses. Why then, a heavy object does not fall faster than a light object ?

How does the force of gravitation between two objects change when the distance between then is reduced to half ?

You have a bag of cotton and an iron bar, each indicating a mass of 100 kg when measured on a weighing machine. In reality, one is heavier than the other. Can you say which one is heavier and why?

You find your mass to be 42 kg on a weighing machine. Is your mass more or less than 42 kg?

Why does an object float or sink when placed on the surface of water?

What do you mean by buoyancy?

Why is it difficult to hold a school bag having a strap made of a thin and strong string?

If the masses of two objects are almost equal then what will be the acceleration due to the force of gravitation ?

What is the reason for the occurrence of tides in the oceans ?

What do you mean by acceleration due to gravity?

What do you mean by free fall?

Write the formula to find the magnitude of the gravitational force between the earth and an object on the surface of the earth.

State the universal law of gravitation.

Chapter 9 NCERT Class 9 Science SOLUTIONS