Mass, Weight, Pressure, Thrust

Question 1 Define the term mass? Give its unit?

Question 2 A man weight 600 N on the earth. What is his mass. If he were taken on the moon, his weight would be 100 N.what is his mass on the moon? What is the acceleration due to gravity on the moon?

Question 3 How much a 70 kg man weigh on the moon. What would be his mass on the  earth and on the moon?

Question 4 Define weight of a body. What is its S.I. unit?

Question 5 What is the relation between mass and weight?

Question 6 On what factors does weight of a body depends?

Question 7 As the altitude of a body increases, do the weight and mass both vary?

Question 8 The weight of a body is 50 N. What is its mass?

Question 9 Define thrust. Give its S.I. unit?

Question 10 Define pressure?

Question 11 What is the relation between pressure, force and area?

Question 12 State the units in which pressure is measured?

Question 13 Explain why school are provided with wide straps to carry them?

Question 14 Why does a sharp knife cut objects more effectively than a blunt knife?

Question 15 Why the tip of a needle is sharp?

Question 16 why a wide steel belt is provided over the wheels of an army tank?

Question 17 Why when a person stands on a cushion, the depression is much more than when he lies down on it?

Question 18 Explain why is it easier to walk on soft sand if you have flat  shoes rather than shoes with sharp heels?

Question 19 Why building  and dams have wide foundation?

Mass

It is the quantity of matter contained in it.
It is a scalar quantity.
Its unit is kg.
The mass remains same whether the object is on the earth, moon or outer space.
Mass of an object is constant and does not change from place to place.
Mass of the body is a measure of inertia of the body. Greater the mass, greater will be the inertia.

Weight

The earth attracts every body towards its center with a force which depends on

(1) Mass of the body
(2) Acceleration due to gravity

The weight of a body is the force with which it is attracted towards the centre of the earth.

The force of attraction of earth on a body is called as weight.

W = m x g

S.I. unit of weight is Newton or kg m/s²

 Weight of 1 kg mass is 9.8 Newton.

At a given place, weight of a body is directly proportional to its mass.(g is constant)

Weight is a vector quantity.

Since the value of g changes from place to place, weight of a body also changes from place to place.

The mass can never be zero but weight can be zero.

The value of g decreases as we down inside the earth and becomes zero at the centre of the earth. Whatever be the weight of a body on surface of earth, its weight become zero when it is taken to the center of earth.

The weight of an object on moon is the force with which the moon attracts the earth.

The gravitational force of moon is about one-sixth that of earth, therefore weight of an object on moon will be one-sixth of what it is on earth.

For Example: A spring balance on earth has weight of 6 N.

A spring balance on moon has weight of 1 N.

Weight of an object on moon is one-sixth of its weight on earth

Weight depends on location whereas mass remain same everywhere.

Thrust and Pressure

Pressure is the force acting on a unit area of object.

The force of thumb produces small pressure when it acts on a large area of wood but same force of thumb produces greater pressure when it acts on a very small area of wood.

The pressure depends on 2 factors.
(1) Force applied
(2) Area over which force acts

The same force can produce different pressure depending on the area over which it acts.
When a force acts over a large area of an object, it produces a small pressure.
If the same force acts over a small area of the object, it produces large pressure. The force acting on a body is perpendicular to its surface is called Thrust.

                  Thrust
Pressure =  ——–
                    Area

School bags have wide Straps: So that weight of bag fall over a large area of the shoulder thus producing less pressure on shoulder. Due to less pressure, it is more comfortable to carry.

Sharp Knife cuts better than blunt knife: Sharp knife has very thin edge due to which force of our hand falls over a small area of object producing a large pressure.

Tip of needle is sharp: Due to its sharp tip, needle may put the force on a small area of cloth, producing a large pressure sufficient to pierce the cloth being stitched.

A nail has pointed tip: When it is hammered, the force of hammer falls on a very small area of wood creating a large pressure which pushes nail into wood.

The pressure on ground is more when a man is walking rather than when he is standing: When a man is walking, then at one time only his foot is on ground. Due to this, the force of weight of man falls on a smaller area of the ground and produces more pressure on the ground. When the man is standing, then both his feet are on the ground. Due to this the force of weight of man falls on a larger area of the ground and produces lesser pressure on the ground.

Tractors have broad tyres so that there is less pressure on the ground and the tyres do not sink into comparatively soft ground in the field.

A wide seat belt is provided over the wheels of army tanks so that they exert less pressure on the ground and do not sink.

It is easier to walk on soft sand if we have flat shoes rather than shoes with sharp heels: This is because a flat shoe has a greater area in contact with the soft sand due to which there is less pressure on the soft ground. Due to this the flat shoe do not sink much in soft sand and it is easy to walk on it. Sharp heel has a small area in contact with the soft sand and so exerts a greater pressure on the soft sand. Due to this greater pressure, the sharp heels tend to sink deep into soft sand making it difficult for the wearer to walk on soft sand.