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The earth attracts each and every object towards it with a force called as gravitational force of the earth.

As the normal force and gravitational pull of the earth are equal, it does not fall down.

Gravitational force.

a) Due to increase of gravitational pull of the earth than the normal force,

b) Gravitational pull of the earth.

Viscosity, Buoyancy.

Cutting instruments have very small area of cross – section, which lead to the increase in pressure. So they can easily penetrate the surface (pressure is inversely proportional to area). That’s why cutting instruments are usually very much sharpened.

1. An upward force is exerted by water on a floating or a partly submerged body. This upward force is called buoyant force. The phenomenon is known as “buoyancy”. 

2. This force is not only exerted by liquids, but also by gases. Liquids and gases together are called fluids. 

3. This upward force decides whether an object will sink or float. If the weight of the object is less than the upward force, then the object will float. If not, it will sink.

At a higher altitude, due to the lack of atmospheric pressure the boiling point of a substance reduces. So, the water boils even at 80° C. At this temperature, the thermal energy that is produced is not sufficient enough for baking or cooking. So, cooking is difficult at higher altitude.

When we stand, our weight acts on relatively small surface area, resulting in increased tension on the muscles of our legs. Hence, we cannot stand at one place for a long time. When we sleep, our weight acts on relatively large surface area, resulting in comparatively reduced tension. Therefore, we can sleep on a place for 8 and odd hours.

(i) Compressed spring possess potential energy. 

(ii) Potential energy of the spring is imparted to the ball in the form of kinetic energy.

1. Equilibrium : “When two or more forces act on a rigid body such that the state of body (rest or of uniform motion) does not change, the body is said to be in equilibrium. 

2. Static

Mass (m) = 10 kg 

Velocity (v) = 2 m/s 

Momentum (p) = ? 

We know that, Momentum (p) = Mass (m) × Velocity (v) 

Therefore, p = 10 kg × 2 m/s = 20 kg m/s 

Thus, the momentum of the stone = 20 kg m/s.

Data : 1 = 10 cm, b = 5 cm,

h = 2 cm, p= 8 × 10 kg/m³, g = 9.8 m/s² , 

A = lb = 10cm × 5cm = 50cm² = 50 × 104m² 

= 5 × 10^-5 m² , force =?, pressure = ? 

Volume of the block = lbh = 10 cm × 5 cm × 2 cm

= 100 cm³ = 100 × 10^-6 m³ = 1 × 10^-4 m³ 

Mass of the block = volume × density 

(∵ density = mass/volume) 

∴Mass of the block, 

m = 1 × 10^-4 m³ × 8 × 10³ kg/m³ = 0.8 kg 

Weight of the block = mg = 0.8 kg × 9.8 m/s² = 7.84 N 

∴ The force exerted by the block on the table = 7.84 N. 

force 7.84 N

Pressure = \(\cfrac{force}{area}\) = \(\cfrac{7.84N}{5\times10^{-3}\,m^2}\)

= 1.568 × 10³ N/m² or 1.568 × 10³ Pa 

The pressure exerted by the block on the table 

= 1.568 × 10 N/m² or 1.568 × 10³ Pa.

Correct option is B) pull

Correct option is C) both

The push or pull of an object is called force. Its S.I. unit is Newton.

1. a push — To1 open a door, we push it. 

2. as pull — To move a grass roller on a lawn, it is pulled by a gardener. 

3. a stretch — Stretching a rubber string. 

4. a squeeze —Change in shape of a sponge on squeezing.

Name the term used for the push or pull Force.

Force is a push or pull acting upon an object as a result of its interaction with another object.

1. F₁ = Static friction 

2. F₂ = Sliding friction 

3. F₃ = Rolling friction 

F₁ > F₂ > F₃ 

F₂ is less than F₁ but greater than F₃.

The examples to illustrate that friction is a necessary evil are: 

1. If friction were absent, we would not be able to walk. 

2. Friction is necessary to burn a matchstick. 

3. It is due to friction that we can write on a board by a chalk.

If friction has disappeared from the earth then

1. we cannot walk or run. 

2. we will not have any cars or bicycles because all of them move due to friction. 3. no building could be constructed. 

4. carpenters will not able to smoothen surfaces. 

5. we could not fix nail on the wall. 

6. it will not possible to write with pen or pencil. 

7. we will not able to hold any appliances such as hammer, soap, etc.

The unit of pressure is N m^-2

The correct answer is:

kgf

Two factors which directly affect the force of friction are : 

1. The smoothness of the surface. 

2. The presence of solid, liquid or gas around the moving body.

When the truck is loaded there will be more friction between the truck and the road.

Wooden surface offers move friction on a body.

(d) both (a) and (c)

(b) decreases

The answer is (A) 2 mv

Given, Velocity of the car (v) = 0.5 m/s

Mass of the car (m) = 1000 kg 

Momentum (p) = ? 

We know that, Momentum (p) = Mass (m) × Velocity (v) 

Therefore, p = 1000 kg × 0.5 m/s = 500 kg m/s 

Thus, momentum of the car = 500 kg m/s.

Rough towelled cloth on the handles of motorbike provides good grip increasing the friction and we have better control in holding the handles.

Vehicles slip/skid on muddy and wet roads as the roads become smooth and offqr less friction to the tyres. Tyres are therefore grooved or treaded to make their surface rough so that the friction between the tyres and the road increases and vehicles may stop easily when desired.

False The S.I. unit of force is newton.

Rod B will go deeper as it has a smaller area of contact, therefore the same force (weight of the rod) produces more pressure. In case of rod A the same force produces less pressure.

The woman wearing sandals with flat soles will feel more comfortable while walking on the sandy beach. The flat soles have larger area compared to the sandals with pointed heels. Since the two women are of the same weight, they will apply same force on the ground. Therefore, the pressure exerted by the pointed heels will be more compared to that with sandals having flat soles. As a result the pointed heel sandals will sink more in the sand than the flat sole sandals. Hence, walking with flat sole sandals will be more comfortable.

B, D, A, C. Because pressure of a liquid column depends upon the height of the liquid column and not on volume of the liquid.

When we prick the surface of an inflated balloon with a needle it exerts a larger pressure because it has a smaller area of contact compared to the finger. The large pressure pierces the surface of the balloon easily.

C) A is true, but R is false

Contact force: The force results when there is a direct physical contact between two interacting objects is called contacting force. Example: Muscular force, Frictional force, Normal force, Tension. Field force or 

Force at a distance : The force which occurs without any physical contact between two objects is known as force at a distance or field force.

Example : Magnetic force, Electrostatic force, Gravitational force.

2. gravitational force 

A good source of energy should have the following characteristics: 

(i) It should do a large amount of work per unit mass or volume. 

(ii) It should have high calorific value. 

(iii) It should be easily accessible over a long period of time. 

(iv) It should be economical and easy to store and transport. 

(v) It should be Safe and convenient to use.

When the body remains in the state of rest under the influence of applied forces, it is in static equilibrium and if it remains in the state of uniform motion, it is in dynamic equilibrium. For example: In a beam balance when the beam is balanced in the horizontal position, the clockwise and anti-clockwise moments balance each other and the beam has no rotational motion i.e., it is in static equilibrium. 

A body moving with uniform velocity in a straight line over a friction less surface is in dynamic equilibrium.