Force and pressure / class 8 / Chaper-11 / New St. Mery English School

1:- What is Force?

Ans:-  The push or pull of an object is called force. It is a physical quantity that can change the 

          speed, state, direction and dimension of an object.

          A force has both magnitude and direction, so it is a vector quantity.

          The SI unit of force is Newton.

          The SI base units‎: ‎kg·m/s²

          The force is represented by 'F'

          Ex:- playing a guitar, scrubbing the floor, rowing the boat, kicking a ball, knocking a nail,

                  launching a rocket, pushing a trolley, opening the door, pulling a box, tug-of-war, 

                  raising a flag, drawing back a bow

2:- What is push?

Ans:-A force that is used to move an object away from the body is called push.

         Ex:-rowing the boat, kicking a ball, knocking a nail, launching a rocket, pushing a trolley   

3:- What is pull?

Ans:- A force that is used to move an object towards the body is called pull.

         Ex:-opening the door,  pulling a box, tug-of-war,  raising a flag, drawing back a bow 

4:- What is acceleration?

Ans:- The rate of change in velocity in per unit of time is called acceleration. It has both fixed 

           direction and magnetude. so it is a vector quantity.

           acceleration is denoted by the symbol 'a'

           The SI unit of acceleration is  m/s²

5:- what is the formula of Force?

Ans:- Since mass is the direct proportional of force and acceleration is also direct proportional of force. therefor

F ∝ m

F ∝ a

F ∝ m × a 

F = k m × a

k is a proportionaly constant whose value in SI unit is 1

 Therefor , F = ma

The SI unit of mass (m) = kg

The SI unit of acceleration (a) = ‎m/s²

^{so the base unit of Force =}  kg. m/s²

6:- What force would be needed to produce an acceleration of  4m/s²  in a ball of mass 6 kg?

Solve:- m = 6kg

            a = 4m/s²

           F = m × a

             = 6 × 4 kg m/s² 

             = 24 kg m/s² 

             = 24 N

 7:- What is mass?

Ans:- Mass is the matter contained in a body. It has magnitude but does not have direction 

          so it is a scaler quantity. 

          Mass is shown by the symbol 'm'.

         The SI symbol of mass is kg. 

 8:- What is Weight?

Ans:- Weight is the force of gravity by which earth attracts any object towards its centre.

          SI unit of weight is Newton and it is denoted by 'N' 

         Weight = mass * acceleration due to gravity near the Earth's surface

                      = mass * g

                         where g = 9.8 m/s²   

example:    If a person has a mass of 100kg, what is his weight?

 

9:- What is a Balanced Force?

Ans:- A force where two forces of equal size, act on a body, in opposing directions is known as a Balanced Force. In Balanced Force, a body continues to be in its position i.e. it may be continuously moving with the same speed and in the same direction or it may be still in its position.

                       

Ex:- in a tug of war, if there is no movement in the rope, the two teams are exerting equal, but opposite forces that are balanced. Again, the resulting force (net force) is zero.                                

 An apple hangs from a tree, the weight of the apple is balanced by the force exerted by the branch on the apple.

 

A stationary object is an example of balanced force. 

 

 

10:- What is  Unbalanced Force?

Ans:- A force where two forces acting in opposing directions on a body, are not equal in magnitude and in size, is known as an unbalanced force. In an unbalanced force, a body changes its position. Unbalanced force also helps an object to change its speed and direction.

           

👉 When unbalanced forces are acted in a same direction Then the resultant force is the sum of the forces applied. and the direction of resultant force is the same of the applied force.

 

👉 When unbalanced forces are acted from different direction Then the resultant force is the difference of the forces applied. and the direction of resultant force is in the direction of greater force.

       

11:- What is  Net Force?

The net force is the force which is the sum of all the forces acting on an object simultaneously. Net force can accelerate a mass. 

Practice Problems

1. A 50 kg on at table mass has an extra 20 N acting down on it and a 20 N force acting to the right. What is the net force on the mass?

2. A mass has a 20 N force acting to the right on it and a 12 N force acting to the left on it. What is the net force on the mass. 

12:- Write five effects of force. 

 Ans:- The five effects of force are

1- A force can cause motion in a stationary object.

2- Force can stop a moving object.

3- Force can change the speed of a moving object.

4-Force can change the direction of motion of a moving object.

5-Force can change the size and shape of an object.

13:- How many types of force? name them. 

Ans:- There are two types of force

1- contact force

2- Non-contact force

14:- What is contact force?

Ans:- The force which act only when there is physical contact, direct or indirect, between the objects, are called contact forces. 

Ex :- muscular force, mechanical force, frictional force etc.

15:- How many types of contact force? name them.

Ans:- There are three types of contact force.

1- muscular force

2-mechanical force

3-frictional force

16:- What is muscular force? give some example.

Ans:- The force resulting due to the action of muscles is known as muscular force.

Ex:- lifting a bucket of water, lifting of weights by a weight lifter, pulling of carts by animals.

17:- What is mechanical force? give some example.

Ans:- The force exerted by a machine is called mechanical force. 

Ex:- Force exerts by car engine, force exerts by wind to move a sailboat

18:- What is frictional force? give some example.

Ans:- Friction is the force that generated when two surfaces rub against one another. It is the force which oppose the relative motion between the two surface that are In contact with each other. 

Ex:- moving a ball on the ground, moving a car on the ground, sliding a sledge on the ground   etc.

19:- Why friction is also called contact force?

Ans:-  Friction is also called a contact force because it occurs when two surface are in contact and one surface tries to move on other surface.

20:- How many types of Non-contact force? Name them.

Ans:- There are three types of Non-contact force

1- magnetic force

2- electrostatic force

3- gravitational force

21:- What is magnetic force?

Ans:- The force exerted by a magnet is called the magnetic force.

22:- Why magnetic force is also called Non-contact force?

Ans:-magnetic force is also called Non-contact force because a magnet can exert a force on another magnet or a magnetic object without being in physical contact with it. 

23:- What is electrostatic force?

Ans:- The force exerted by a charged body on another charged or uncharged body is known as the electrostatic force.

24:- Why electrostatic force is also called Non-contact force?

Ans:- Electrostatic force is also called Non-contact force because this force exerts even when the bodies are not in physical contact.

25:- What is force of gravity?

Ans:- Force of gravity is the  force that occurs between the earth and other bodies. It is the force acting to pull objects towards the earth.

26:- What is gravitational force?

Ans:- The mutual force of attraction between two bodies because of their masses is called gravitational force. Grater the mass of object, greater will be the gravitational force.

27:- Who was the first person who identify gravitational force?

Ans:- Sir Isaac Newton

Pressure

1:- What is pressure?

Ans:- the force acting on a unit area is called pressure. The SI unit of pressure is newton per square metre or N/m²   . It is also called Pascal and it is denoted by 'P'.

 

2:- Write two factors on which the effect of pressure depends.

Ans:- The two factors on which the effect of pressure depends are

1- If the magnitude of the force applied is increased then the pressure will also increased. 

2- If the area over which the force is applied is increased then the pressure will decrease.

 

3:- What is the formula of pressure?     

Ans:- 

Pressure is directly proportion to Force.

P ∝ F

Pressure is inversely proportion to area

P ∝ 1 / A

P ∝ F × 1/A

P = k × F × 1/A

In SI unit the value of  " k " is 1

P = F/A

Pressure = Force / Area

 

4:- What is 1 Pascal?

Ans:- 

 1 Pascal is a pressure that is exerted by 1 newton of force on 1 square metre of area.

1P = 1N × 1m² 

5:- Why porters place a round piece of cloth on their heads before carrying heavy loads?

Ans:-porters place a round piece of cloth on their heads before carrying heavy loads because round clothe increases the area of contact of the load with their head which helps to decrease pressure on their head.

6:- Write five applications where pressure is reduced by increasing area.

Ans:-The five applications where pressure is reduced by increasing area are

1- The buses and trucks are provided with broad tyres and double wheels, to reduce pressure on the road.

2- The base of dam is kept wide which increases the strength of dam.

3- building have wide foundation to reduce the pressure on the base of the building.

4- The straps of the soldier in which we carry luggage are made broad. This decrease the pressure on the soldier.

5- Heavy army tanks move on steel tracks to reduce the pressure on the ground.

7:- Why all cutting tools have pointed tips and sharp edge?

Ans:- All cutting tools have pointed tips and sharp edges to increase pressure by reducing area for easy cut.

8:- A solid weighs 80 N. when placed on surface, the area of contact is found to be 1.6 square metre. find the pressure exerted by the solid on the surface? 

Solve:-

Force = weight = 80 N

Area = 1.6 m²   

Pressure ( P ) = Force( F ) / Area ( A )

                       = 80 / 1.6 N/ m²  

                       = 50 N/m²   = 50 Pa

9:-A rectangular wooden piece weighs 25 N. If it is placed in such a way that its 25 cm length and 10 cm width are in contact with the surface, What will be the exerted pressure?

10:- Explain pressure due to liquids.

Ans:- Liquid do not have definite shape. They take the shape of the container in which they are kept. So, their area of contact changes with the shape of the container. A liquid exerts pressure on the base of the container due to its weight and also exerts pressure on the walls of the container due to collisions of its molecules with the wall.Since the molecules of a liquid are regularly in motion, they strike continuously with the walls of the container, thus exerting a pressure.

11:- What is lateral pressure?

Ans:- The pressure that is exerted by liquid or gas on the wall of container is called lateral pressure.

 

12:- Write an activity to show that pressure exerted by a given liquid increases with depth​.

Ans:-              

◆Take a plastic bottle or a beaker which has tube like structure.

◆Fill the container full with water.

◆Now with any sharp object make holes to the container one by one from top to bottom.

◆You can observe that water flowing out from top holes are coming with less force But the water flowing out from bottom holes are coming with higher force and falling far from container.

◆This shows that the pressure of a liquid increases with depth.

13:- WRITE AN ACTIVITY TO SHOW THAT A LIQUID EXERTS PRESSURE AT THE BOTTOM OF ITS CONTAINER :

Ans:- take a balloon and tie it at the lower end of a glass tube and hold vertically.  pour some water in the tube , balloon bulges Because water column exerts pressure due to its weight at its bottom. This shows that a liquid exerts pressure at the bottom of the container in which it is kept.

                        

14:- WRITE AN ACTIVITY TO SHOW THAT A LIQUID EXERTS PRESSURE ON THE WALL OF CONTAINER .

Ans:- Take a plastic bottle.  Fix a cylindrical glass tube, a few cm long near its bottom. Make sure that the water does not leak from the joint. If there is any leakage, seal it with molten wax. Cover the mouth of the glass tube with a thin rubber sheet. Now fill the bottle with water. We will notice that There is a change in the bulge of the rubber sheet. The bulging of the rubber sheet in this case indicate that water exerts pressure on the sides of the container.

15:-  WRITE AN ACTIVITY TO SHOW THAT A LIQUID EXERTS EQUAL PRESSURE ON THE WALL OF CONTAINER AT THE SAME DEPTH

Ans:-             

Take a plastic bottles. Make two small holes of equal size on the two opposite sides of the plastic bottles some distance above the bottom of the bottle. The holes should be at the exactly the same height from the bottom of the plastic bottles. Now fill the bottle with water. The two jets of water comming out of the two holes fall at the same distance away from the base on its either side. The two jets of water can fall at equal distance on the two sides of the bottle only if the pressure of water at the depth  of two holes in the bottle is equal. This prove that a liquid exerts equal pressure on the wall of container at the same depth.  .

16:- What is pressure gauge?

Ans:- An instrument which is used to measure liquid pressure is called a pressure gauge. manometer is the simplest form of pressure gauge which measures pressure difference.

             

17:- What is atmosphere of the earth?

Ans:- The earth is surrounded by a layer of air up to height nearly 300 km and this layer of air around the earth is known as the atmosphere of the earth.

18:- What is atmospheric pressure?

Ans:- Atmospheric pressure is the air pressure which is exerted by the weight of air present in the atmosphere. In SI system the value of atmospheric pressure is 10,000 N/ m²   or 10,000 Pa. This pressure is equivalent to the pressure that is generated by two elephants weight.

19:- How does our body sustains in highest atmospheric pressure that is 10,000 N/ m²   or 10,000 Pa?

Ans:-our body sustains in highest atmospheric pressure that is 10,000 N/ m²   or 10,000 Pa because our our blood pressure balances the atmospheric pressure and cancles the pressure from outside.