Class - 8, Science, Chapter:- 11 Force & Pressure Notes, NCERT Solutions & Frequently Asked Questions

  Science

Class:-8, Chapter:- 11

Force & Pressure 

Notes, NCERT Solutions & Frequently Asked Questions

-- Notes --

⭐Force:- Force is defined as a push or a pull on an object, which changes or tends to change the state of rest or uniform motion of the object. 

• It may lead to a change in the direction or shape of the object. 

• A push or pull is being exerted on the object to change their state of motion. Like the closed pencil box is being opened, the football moves on kicking etc.

⭐Unit of force:- SI units of force is Newton.

⭐Effects of Force:-

• A force is a resultant of the interaction between two objects. If we consider football, then it will not move unless a person kicks it. So, it is an interaction between the person and the football which creates a force that results in its movement.

• A force has strength/magnitude as well as direction. 

• The magnitude of the applied force can be large, small or it can also be equal. 

• The force can be applied in the same or opposite direction to each other. 

• It is easy to push a heavy object in the same direction by two people instead of two people pushing it in opposite directions to move it.

• A force may bring about a change in the speed or the direction of motion or both of an object. This implies that the object has undergone a change in its state of motion. Eg.- a football being kicked by a player, then the kick sets the stationary ball into motion towards the direction in which it is being kicked. The amount of force exerted on the object decides the speed of the object.

• A force that is acting on an object may tend to bring a change in its shape. Eg.- dough can be rolled into chapatis by applying force and rolling them.

• A force can act on an object by being in contact with it or by not being in contact with it.

⭐Types of Forces:- There are two types of forces based on whether the force acting on the object is by being in contact with it or not.

a) Contact Forces:- A force that acts on an object when it is in contact with the body is called a contact force. 

➤ The point of application of force on the object is called the point of contact.

➤ The contact forces can be categorised as follows:

i) Muscular Force:- A force that is exerted as the result of the action of the muscles in our body is termed a muscular force. 

• We use the muscular force to carry out some of our bodily actions like breathing, digestion etc. 

• The animals like buffalo, camels or horses use their muscular force to get activities done for us like ploughing, pulling, or carrying the load.

ii) Frictional Force:- A frictional force is one that opposes the motion of an object and it always acts in the direction opposite to the motion of the object. 

• This can be understood when a boat stops moving when we stop rowing. 

• This is due to the frictional force between the boat and the water.

b) Non-Contact Forces:- A force that does not involve any kind of physical contact between two objects on which they act is called a non-contact force.

➤ The non-contact forces can be categorised as follows:-

i) Magnetic Force:- A force exerted by a magnet on objects made of iron, steel, cobalt, or nickel without being in contact with them is termed as a magnetic force.

ii) Electrostatic Force:- A force that is exerted by a charged body on another charged or uncharged body is termed as an electrostatic force. 

• It may result in the repulsion of similar charges like if we bring two straws rubbed with paper charged near each other and attraction of opposite charges if we bring a charged straw near an uncharged straw.

iii) Gravitational Force:- A force that is exerted by each and every object in the world on another object which is an attractive force, is termed as gravitational force.

⭐Pressure:- The unit force that acts on the per unit area of a surface is called pressure. 

• The relationship between pressure and force can be defined as

     Pressure = force / area on which it acts

• It is easy to push a nail into a wooden board with its pointed end instead of the head as the smaller area exerts a larger pressure for the same amount of force and the nail goes into the wood easily with one push.  

• SI system of pressure is Pascal (Pa) which is equal to newton per square metre (N/m²). 

⭐Distribution of pressure:- Force acting on a smaller area applies more pressure than the same force acting on a larger area.

• A sharp knife cuts better as more pressure is exerted over a smaller area.

⭐Pressure in fluids:- Fluids and gases also exert pressure like solids. 

• A solid generally exerts pressure in the downward direction only, which is mostly due to its weight. 

• Liquids and gases exert pressure in all directions. Eg.- when a water pipe leaks, you can see fountains of water coming in all directions. This is because the water exerts pressure on the wall of the pipes. Similarly, when a balloon has holes, the air escapes in all directions.

⭐Upthrust:- The upward force exerted by a fluid on an object is known as upthrust or buoyant force.

⭐Atmosphere:- The thick blanket of air that surrounds the earth above its surface is termed as atmosphere.

• The pressure that is exerted by this column of air in the atmosphere is called atmospheric pressure. It is this pressure that allows a rubber sucker to stay attached to the surface.

• We cannot feel this tremendous atmospheric pressure that surrounds us. This is so because the fluid pressure inside our bodies actually counter-balances the atmospheric pressure that surrounds us.

-- NCERT Solutions --

Question 1.- Give two examples each of the situations in which you push or pull to change the state of motion of objects.

Answer:-

(i) Push:-  

• We close drawer by pushing. 

• We move a wooden box by pushing.

(ii) Pull:-  

• We draw water from a well by pulling the rope.

• A horse pulls a cart.

Question 2.- Give two examples of situations in which applied force causes a change in the shape of an object.

Answer:- When we apply force on a rubber band to stretch it and on clay to change its shape.

Question 3.- Fill in the blanks in the following statements.

(a) To draw water from a well we have to ______ at the rope.

(b) A charged body ______ an uncharged body towards it.

(c) To move a loaded trolley we have to ______ it.

(d) The north pole of a magnet _______ the north pole of another magnet.

Answer:-

(a) pull

(b) attracts

(c) push

(d) repels

Question 4.- An archer stretches her bow while taking aim at the target. She then releases the arrow, which begins to move towards the target. Based on this information fill up the gaps in the following statements using the following terms:

muscular, contact, non-contact, gravity, friction, shape, attraction

(a) To stretch the bow, the archer applies a force that causes a change in its ______

(b) The force applied by the archer to stretch the bow is an example of ______ force.

(c) The type of force responsible for a change in the state of motion of the arrow is an example of a ______ force.

(d) While the arrow moves towards its target, the forces acting on it are due to _______ and that due to _____ of air.

Answer:-

(a) shape

(b) muscular

(c) contact

(d) gravity, friction

Question 5.- In the following situations identify the agent exerting the force and the object on which it acts. State the effect of the force in each case.

(a) Squeezing a piece of lemon between the fingers to extract its juice.

(b) Taking out paste from a toothpaste tube.

(c) A load suspended from a spring while its other end is on a hook fixed to a wall.

(d) An athlete making a high jump to clear the bar at a certain height.

Answer:-

(a) Agents are fingers, object is lemon, effect of force changes the shape of lemon.

(b) Agents are fingers of the person squeezing the tube, object is toothpaste tube and effect of the force can be observed as the paste coming out of the tube (change in shape).

(c) Agent is the load suspended, object is the spring and effort can be seen in the form of elongation of spring on suspension of load (change in shape).

(d) Agent is muscles of athlete, object is athlete himself and effect of the force changes the state of motion of the athlete.

Question 6.- A blacksmith hammers a hot piece of iron while making a tool. How does the force due to hammering affect the piece of iron?

Answer:- The force due to hammering causes the change in the shape of the iron and iron can be moulded in the shape of the required tool.

Question 7.- An inflated balloon was pressed against a wall after it has been rubbed with a piece of synthetic cloth. It was found that the balloon sticks to the wall. What force might be responsible for the attraction between the balloon and the wall?

Answer:- Electrostatic force.

Question 8.- Name the forces acting on a plastic bucket containing water held above ground level in your hand. Discuss why the forces acting on the bucket do not bring a change in its state of motion.

Answer:- Forces acting on bucket are as follows:

(i) Muscular force of arms acting upward.

(ii) Force of gravity acting downward.

Both the forces do not bring any change in the state of motion because both of them are acting in equal and opposite directions and thus they cancel each other’s effect.

Question 9.- A rocket has been fired upwards to launch a satellite in its orbit. Name the two forces acting on the rocket immediately after leaving the launching pad.

Answer:- The forces that act when a rocket leaves launching pad are as follows:

(i) Gravitational force of the earth (downward)

(ii) Frictional force of air (in opposite direction)

Question 10.- When we press the bulb of a dropper with its nozzle kept in water, air in the dropper is seen to escape in the form of bubbles. Once we release the pressure on the bulb, water gets filled in the dropper. The rise of water in the dropper is due to

(a) pressure of water

(b) gravity of the earth

(c) shape of rubber bulb

(d) atmospheric pressure

Answer:- (d) atmospheric pressure

-- Frequently Asked Questions --

Question 1.- What is ‘Spring force’?

Answer:- Spring force is a restoring force that tends to retract the spring back to its original position.

Question 2.- What are ‘Non contact’ forces?

Answer:- A non-contact force is a force which acts on an object without coming physically in contact with it.

Question 3.-  What is ‘Thrust’?

Answer:- Thrust is defined as to quickly push with force.