FORCE AND PRESSURE

 NOTES 

A push or pull exerted by an object on another is a force.

Force arises due to the interaction between at least two objects.

Force has both Magnitude and Direction

• Force applied on an object in the same direction add to one another.
• If the two forces act in the opposite direction on an object, the net force acting on it is the difference between the two forces.
• In general, more than one force may be acting on an object at any given point. However, a force acting on an object is always the mean net force acting on it.

Two friends pushing a heavy load (a) in the same direction and (b) in the opposite direction

A Force can Change the State of Motion

• A change in the speed of an object or the direction of its motion or both implies a change in its state of motion.
• Force acting on an object may bring a change in its state of motion or a change in its shape.

To move a tyre faster it has to be pushed repeatedly

A magnet can exert a force without being in contact with it. The force exerted by a magnet is an example of a non-contact force.

Pressure: Thrust per unit area is called pressure. So, pressure = force/area on which it acts.

Liquids exert pressure on the walls of the container in which they are kept.

Gases exert pressure in all directions.

Electrostatic force: A force exerted by a charged body on another charged or uncharged body is known as electrostatic force.

Atmospheric pressure is defined as the pressure exerted on a surface by the weight of air above that surface.

Atmospheric Pressure: The pressure exerted by atmospheric air around us is known as atmospheric pressure.

Contact Force: A force that can be applied only when it is in contact with an object is called a contact force. For example, hammering a nail.

Electrostatic Force: When a charged body, either having a (+) or (-) charge, exerts force on another charged or uncharged body, that force is known as electrostatic force.

Force: Simply a push or pull exerted by an object on another is a force.

Friction: The force of friction always acts on all the moving objects and its direction is always opposite to the direction of motion.

Gravitational Force: The force exerted by the earth to pull the objects towards itself is called the force of gravity.

Gravity: Every object in the universe, whether small or large, exerts a force on every other object. This force is called the force of gravity or just gravity.

Magnetic Force: The force exerted by a magnet to pull/push a metallic object is called magnetic force.

Muscular Force: The force resulting due to the action of muscles is known as muscular force. For example, writing, cutting vegetables.

Non-Contact Forces: A force that can be applied without any contact between two objects is called non-contact force. For example, the magnet pulls the iron-pieces.

Pressure: The force acting on per unit area, applied to an object in the direction perpendicular to the surface is called pressure.

QUESTION ANSWER

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.

uestion 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