CBSE Class 8 Science Chapter 8 Chapter 8 Force And Pressure Force: A Push Or A Pull
To move an object, it has to be pushed or pulled. A push or a pull acting on an object is known as force. Thus, any object is in motion due to the application of force.
The direction in which the body is pushed or pulled is called the direction of force.
Actions like kicking, opening, shutting, lifting, hitting, etc., are examples of push or pull. These actions usually results in some kind of change in the state of motion of an object.
Best Short Notes For CBSE Class 8 Force And Pressure Chapter Force Is Due To An Interaction
The force between the objects is caused by an interaction of one object with another object. If there is no interaction between two objects, then no force can show its effect. Assume that a man is standing behind a stationary car where there is no interaction between the man and the car, so no force is acting on the car, therefore, the car does not move.
Note, consider the second diagram in which the man pushes the car and it starts moving. This is possible because the man interacts with the car.
From this example, we can conclude that at least two objects must interact for a force to come into play.
Magnetic Force And Electrostatic Force Class 8 Comparison Notes Exploring Forces
The strength of the force is expressed by its magnitude and direction. So, the change in the magnitude or direction of the applied force always results in a change in the effect produced by an applied force.
If forces are applied on an object in the same direction, then they add to one another. For example, two men apply a pushing force together in the same direction on the heavy box; the two forces add upto provide a much bigger force.
If the two forces act in opposite directions on an object, the net force acting on it is the difference between the two forces. For example, the box will move in the direction where the stronger push is applied.
Another example is the game ‘tug of war’ in which two teams pull at a rope in opposite directions. When the two teams pull the rope equally hard by applying equal forces, then the net force acting on the rope is zero, and hence, the rope does not move. In general, if more than one force is acting on an object, then the effect on the object will be due to the net force.
Free Printable Force And Pressure Notes For Class 8 CBSE Effects Of Force
If we apply force on an object, it can have the following effects, depending on the type of object.
- It can change the state of motion of an object.
- It can change the size or shape of an object.
Force can change the State of Motion
The state of motion is described by its speed and the direction of motion. The state of rest is considered to be the state of zero speed.
A force may bring a change in the state of motion of an object.
Force can increase the speed if the force applied on the object is in the direction of its motion and decrease the speed, if the force is applied in the opposite direction to the direction of motion. Even, it can stop the moving object or change the direction of motion ofan object.
For Example;
- A goalkeeper dives or jumps up to save the goal. He applies force to the moving ball. This force can stop or deflect the ball.
- In the following figure, the child applies force in the direction of motion of the tyre to increase its speed.
Sometimes the application of force does not change the state of motion. For example, a Heavy box may not move even if you apply the maximum force that you can exert. Similarly, you cannot displace a wall by pushing that.
Force can change the Shape of an Object.
The application of force on an object may change its shape. For Example
- The shape of an inflated balloon changes when by press it.
- The shape of the ball of dough changes when it is rolled to make a chapati.
We can summaries that applying force
- May make an object move from rest.
- May change the speed of an object if it is moving.
- May change the direction of motion of an object.
- May bring about a change in the shape of an object.
- May cause some or all of these effects.
Types Of Force
There are different types of force which are categorised into two groups
1. Contact Force
These forces come into action only when two objects are in actual contact with each other. These are also of two types
Muscular force. This force is caused by the action of muscles in our body. It enables us to perform all activities involving movement or bending of our body.
For Example. In the process of digestion, the food gets pushed through the alimentary canal, expansion and contraction of lungs while breathing, physical activities done by animals, etc.
Frictional force. This force always acts opposite to the motion of any moving object, for Example. Ball rolling on the ground slows down, boat comes to a rowing, etc.
Non-Contact Forces
These forces came into action without any contact with one another, for Example, Magnetic force, electrostatic force, and gravitational force.
Magnetic force: The force exerted by the magnet is called the magnetic force. Like poles of two magnets repel each other and unlike poles attract each other. A magnet can attract iron nails without touching them, or a magnet exerts a force even without touching an object. So, it is considered a non-contact force.
Electrostatic force The force exerted by a charged body on another charged body is known as electrostatic force, Example; When a plastic comb is rubbed in dry hair, the comb gets electric charges by friction and if this electrically charged comb is brought near the small pieces of paper, then it exerts an electrostatic force of attraction on them (pieces of paper) even if they are not in touch with each other.
Gravitational force: The force due to which every body of the universe attracts another body, whether the object is small or large, is known as gravitational force. Objects or things fall towards the Earth because it pulls them. This force is called the force of gravity or gravity, and this is an attractive force. Example. The earth attracts leaves of trees, water begins to flow towards the ground as soon as we open a tap, etc.
Science Insight The Gravitational force of the moon is (l/6)th of the gravitational force of the Earth. Due to this fact, the moon does not have its atmosphere.
Pressure
The force acting on a unit area of surface is defined as pressure. It can be represented as,
⇒ \(\text { Pressure }=\frac{\text { Force }}{\text { Area }}\)
For example, if we push our thumb into a wooden plank, it does not go in. But, if we push a nail by its pointed end, it goes in easily. This is because in a nail, the force acting on per unit area of wood is very large.
Thumb produces a smaller pressure because it acts on a large area of wood, but the same force produces a much larger pressure when it is applied on a small area in a nail.
Applications of pressure in our daily life
A porter reduces pressure by placing a cloth on his head while carrying a heavy load. This increases the area of contact of the load with their head, due to which the pressure on the head decreases, and it becomes easier for a porter to carry the load.
So, the larger the area, the smaller the pressure on a surface for the same force.
School bags have broad straps, so that the weight of the bag falls over a larger area, producing less pressure over the shoulder.
The tip of the needle is sharp as it produces more pressure on a small area of the cloth. A sharp needle can pierce through the cloth while sewing.
Pressure Exerted By Liquids And Gases
Like solid, fluid also exerts pressure, and fluid can be liquids or gases.
Pressure Exerted By Liquids
- Liquids exert pressure due to the weight that liquids possess.
- Liquids always possess pressure in the downward direction.
- The pressure exerted by liquids at the bottom depends on the height of the liquid column.
- The pressure exerted by liquids increases with depth.
- Liquids exert pressure in all directions on the walls of the container.
- Liquids exert equal pressure at the same height.
Pressure Exerted By Gases
- Gases also exert pressure in all directions on the walls of containers like liquids.
- Gases exert pressure due to constant collisions of molecules
Common Mistakes In Force And Pressure Chapter Class 8 Atmospheric Pressure
The envelope of air around us is known as the atmosphere. It extends up to many kelometres above the surface of the Earth. The pressure exerted by the atmosphere is known as atmospheric pressure. It is the force that air exerts on per unit area. In other words, it is the weight of air present in a very tall column of unit area.
Science Insight: The maximum atmospheric pressure. The Earth is at sea level because the column of air is the largest above the sea. The pressure decreases as we go above the Earth’s surface because the length of the air column decreases. That’s why the atmospheric pressure above the mountain is less as compared to that on the Earth’s surface.
Some facts about atmospheric pressure are
- The weight of air above our heads is about 225 kg. But, we are not crushed under the weight of this air because the pressure inside our body is equal to the atmospheric pressure outside.
- A suction cup sticks to a flat surface. When we press the suction cup against a flat surface, it flattens and expels most of the air from under it.
- This creates a partial vacuum inside the suction cup. The atmospheric pressure outside is much greater, and this holds the cup firmly to the surface.
Magdeburg Hemispheres
In the 17th century, a German scientist named to Von Guericke invented a pump to extract all the air from a vessel. This pump is called a vacuum pump. So, he demonstrated an experiment in the town of Magdeburg to show the large atmospheric pressure.
Force refers to a pull or push on an object.
Push It is moving an object away from the source of force.
Pull: It is moving an object towards the source of force. Contact Force Force that is exerted by one object on another object when they are in contact with each other.
Muscular Force: The force exerted by the muscles of a body.
Frictional Force: The force exerted by one object to oppose its motion over another object.
Non-contact Force: The force exerted by one object on another without being in contact.
Magnetic Force: The force exerted by a magnet.
Electrostatic Force: The force exerted by an electrically charged object.
Gravitational Force: The force of attraction (pull) exerted by the Earth.
Pressure refers to the force that is exerted over a unit area of a surface.
Atmospheric Pressure: It refers to the pressure exerted by the atmosphere.