Important Notes for CBSE Class 8 Science Chapter 11 – Force and Pressure. Force and Pressure Class 8 Notes and Study Material Download in PDF form.
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Force and Pressure Class 8 Notes
- Force – It is a ‘Push’ or a ‘Pull’ on an object is called force.
- Motion is imparted to objects due to the action of force.
Force is generated due to interaction:
- When ever force is acting on a body, it causes the displacement in direction of the applied force.
- Two objects must interact for a force to come into play. Thus, an interaction of one object with another object results in a force between the two objects.
- Forces applied on an object in the same direction add to one another.
- If the two forces act in the opposite directions on an object, the net force acting on it is the difference between the two forces.
- Magnitude: The strength of a force is usually expressed by its magnitude
- Force applied could be larger or smaller than the other or equal to each other.
- If the direction or the magnitude of the applied force changes, its effect also changes.
- The net force on an object is zero if the two forces acting on it in opposite directions are equal.
Force can Change the State of Motion:
- Force applied on an object may change its speed.
- If the force applied on the object is in the direction of its motion, the speed of the object increases.
- If the force is applied in the direction opposite to the direction of motion, then it results in a decrease in the speed of the object.
- A change in either the speed of an object, or its direction of motion, or both, is described as a change in its state of motion.
- Thus, a force may bring a change in the state of motion of an object
- State of motion: an object is described by its speed and the direction of motion.
- The state of rest is considered to be the state of zero speed.
- An object may be at rest or in motion; both are its states of motion.
- Many times, application of force does not result in a change in the state of motion.
Force can lead to change in shape:
- Force on an object may change its shape.
- An object cannot move by itself, it cannot change speed by itself, it cannot change direction by itself and its shape cannot change by itself.
Contact Forces:
1) Muscular Force:
- The force is caused by the action of muscles in our body.
- The force resulting due to the action of muscles is known as the muscular force.
- The muscular force that enables us to perform all activities involving movement or bending of our body.
- Animals make use of muscular force to carry out their physical activities and other tasks.
- Animals like bullocks, horses, donkeys and camels are used to perform various tasks for us.
- Muscular force can be applied only when it is in contact with an object, is also called a contact force.
- Example: Friction: The force responsible for changing the state of motion of objects in all these examples is the force of friction.
- The force of friction always acts on all the moving objects and its direction is always opposite to the direction of motion.
- Since the force of friction arises due to contact between surfaces, it is also an example of a contact force.
Non-contact force:
- Attraction or repulsion between objects is also a type of non-contact force.
- Magnet are a type of force which act without being in contact with one another.
- It is a non-contact force.
- The force exerted by a magnet on a piece of iron is also a non contact force
I. Electrostatic Force:
- The force exerted by a charged body on another charged or uncharged body is known as electrostatic force.
- This force comes into play when the bodies are not in contact.
- The electrostatic force is example of a non-contact force.
II. Gravitational Force:
- It is an attractive force that acts on all object.
- Things fall towards the earth because it pulls them.
- This force is called the force of gravity.
- Every object in the universe exerts a force on every other object and is known as the gravitational force.
III. Pressure:
- Pressure is the force per unit area.
- Pressure is calculated with the forces which act perpendicular to thesurface.
- For a smaller the area, larger thepressure on a surface for the same force.
- Gases and other liquids exert pressure onthe walls of their container’
Atmospheric Pressure:
- Air envelope around the earth is the atmosphere.
- The pressure exerted by this air is known as atmospheric pressure.
- The force of gravity on the air is the atmospheric pressure.