Force and Pressure – Quick Notes

Force

Definition: A push or pull acting on a body which tends to change its state of rest or motion.

Types:

1. Force as a Pull: Changes the direction of the object towards the source.
   • Examples: Opening a door, pulling a cart, lifting a pen.
2. Force as a Push: Changes the direction of the object away from the source.
   • Examples: Kicking a ball, riding a bicycle, moving a stationary object.

Effects of Force:

1. Can change the state of motion of an object.
2. Can change the direction of a moving object.
3. Can change the shape and size of an object.

Unit of Force

1. SI Unit: Newton (N)
2. Other Units: Kilogram-force (kgf), gram-force (gf), etc.
3. Relation between forces
   • 1 kgf = 1000 gf
   • 1 kgf ≈ 9.8 N

Resultant Force

• The single force which acts on a body to produce the same effect as all the forces acting on it collectively.
• If forces act along the same line in the same direction, the resultant force is equal to the sum of the individual forces.
• If forces act along the same line but in opposite directions, the resultant force is equal to the difference between the two forces.

Balanced and Unbalanced Force

• Balanced Force: When the resultant of the forces acting on a body is zero.
• Unbalanced Force: When the resultant of the forces acting on a body is not zero.

Types of Forces

1. Contact Force: Two objects physically interact with each other with actual contact.
   • Examples:
     • Muscular Force: Force exerted by the muscles of a human or animal body.
     • Frictional Force: Force acting between two surfaces in contact, opposing the motion of one body over the other.
2. Noncontact Force (Action-at-a-Distance Force): Force experienced without any actual contact between the two objects.
   • Examples:
     • Magnetic Force: Force exerted by a magnet on another magnet or magnetic substance. It is either attractive or repulsive.
     • Electrostatic Force: Force exerted by a charged body on another charged or uncharged body. It is either attractive or repulsive.
     • Gravitational Force: Force of attraction existing between all objects by virtue of their masses. It is always attractive.

Pressure

Definition: Force acting on a unit area. \[\text {Pressure} = \frac {Force}{Area}\]
Unit: Pascal (\(\text{Pa}\)), also called newton per square meter (N/m²).

Factors Affecting Pressure:

• Force: More force more pressure, less force less pressure.
• Area: Smaller area of contact results in higher pressure; larger area of contact results in lower pressure.

Pressure Exerted by Liquids

• Pressure at a point in a liquid depends on its depth.
• Pressure is equal in all directions at the same depth.
• Pressure of a liquid does not depend on the shape and size of the container.
• Pascal’s Law: Pressure applied to a confined fluid is transmitted equally in all directions.