Chapter 8: Force and Motion

8.1 Force

A force is a push or a pull exerted on an object as a result of its interaction with another object. Force is a vector quantity having both magnitude and direction.

SI Unit and Measurement

• SI Unit: Newton (N)
• Measuring Instrument: Spring balance (spring dynamometer)

Types of Forces

• Gravitational Force: Pulls all objects towards the center of Earth.
• Frictional Force: Resists the motion of two touching surfaces moving past each other (acts opposite to the direction of motion).
• Normal Force: Supporting force exerted by a surface perpendicular to the object resting on it.
• Buoyant Force: Upward force exerted by a fluid on an object immersed in it.
• Elastic Force: Force present when a stretched or compressed elastic material returns to its original shape.
• Electrostatic / Magnetic Force: Non-contact attractive or repulsive forces between charged objects or magnetic poles.

Effects of Force

A force applied to an object can change its:

1. Shape or size (e.g., stretching a rubber band, crushing a can).
2. State of rest or motion (starts moving a stationary object, or stops a moving object).
3. Speed (accelerates or decelerates an object).
4. Direction of motion (e.g., hitting a rolling ball to change its path).

8.2 Effects of Force (Pivots, Levers, and Pressure)

Moment of Force

The moment of force is the turning effect of a force about a fixed point called the pivot (fulcrum).

$$\text{Formula: } \text{Moment of Force (N m)} = \text{Force (N)} \times \text{Perpendicular distance from pivot to force (m)}$$

• Directions of moment: Clockwise or Anticlockwise.
• Principle of Moments: At equilibrium, Total Clockwise Moment = Total Anticlockwise Moment.

Levers

A lever is a simple machine consisting of a rigid bar that rotates around a pivot (F) to lift a load (L) using an effort (E).

• First Class Lever: Pivot is between Load and Effort (L - F - E).
  Examples: Scissors, crowbar, seesaw, pliers.
• Second Class Lever: Load is between Pivot and Effort (F - L - E).
  Examples: Wheelbarrow, nutcracker, bottle opener.
• Third Class Lever: Effort is between Pivot and Load (F - E - L).
  Examples: Fishing rod, tweezers, broom, ice tongs.

Pressure

Pressure is defined as the force acting perpendicularly per unit surface area.

$$\text{Formula: } P = \frac{F}{A} \quad \text{where } P = \text{Pressure (N/m}^2\text{ or Pa)}, F = \text{Force (N)}, A = \text{Area (m}^2\text{)}$$

• Increasing Pressure: Reduce surface area $A$ (e.g., sharp knife edge, push pins, ice skate blades).
• Decreasing Pressure: Increase surface area $A$ (e.g., wide tractor tires, skis, snowshoes).

Gas Pressure and Atmospheric Pressure

• Gas Pressure: Caused by gas particles colliding continuously against the inner walls of a container.
• Atmospheric Pressure: The weight of atmospheric air pressing down on Earth's surface ($1 \text{ atm} \approx 101,325 \text{ Pa}$).
• Factors affecting gas pressure:
  1. Volume: Decreasing volume increases gas particle collision frequency, increasing pressure.
  2. Temperature: Increasing temperature increases particle kinetic energy, increasing collision force and pressure.
• Applications: Syringe, suction cups, drinking straw, siphon.

Liquid Pressure

Liquids exert pressure due to the weight of the liquid above.

• Pressure in liquids increases with depth.
• Pressure in liquids increases with liquid density.
• Liquid pressure acts equally in all directions at a given depth.