Comprehensive Study Notes on Forces, Pressure, Work, and Energy

Definition of Force: A force is defined as a push or a pull.
SI Unit: The standard international (SI) unit of force is the Newton ( $N$ ).
Measurement: Force is measured using instruments such as a spring balance or a dynamometer.
General Classification: Forces can be broadly categorized into two types: contact forces and noncontact forces.

Contact force is a force acting between two objects that are in direct physical contact with each other. Major types include:

Friction:
- Definition: A contact force that opposes motion between two surfaces in contact.
- Surface Texture Influence: Friction is greater when an object moves on a rough surface compared to a smooth surface.
- Disadvantages:
  - Wear and tear of surfaces.
  - Energy wastage due to the production of heat, which results in a waste of energy.
  - As friction opposes motion, more energy is required to move the object.
- Methods to Overcome Friction:
  - Using lubricating oils or grease.
  - Using ball bearings or rollers.
  - Using wheels.
  - Using a cushion of air, which reduces direct contact and friction between surfaces.
Tension / Elastic Force:
- Definition: A contact force that comes into play when an object is stretched or compressed from its original shape.
- Directionality: Elastic forces exerted by an object act in opposite directions to the force applied to the object, whether that applied force is compression (a push) or stretching (a pull).
Applied Force:
- Definition: A thrust exerted through external means.
- Trigger: This occurs whenever an external force, known as a push or a pull, is exerted on an object.

Noncontact force is a force acting between two objects that are not touching each other. Major types include:

Electrical Force:
- Definition: A noncontact force occurring through attraction or repulsion when electrical charges interact with each other.
- Context: Relevant to all conductors.
Magnetic Force:
- Definition: The attraction or repulsion that occurs when magnets interact with each other or with magnetic materials.
Gravitational Force (Weight):
- Definition: A noncontact force of attraction between two bodies of mass.
- Mass Dependency: The magnitude varies according to the sizes of the masses. All objects with mass possess a gravitational field around them. Bigger masses have a stronger gravitational field.

It is critical to note that mass is not equal to weight.

Nature of Quantity:
- Mass: Amount of matter in an object. Mass is not a force.
- Weight: Amount of gravitational force acting on a mass object. Weight is a type of force.
SI Units:
- Mass: kilogram ( $kg$ ).
- Weight: Newton ( $N$ ).
Measurement Instruments:
- Mass: Beam balance or electronic balance.
- Weight: Spring balance.
Constancy and Location:
- Mass: Constant regardless of location; it is never zero.
- Weight: Changes depending on location and can be zero if the gravitational force is zero.
Formula:
- $\text{Weight} = \text{mass} \times \text{gravitational force}$

1. Changing the Motion of Objects:
- Includes starting a motion, stopping a motion, and changing an object's speed or direction.
- Definition of Speed: Speed is defined as the distance traveled per unit time. It describes the state of motion in terms of how fast the object is moving.
- Speedometer: Instrument used to measure speed.
- SI Unit of Speed: meter per second ( $m/s$ ).
2. Changing the Size and Shape of Objects:
- Force can change the size and shape of objects through compression and stretching.
- While the shape/size changes, the mass of the object remains the same.
3. Creating a Turning Effect (Rotation):
- When a force is applied about a pivot, it can create a turning effect which causes rotation.
4. Causing Pressure:
- When a force is applied over an area, the area in contact with the force experiences pressure.

Definition: Pressure is the force acting per unit area.
Formula: $\text{Pressure} = \frac{\text{Force}}{\text{Contact Area}}$
Units:
- SI unit: Pascal ( $Pa$ ).
- Force is in Newton ( $N$ ).
- Contact area is meter cube ( $m^3$ ).
Relationship Between Force, Area, and Pressure:
- Constant Force: Higher contact area leads to less pressure; lower contact area leads to more pressure.
- Constant Area: Higher force leads to more pressure; lower force leads to less pressure.
Atmospheric Pressure:
- Definition: The pressure exerted by air in the Earth's atmosphere.
- Mechanism: Atmospheric pressure increases with the presence of more air molecules.
- Drinking Straw Scenario: When sucking through a straw, some air is removed from inside the straw. This creates lower air pressure inside the straw. The higher atmospheric pressure outside the straw then pushes the liquid up into the straw to enter the mouth.
Liquid Pressure:
- Definition: The pressure exerted by liquids.
- Depth Relationship: Liquid pressure increases with the depth of the liquid.
- Bubble Behavior: Bubbles increase in size as they rise because they experience less pressure acting on them as depth reduces.

Definition of Work Done: Work done refers to the measure of energy transfer that occurs when an object is moved over a distance by an external force applied in the direction of displacement.
Conditions for Work to be Done:
1. A force must be exerted.
2. The object must move in the direction of the applied force.
Formula:
- $\text{Work done} = \text{force} \times \text{distance}$
- Force is measured in Newton ( $N$ ) and distance in meters ( $m$ ).

Definition of Energy: Energy is the capacity or ability to do work.
SI Unit: Joules ( $J$ ).
Energy Stores:
- Chemical Potential Energy: Energy stored in chemical bonds of substances; can be released through chemical reactions.
- Gravitational Potential Energy (GPE):
  - Description: Energy stored in an object as a result of its position.
  - Formula: $\text{mass} \times \text{gravitational force} \times \text{height}$
  - Mass is in $kg$ , gravitational force in $N$ , and height in $m$ .
- Elastic Potential Energy: Energy stored in an elastic object when it is stretched or compressed.
- Kinetic Energy (KE):
  - Description: Energy stored associated with moving objects.
  - Formula: $\frac{1}{2} \times \text{mass} \times \text{velocity}^2$
  - Mass is in $kg$ and velocity is in $m/s$ .
- Internal Thermal Energy: Energy stored in objects as a result of their temperature.
- Nuclear Energy: Energy stored in the nucleus of an atom; released in the fusion of an atomic nucleus.

Fundamental Law: Energy cannot be created or destroyed.
Energy Transfer: Energy can only be transferred from one energy store to another.
System Integrity: The total amount of energy in a closed system always remains the same.

Question: What is meant by gravitational field strength?
Contextual definition: It refers to the force of gravity that acts per unit mass on an object within its gravitational field.