Grade 7 Chapter 7: Motion, Force and Work

Introduction to the Physical World

The world around us is filled with constant activity. From a gentle breeze to a soaring aeroplane, everything is in motion. To truly understand how and why things move, we delve into three fundamental concepts in physics: Motion, Force, and Work. These concepts are interconnected; forces cause motion, and when motion occurs due to a force, work is done. This chapter will provide a detailed exploration of these core ideas, illustrated with everyday examples.

Understanding Motion

Motion is fundamentally defined as the change in the position of an object over a period of time, relative to a fixed point or object, known as a reference point. If an object's position changes with respect to its surroundings, it is said to be in motion.

Detailed Types of Motion

1. Linear Motion (Rectilinear Motion):
   • Definition: This is the simplest type of motion where an object moves along a straight line path. The direction of motion remains constant.
   • Examples:
     • A car travelling on a perfectly straight highway.
     • A ball rolling on a flat, straight surface without veering.
     • An object falling vertically downwards under gravity (ignoring air resistance).
     • A person walking in a straight line.
2. Non-linear Motion:
   • Definition: Any motion that does not follow a straight line path. The direction of motion continuously changes.
   • Sub-types of Non-linear Motion:
     • Circular Motion:
       • Definition: Motion of an object along the circumference of a circle or a circular path. The object's distance from a fixed center point remains constant.
       • Examples:
         • The blades of a rotating fan.
         • A merry-go-round in operation.
         • An athlete running on a circular track.
         • The motion of Earth around the Sun (approximately circular).
     • Oscillatory Motion (Vibratory Motion):
       • Definition: A repetitive back-and-forth (to and fro) movement of an object about a fixed central position (mean position). This motion is often characterized by a regular swing.
       • Examples:
         • A swinging pendulum in a clock.
         • A child on a swing moving back and forth.
         • The strings of a guitar or violin vibrating when plucked.
         • The balance wheel in a watch.
     • Random Motion:
       • Definition: Motion that has no definite pattern or predictable path. The direction and speed of the object change irregularly and unpredictably.
       • Examples:
         • A butterfly flying erratically from flower to flower.
         • The movement of gas molecules in a container (Brownian motion).
         • The chaotic movement of a crowd of people.
3. Periodic Motion:
   • Definition: Any motion that repeats itself at regular intervals of time. This means the object returns to its initial position and velocity after a fixed period.
   • Examples:
     • The movement of the hands of a clock.
     • The revolution of Earth around the Sun (completes in approximately 365 days).
     • The swinging of a pendulum (as it repeats its swing in a fixed time).
     • The heartbeat of a person.
   • Important Note: While all oscillatory motions are periodic (as they repeat), not all periodic motions are oscillatory. For instance, the Earth's revolution around the Sun is periodic but not oscillatory. A fan's rotation is also periodic but not oscillatory.

Quantifying Motion: Speed and Velocity

To describe motion more precisely, we use quantities like speed and velocity.

• Speed:
  • Definition: Speed is a scalar quantity that measures how fast an object is moving. It is the rate at which an object covers distance. It only considers the magnitude (amount) of movement, not its direction.
  • Formula: Speed = $\frac{\text{Distance Covered}}{\text{Time Taken}}$
  • SI Unit: metre per second (m/s). Other common units include kilometres per hour (km/h).
  • Example: If a car travels 100 km in 2 hours, its speed is 50 km/h. We don't know its direction.
• Velocity:
  • Definition: Velocity is a vector quantity that measures both how fast an object is moving and in what specific direction it is moving. It is the rate of change of displacement.
  • Formula: Velocity = $\frac{\text{Displacement}}{\text{Time Taken}}$
  • SI Unit: metre per second (m/s). The unit is the same as speed, but direction must always be specified for velocity.
  • Example: If a car travels 100 km to the East in 2 hours, its velocity is 50 km/h East.

Exploring Force

A force is fundamentally a push or a pull exerted on an object. Forces are what cause changes in motion, initiate motion, stop motion, change the direction of motion, or even change the shape of an object.

Detailed Types of Forces

1. Muscular Force:
   • Definition: This force is generated and exerted by the contraction of muscles in living organisms, including humans and animals.
   • Examples:
     • A person lifting a heavy box.
     • A horse pulling a cart.
     • Kicking a football.
     • Walking, running, or jumping.
     • Chewing food.
2. Gravitational Force (Gravity):
   • Definition: This is the universal force of attraction between any two objects with mass. On Earth, it is the force that pulls objects towards the Earth's center. It is responsible for objects falling downwards.
   • Examples:
     • An apple falling from a tree to the ground.
     • Raindrops falling from the sky.
     • The Earth orbiting the Sun and the Moon orbiting the Earth.
     • Objects having weight.
3. Elastic Force:
   • Definition: The restoring force exerted by a material when it is deformed (stretched, compressed, bent, or twisted) that attempts to return the material to its original shape and size.
   • Examples:
     • Stretching a rubber band, and it pulls back to its original length.
     • Compressing a spring, and it pushes back.
     • A trampoline bouncing a person upwards.
     • A slingshot propelling a stone.
4. Friction Force:
   • Definition: A force that opposes the relative motion or tendency of motion between two surfaces that are in contact with each other. It always acts in the opposite direction to the motion.
   • Examples:
     • A ball rolling on the ground eventually slowing down and stopping.
     • The resistance you feel when pushing a heavy box across a rough floor.
     • The force that allows cars to stop when brakes are applied.
     • Rubbing your hands together to create warmth.
     • Walking without slipping (friction between shoes and ground).
5. Mechanical Force:
   • Definition: Force applied or generated by mechanical devices or machines. It often involves the transfer of force through levers, gears, pulleys, etc.
   • Examples:
     • The force exerted by a crane to lift heavy loads.
     • The force generated by a hydraulic jack to lift a car.
     • The force exerted by a blender or mixer on food.
     • The force from a bicycle's chain driving the wheels.

Understanding Work in Physics

In everyday language, "work" refers to any activity that requires effort. However, in physics, the term Work has a very specific and precise definition. Work is done only when a force causes a displacement of an object in the direction of the force.

Conditions for Work to be Done (Scientifically):

For work to be considered done in a scientific sense, two conditions must be met simultaneously:

1. A Force Must Be Applied: There must be an external push or pull acting on the object.
2. The Object Must Be Displaced: The object must move from its initial position to a new position as a direct result of the applied force. Furthermore, the displacement must have a component in the direction of the applied force.

If either of these conditions is not met, no work is done, regardless of the effort expended.

Examples Illustrating Work Done vs. No Work Done:

• Examples of Work Done:
  • Pushing a box across the floor: You apply a force, and the box moves in the direction of your push.
  • Lifting a book from a table: You apply an upward force, and the book moves upwards.
  • Pulling a trolley: You apply a force, and the trolley moves.
  • A horse pulling a cart: The horse applies force, and the cart moves.
• Examples of No Work Done (Scientifically):
  • Pushing a wall that does not move: A force is applied, but there is no displacement, so no work is done on the wall.
  • Standing still holding a heavy bag: You are applying an upward force to support the bag, but if you are stationary, the bag is not being displaced. Therefore, no work is done on the bag by you.
  • A satellite orbiting Earth: The gravitational force acts towards the center of Earth, but the satellite's displacement is tangential to the circle. Since the force is perpendicular to the displacement, no work is done by gravity on the satellite (in an ideal circular orbit).

Units of Work

• SI Unit: The SI unit of work is the Joule (J).
• Definition of Joule: One Joule of work is done when a force of one Newton (N) displaces an object by one metre (m) in the direction of the force.

The Concept of Energy

Energy is closely related to work. In simple terms, energy is defined as the capacity or ability to do work.

• When work is done, energy is transferred from one object to another or converted from one form to another.
• Objects possess energy due to their state or position (e.g., potential energy) or due to their motion (e.g., kinetic energy).
• Energy exists in various forms:
  • Kinetic Energy: Energy possessed by an object due to its motion (e.g., a moving car, a flying bird).
  • Potential Energy: Energy stored in an object due to its position or state (e.g., a book on a high shelf, a stretched spring).
  • Heat Energy: Energy associated with the random motion of atoms and molecules.
  • Light Energy: A form of electromagnetic radiation that we can see.
  • Chemical Energy: Energy stored in the bonds of chemical compounds.
  • Electrical Energy: Energy associated with the flow of electric charge.
  • Sound Energy: Energy produced by vibrations.
• The principle of conservation of energy states that energy cannot be created or destroyed, only transformed from one form to another.

Summary of Key Concepts

• Motion: Change in position of an object over time relative to a reference point.
  • Types: Linear (straight line), Non-linear (Circular, Oscillatory, Random), Periodic (repeats over time).
  • Speed: Distance covered per unit time (scalar). Speed = Distance / Time.
  • Velocity: Displacement per unit time in a given direction (vector). Velocity = Displacement / Time.
• Force: A push or pull that can change an object's state of motion, direction, or shape.
  • Types: Muscular, Gravitational, Elastic, Friction, Mechanical.
• Work: Done only when a force causes displacement of an object in the direction of the force.
  • SI Unit: Joule (J).
  • No displacement = No work.
• Energy: The capacity to do work. Energy can be transferred or transformed.