Moving Things — Force, Motion, Friction & Pressure | CTET Science P2

Types of Motion

Motion is the change in the position of an object with respect to a reference point over time. An object is said to be at rest when its position does not change, and in motion when its position changes.

There are several types of motion:

• Rectilinear (linear) motion: movement along a straight line. Example: a car moving on a straight road, a ball falling vertically.
• Circular motion: movement along a circular path. Example: a stone tied to a string and whirled, the Earth revolving around the Sun.
• Rotational motion: spinning of an object about its own axis. Example: a spinning top, the Earth rotating on its own axis.
• Periodic motion: motion that repeats itself at regular intervals of time. Example: a pendulum, vibrating strings of a guitar, the Earth's revolution around the Sun.
• Oscillatory (vibratory) motion: a type of periodic motion in which an object moves to and fro about a fixed position. Example: a swing, a pendulum, the piston in an engine.
• Random motion: motion with no fixed path or direction. Example: the Brownian motion of gas molecules, the movement of a butterfly.

An object can exhibit more than one type of motion simultaneously. For example, a wheel rolling along a road undergoes both rotational and rectilinear motion, while the hands of a clock show circular motion.

The concept of a reference frame is crucial: an object's motion is described relative to a chosen reference point. A passenger sitting in a moving train is at rest relative to the train but in motion relative to a person standing on the platform.

For CTET, be able to classify examples of motion from everyday life and understand that motion is always relative — a key idea for upper-primary students to grasp through observation activities.

Speed, Velocity and Acceleration

Speed is the distance covered by an object per unit time. It is a scalar quantity (only magnitude, no direction).
Speed = Distance / Time. SI unit: metre per second (m/s).

Average speed = Total distance / Total time taken. This is used when speed varies during a journey. A speedometer in a vehicle measures instantaneous speed (speed at a particular moment). An odometer measures the total distance covered by the vehicle — it does not measure speed or acceleration.

Velocity is the displacement (change in position in a specific direction) per unit time. It is a vector quantity (both magnitude and direction).
Velocity = Displacement / Time. SI unit: m/s.

An object can have constant speed but changing velocity if its direction changes — for example, a car moving at constant speed in a circle has uniform speed but changing velocity (because direction continuously changes), which means it is accelerating.

Acceleration is the rate of change of velocity. It is also a vector quantity.
Acceleration = (Final velocity − Initial velocity) / Time = (v − u) / t. SI unit: m/s².

• Positive acceleration: velocity is increasing (speeding up).
• Negative acceleration (retardation/deceleration): velocity is decreasing (slowing down).
• Uniform acceleration: velocity changes by equal amounts in equal time intervals.

Three equations of motion (for uniform acceleration):

• v = u + at
• s = ut + ½at²
• v² = u² + 2as

(where u = initial velocity, v = final velocity, a = acceleration, t = time, s = displacement)

Distance–time graphs and speed–time graphs are important tools. A straight horizontal line on a speed–time graph indicates constant speed (zero acceleration); a sloping line indicates acceleration.

Force and Its Effects

A force is a push or pull that can change the state of rest or motion of an object, change its speed or direction, or change its shape. Force is a vector quantity. SI unit: Newton (N). 1 Newton = 1 kg·m/s².

Effects of force:

• A force can set a stationary object in motion (e.g., kicking a football).
• A force can bring a moving object to rest (e.g., brakes on a bicycle).
• A force can change the speed of a moving object (e.g., pressing the accelerator).
• A force can change the direction of a moving object (e.g., turning a steering wheel).
• A force can change the shape of an object (e.g., pressing clay, stretching a rubber band).

Newton's Laws of Motion:

• First Law (Law of Inertia): Every object remains at rest or in uniform motion in a straight line unless acted upon by an external unbalanced force. Inertia is the tendency to resist change in state. Greater mass = greater inertia.
• Second Law: The rate of change of momentum (or acceleration) of an object is directly proportional to the applied force and in the direction of the force. F = ma. This explains why heavy trucks need more force than cars to accelerate at the same rate.
• Third Law: For every action there is an equal and opposite reaction. Example: a rocket expels gas downwards (action); the rocket moves upward (reaction).

Balanced and unbalanced forces: When forces acting on an object cancel each other out (net force = 0), they are balanced and no change in motion occurs. Unbalanced forces produce acceleration.

Gravitational force: the attractive force between masses. Weight (W = mg) is the gravitational force Earth exerts on an object; it varies with location. Mass remains constant everywhere.

Magnetic and electrostatic forces can act at a distance without contact — these are called non-contact forces. Muscular force, friction, and normal reaction are contact forces.

Friction — Causes and Consequences

Friction is the force that opposes the relative motion (or tendency of motion) between two surfaces in contact. It acts opposite to the direction of motion.

Causes of friction: irregularities (roughness) on the surfaces of objects interlocking with each other. Even surfaces that appear smooth have microscopic asperities. The nature and texture of the surfaces, and the force pressing them together (normal force), determine the magnitude of friction.

Types of friction:

• Static friction: the friction that prevents a stationary object from starting to move. It adjusts itself up to a maximum value (limiting friction). Static friction is generally greater than kinetic friction.
• Kinetic (sliding) friction: friction between surfaces that are in relative motion. Example: a book sliding on a table.
• Rolling friction: occurs when an object rolls over a surface. It is much less than sliding friction, which is why wheels and ball bearings are used.
• Fluid friction (drag): friction experienced by objects moving through fluids (liquids and gases). Streamlined shapes reduce fluid friction — used in the design of ships, aircraft, and fish bodies.

Advantages of friction:

• Walking is possible because of friction between feet/shoes and the ground.
• Brakes on vehicles rely on friction to stop.
• We can write on paper, hold objects, and strike matches because of friction.

Disadvantages of friction:

• Friction causes wear and tear of machine parts, tyres and soles of shoes.
• It generates unwanted heat, reducing efficiency of engines.
• It slows down moving parts.

Reducing friction: lubricants (oil, grease) are applied between moving parts; smooth surfaces reduce friction; rollers and ball bearings replace sliding contact with rolling contact; streamlining reduces fluid drag.

Increasing friction: grooving on tyres, treads on shoe soles, using sand on icy roads, and applying chalk on hands for grip in sports.

Pressure in Solids and Fluids

Pressure is the force applied per unit area. P = F / A. SI unit: Pascal (Pa) = N/m².

For a given force, pressure increases as area decreases and decreases as area increases. This explains why:

• A sharp needle pierces skin easily (small area → high pressure), while a blunt needle does not.
• A school bag with broad straps is more comfortable than one with thin straps (greater area → less pressure on shoulders).
• Camels have large, flat feet to spread their weight over a larger area on sand (reducing pressure so they don't sink).
• Tractors have wide tyres to avoid sinking into soft soil.

Pressure in fluids (liquids and gases):

• Fluids exert pressure in all directions.
• Pressure increases with depth: P = ρgh (where ρ = density of fluid, g = acceleration due to gravity, h = depth). This is why the bottom hole in a water bottle shoots water with greater force and farther distance than the top hole — higher water column above means higher pressure.
• At the same depth, pressure is equal in all directions (Pascal's principle).
• Pascal's Law: pressure applied to an enclosed fluid is transmitted undiminished in all directions. Application: hydraulic brakes, hydraulic lift — a small force on a small piston creates large force on a large piston.

Atmospheric pressure: the weight of the air column above exerts pressure on all surfaces. At sea level it is approximately 101,325 Pa (1 atm). Atmospheric pressure decreases with altitude — that is why mountain climbers use oxygen cylinders.

A barometer measures atmospheric pressure. A manometer measures the pressure of enclosed gases. These instruments and the concept of pressure in fluids are directly tested in CTET Paper 2.

Archimedes' Principle and Buoyancy

When an object is immersed in a fluid (liquid or gas), the fluid exerts an upward force on it called the buoyant force (or upthrust). This is the force responsible for objects appearing lighter in water.

Archimedes' Principle: When a body is wholly or partially immersed in a fluid, it experiences an upward buoyant force equal to the weight of the fluid displaced by the body.

Buoyant force = Weight of fluid displaced = ρ_fluid × V_displaced × g

Conditions for floating and sinking:

• If buoyant force > weight of object → object floats.
• If buoyant force = weight of object → object floats at the surface (in equilibrium).
• If buoyant force < weight of object → object sinks.

A floating object displaces a volume of fluid whose weight equals the object's own weight. Therefore the fraction of the object submerged depends on the ratio of the object's density to the fluid's density.

Comparing densities using floating: If an object floats with only 1/4 of its volume above Liquid A (3/4 submerged), the buoyant force equals the object's weight when 3/4 is submerged — this means Liquid A has a moderate density. If the same object floats with 3/4 above Liquid B (only 1/4 submerged), a much smaller volume of Liquid B is displaced to support the same weight — so Liquid B must be denser. Therefore, Liquid A has greater density than Liquid B.

Applications of Archimedes' Principle:

• Ships: made of steel (denser than water) but hollow, so the average density of the ship + air is less than water → ship floats.
• Submarines: ballast tanks are filled with water to sink and emptied (filled with air) to rise.
• Hydrometers: measure the density of liquids using the principle that a denser liquid pushes the hydrometer up higher.
• Swimming: the human body can float because its average density is close to that of water.

The apparent weight of an object in a fluid = actual weight − buoyant force. This explains why lifting a stone from water feels heavier the moment it clears the water surface.

Potential and Kinetic Energy

Energy is the capacity to do work. Work is done when a force moves an object in the direction of the force. W = F × d. SI unit of energy and work: Joule (J). 1 Joule = 1 Newton × 1 metre.

Kinetic Energy (KE) is the energy possessed by an object due to its motion.
KE = ½mv² (where m = mass, v = velocity). Kinetic energy depends on both mass and the square of velocity — doubling the speed quadruples kinetic energy.

Potential Energy (PE) is the energy stored in an object due to its position or configuration.

• Gravitational PE: energy due to height above a reference level. PE = mgh. A stone held high has more gravitational PE; as it falls, this converts to KE.
• Elastic PE: energy stored in a deformed elastic object — a compressed or stretched spring. The spring has maximum elastic PE when maximally compressed or stretched, and minimum PE (= 0) when it is in its natural (undeformed) state at rest on the ground. A spring resting undeformed on the ground has zero elastic PE and zero gravitational PE — making it the position of least total potential energy.

Law of Conservation of Energy: energy cannot be created or destroyed; it only changes from one form to another. The total mechanical energy (KE + PE) of an object is conserved in the absence of friction and other non-conservative forces.

At the top of a swing, all energy is PE; at the bottom, all is KE. During the fall of a ball, PE converts to KE. When a car brakes, KE converts to heat and sound.

Unit pairs: Work and energy share the unit Joule (J = kg·m²/s²). Force uses Newton (N = kg·m/s²) and Nm (= J). So kg·m/s² and Nm both measure force. Power (energy/time) is measured in Watts (W = J/s = kg·m²/s³). The pair kg·m²/s² and J measure the same quantity (energy/work); kg·m/s² and Nm also measure the same quantity (force).

Simple Machines

A machine is a device that helps us do work more easily by multiplying force, changing the direction of force, or increasing speed. Simple machines are the basic building blocks of all complex machines.

The six classical simple machines:

• Lever: a rigid bar that pivots about a fulcrum. Examples: seesaw, scissors, crowbar, human arm, tongs.
• Wheel and axle: a wheel attached to a smaller axle; turning the wheel applies force to the axle. Examples: steering wheel, screwdriver, tap handle.
• Pulley: a grooved wheel that redirects rope force. A single fixed pulley changes direction; a movable pulley gives mechanical advantage. Examples: flagpole, well bucket, crane.
• Inclined plane: a sloping surface that reduces the force needed to lift objects by increasing the distance. Examples: ramp, staircase, screw thread.
• Wedge: two inclined planes back-to-back; converts a downward force into splitting forces. Examples: knife, axe, chisel, nail.
• Screw: an inclined plane wrapped around a cylinder. Examples: screw fastener, jackscrew, auger.

Key terms:

• Effort (E): the force applied to the machine.
• Load (L): the force (resistance) overcome by the machine.
• Fulcrum: the pivot point of a lever.
• Mechanical Advantage (MA): MA = Load / Effort. MA > 1 means the machine multiplies force.

Classes of levers:

• Class I: fulcrum between effort and load. Example: seesaw, scissors, crowbar.
• Class II: load between fulcrum and effort. Example: wheelbarrow, bottle opener, nutcracker.
• Class III: effort between fulcrum and load. Example: tweezers, broom, human forearm while lifting.

No machine is 100% efficient because friction always causes some energy loss. Efficiency = (useful work output / total work input) × 100%. Understanding simple machines helps upper-primary students see physics in everyday tools, making it an excellent topic for activity-based learning in school.