Science: Class 7 — Chapter 8: Force and Pressure

8.1 Force – A Push or a Pull

Force: A push or a pull on an object.
Actions: Picking, opening, shutting, kicking, hitting, lifting, flicking, pushing, pulling are common actions.
Effect of Actions: These actions usually result in a change in the state of motion of an object.
Table 8.1 Examples: Identifying actions as push or pull (e.g., moving a book - push, opening a door - push/pull, drawing water - pull).

8.2 Forces are due to an Interaction

Interaction required: A force comes into play due to the interaction between at least two objects.
Example: A man pushing a car (interaction between man and car).
Mutual Interaction: Forces are applied on each other (e.g., girls pushing/pulling each other, cow and man pulling each other).

8.3 Exploring Forces

Adding forces: When forces are applied in the same direction, they add up.
Subtracting forces: When forces are applied in opposite directions, the net force is the difference between them.
Tug-of-war analogy: If teams pull equally hard, the rope doesn't move; the team pulling harder wins.
Magnitude and Direction: The strength of a force is expressed by its magnitude, and its direction must also be specified.
Net Force: The effect on an object is due to the net force acting on it, even if multiple forces are involved.

8.4 A Force can Change the State of Motion

Changing Speed: A force can increase or decrease the speed of a moving object.
Direction of Motion: A force can change the direction of motion of an object.
State of Motion: A change in speed or direction of motion, or both, is a change in the state of motion.
Rest is a state of motion: An object at rest has zero speed.
Limitations: Force may not always result in a change in the state of motion (e.g., pushing a heavy wall).

8.5 Force can Change the Shape of an Object

Examples: Dough, spring, rubber band, scale can change shape when force is applied.
Effects of Force Summary:
- Make an object move from rest.
- Change the speed of a moving object.
- Change the direction of motion.
- Change the shape of an object.
- Cause some or all of these effects.
Necessity of Force: None of these effects can happen without the action of a force.

8.6 Contact Forces

Muscular Force

Definition: Force resulting from the action of muscles.
Requirement: Usually requires direct contact with the object.
Examples: Lifting, pushing, bending the body; digestion, breathing; animals pulling loads.
Contact Force: Also known as a contact force because it requires contact.

Friction

Cause of slowing down: The force responsible for slowing down moving objects (e.g., rolling ball, bicycle, boat).
Direction: Always acts opposite to the direction of motion.
Contact requirement: Arises due to contact between surfaces.
Contact Force: An example of a contact force.

8.7 Non-contact Forces

Magnetic Force

Interaction without contact: Force exerted by magnets on each other or on magnetic materials without touching.
Examples: Attraction and repulsion between like/unlike poles; magnet attracting iron.
Non-contact Force: Force that acts without direct physical contact.

Electrostatic Force

Charged bodies: Force exerted by charged bodies on other charged or uncharged bodies.
Examples: Rubbed straw attracting/repelling another straw.
Non-contact Force: Acts even when bodies are not in contact.

Gravitational Force

Attraction to Earth: Objects fall towards Earth due to its pull (gravity).
Universal Force: Every object in the universe exerts this force on every other object.
Examples: Falling coin, leaves, fruits; water flow in rivers.
Attractive Force: Always pulls objects towards each other.

8.8 Pressure

Definition: Force acting on a unit area of a surface.
Formula: pressure = force / area
Area effect: Smaller area leads to larger pressure for the same force.
Examples:
- Pushing a nail (pointed end vs. head).
- Cutting with sharp vs. blunt knife.
- Porters placing cloth on heads to increase area and reduce pressure.
- Shoulder bags with broad straps.

8.9 Pressure Exerted by Liquids and Gases

Liquids

Pressure at bottom: Depends on the height of the liquid column (Activity 8.8).
Pressure on walls: Liquids exert pressure on the sides of the container (Activity 8.9).
Equal pressure at same depth: Liquids exert equal pressure at the same depth (Activity 8.10).

Gases

8.10 Atmospheric Pressure

Atmosphere: The envelop of air extending many kilometres above the Earth's surface.
Definition: The pressure exerted by atmospheric air.
Magnitude: Significant force due to the weight of the air column.
Rubber sucker example: Stays stuck due to atmospheric pressure acting on it.
Balancing pressure: Internal body pressure balances external atmospheric pressure, preventing crushing.
Otto von Guericke experiment: Demonstrated the immense force of air pressure using hemispheres and horses.

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