📖 Chapter Overview
Force is a push or a pull that can change the state of motion, shape, or direction of an object. Pressure is the force acting per unit area. This chapter introduces types of forces (contact and non‑contact), effects of force, pressure in solids and fluids, and atmospheric pressure, with daily‑life applications.
Force (F)
Pressure (P)
Contact forces
Non‑contact forces
Atmospheric pressure
Remember: Greater the area, lesser the pressure for the same force — that’s why snowshoes are wide and knife edges are sharp.
🔑 Key Points
- Force is a push or pull; SI unit: newton (N).
- Effects of force: change in speed, direction, and shape of objects.
- Contact forces: muscular force, friction.
- Non‑contact forces: gravitational, magnetic, electrostatic.
- Pressure (P) = Force/Area; SI unit: pascal (Pa) = N/m².
- Fluid pressure: liquids and gases exert pressure in all directions; increases with depth.
- Atmospheric pressure: air exerts pressure; demonstrated by suction, drinking straw, and barometer.
- Applications: syringes, hydraulic brakes (Pascal’s principle – class intro), wide straps, nails, camels’ broad feet.
📐 Formulae & Units
| Quantity | Symbol | Definition / Relation | SI Unit |
|---|---|---|---|
| Force | F | Push or pull | newton (N) |
| Pressure | P | P = F / A | pascal (Pa) = N/m² |
| Area | A | Surface area on which force acts | m² |
Pascal’s Principle (idea level): Pressure applied to an enclosed fluid is transmitted equally in all directions. (Used in hydraulic lifts and brakes.)
🧪 Everyday Examples
| Situation | Concept | Reason |
|---|---|---|
| Knife cuts better when sharp | High pressure | Small area increases P for same F |
| Camel walks easily on sand | Low pressure | Broad feet increase area, reduce P |
| Drawing ink with a syringe | Fluid pressure | Lower pressure inside, liquid pushed in |
| Suction cup sticking to wall | Atmospheric pressure | Outside air pushes cup against wall |
| Wide school bag straps | Low pressure | Greater area reduces pressure on shoulder |
📚 30 Important Questions & Answers
Q1. Define force.
Force is a push or a pull that can change the state of motion or shape of an object.
Q2. State the SI unit of force.
Newton (N).
Q3. List any two effects of force.
It can change (i) the speed of an object and (ii) its direction or shape.
Q4. What are contact forces?
Forces that act only when bodies are in physical contact, e.g., muscular force, friction.
Q5. Give two examples of non‑contact forces.
Gravitational force and magnetic force.
Q6. Define pressure with formula.
Pressure is force per unit area: P = F / A.
Q7. SI unit of pressure?
Pascal (Pa) which equals N/m².
Q8. Why do sharp nails get driven in easily?
Small tip area gives high pressure for the same hammering force.
Q9. Why are bag straps made wide?
To increase area and reduce pressure on shoulders.
Q10. Name the force opposing motion between surfaces in contact.
Frictional force.
Q11. What type of force is friction—contact or non‑contact?
Contact force.
Q12. Explain why camels can walk on sand easily.
Broad feet increase area, reducing pressure on sand, so feet do not sink much.
Q13. What is atmospheric pressure?
The pressure exerted by the air (atmosphere) on all objects.
Q14. Mention one device that works due to atmospheric pressure.
Drinking straw / suction pump.
Q15. Do liquids exert pressure? In which directions?
Yes. Liquids exert pressure in all directions and increase with depth.
Q16. What happens to pressure if the same force acts on a smaller area?
Pressure increases.
Q17. What is the relation between depth and pressure in a liquid?
Pressure increases with depth.
Q18. Name two examples of non‑contact forces other than gravity.
Magnetic force and electrostatic force.
Q19. Why do tractors have wide rear tyres?
Wide tyres increase area and reduce pressure on soil, preventing sinking.
Q20. What is Pascal’s principle (intro)?
Pressure applied to an enclosed fluid is transmitted equally in all directions.
Q21. Name two applications of Pascal’s principle.
Hydraulic lifts and hydraulic brakes.
Q22. Explain why a thumbtack has a large head.
Large head provides greater area, reducing pressure on your finger while pressing.
Q23. State one difference between contact and non‑contact forces.
Contact forces need physical contact to act; non‑contact forces act at a distance.
Q24. A force of 50 N acts on an area of 0.5 m². Calculate the pressure.
P = F/A = 50 / 0.5 = 100 Pa.
Q25. How does a syringe draw liquid when the plunger is pulled?
Pulling creates lower pressure inside; higher outside pressure pushes liquid in.
Q26. What is the effect of force on a rubber ball when squeezed?
Its shape changes (deformation).
Q27. Name the force responsible for falling of an apple from a tree.
Gravitational force.
Q28. Give one example showing that gases exert pressure.
An inflated balloon is taut because air inside exerts pressure on its walls.
Q29. State the relationship between pressure and area for constant force.
Pressure is inversely proportional to area (P ∝ 1/A).
Q30. Why is it easier to cut vegetables with a sharp knife than a blunt one?
A sharp edge has smaller area; for the same force, pressure is higher, making cutting easier.