Force and Pressure — Class 8 Science (Samacheer Kalvi)
TN State Board (Samacheer Kalvi) Class 8 Science, Physics — Chapter 2. Pushes, pulls, pressure and the properties of liquids.
1. About this chapter
This chapter covers force and its effects, friction, pressure in solids and liquids, atmospheric pressure, Pascal's law, and two liquid properties — viscosity and surface tension.
2. Force and friction
- A force is a push or a pull; it can change the state of motion, speed, direction or shape of a body.
- Contact forces: muscular force, friction. Non-contact forces: gravitational, magnetic, electrostatic.
- Friction opposes motion between surfaces. Types: static, sliding and rolling friction — and rolling friction is slightly less than sliding friction (so wheels and ball bearings reduce friction).
3. Pressure
- Pressure = Force / Area → P = F / A. Its unit is the pascal (Pa) = N m⁻².
- The same force over a smaller area gives greater pressure (a sharp knife cuts better).
4. Pressure in liquids and the atmosphere
- The pressure of a liquid column increases with the depth and with the density of the liquid; liquids exert pressure in all directions.
- Atmospheric pressure at sea level = 76 cm of mercury column.
- Pascal's law: pressure applied to an enclosed fluid is transmitted equally in all directions — used in the hydraulic lift, brake system and pressing heavy bundles.
5. Viscosity and surface tension
- Viscosity is the resistance of a liquid to flow. Thick liquids like grease and honey have high viscosity; water has low viscosity.
- Surface tension is the property of a liquid surface that makes it behave like a stretched film. It lets water droplets rise in narrow tubes and helps water move upward in plants.
6. Worked examples
Example 1. A force of 100 N acts on an area of 2 m². Find the pressure. P = F/A = 100/2 = 50 Pa.
Example 2. Why do school bags have wide straps? Wide straps increase the area, reducing the pressure on the shoulders.
Example 3. Why does a dam have a thicker wall at the bottom? Because liquid pressure increases with depth, so the base bears more pressure.
7. Book-back questions (Samacheer Kalvi)
I. Choose the correct answer
- If we apply a force against the direction of motion of a body, it will — (a) stop moving / (b) speed up / (c) change shape. Ans: (a) stop moving.
- The pressure exerted by a liquid increases with — (a) area / (b) colour / (c) the density and the height of the liquid column. Ans: (c).
- The unit of pressure is — (a) pascal / (b) N m⁻² / (c) both pascal and N m⁻². Ans: (c).
- The atmospheric pressure at sea level is — (a) 76 cm of mercury / (b) 36 cm / (c) 100 cm. Ans: (a) 76 cm of mercury.
- Pascal's law is used in — hydraulic lift, brake system, pressing heavy bundles → (d) all the above.
II. Fill in the blanks 6. The pressure of a liquid column increases with the depth of the column. 7. Hydraulic lift works under the principle of Pascal's law. 8. The property of surface tension of a liquid surface helps water move upward in plants.
III. True or False 9. Force acting on a given area is called pressure. — True. 10. Rolling friction is slightly greater than sliding friction. — False (it is slightly less).
8. Common mistakes
- Mistake: Thinking a larger force always means larger pressure. Fix: Pressure also depends on area (P = F/A).
- Mistake: Saying rolling friction is greater than sliding friction. Fix: Rolling friction is less than sliding friction.
- Mistake: Confusing viscosity and surface tension. Fix: Viscosity = resistance to flow; surface tension = the stretched-film property of the surface.
9. Quick revision
- Physics Ch 2 · force, friction, pressure, liquids.
- Force changes motion, speed, direction or shape; friction: static, sliding, rolling (rolling < sliding).
- P = F/A (pascal = N m⁻²); liquid pressure rises with depth and density.
- Atmospheric pressure = 76 cm of mercury; Pascal's law → hydraulic lift/brakes.
- Viscosity = resistance to flow (grease high); surface tension lifts water in plants.
