CBSE Class 8 > Class 8 Notes > Science Curiosity - New NCERT > Cheat Sheet: Exploring Forces
Cheat Sheet: Exploring Forces
1. Force - Definition & Properties
1.1 Definition
| Term | Definition |
|---|---|
| Force | Push or pull applied to an object resulting from interaction with another object |
1.2 Effects of a Force
- Start or move an object at rest
- Stop or slow down a moving object
- Change the speed of a moving object
- Change the direction of motion
- Change the shape of an object
1.3 Interaction and Units
| Term | Definition |
|---|---|
| Interaction | Every force involves two interacting objects |
| Force pairs | Applying a force produces a force back on the actor; both disappear when interaction stops |
| SI unit | Newton (N) |
| One newton | Force that gives 1 kg mass acceleration of 1 m/s² |
2. Types of Forces
2.1 Classification
- Contact forces
- Non-contact forces
2.2 Contact Forces
| Term | Definition |
|---|---|
| Muscular force | Force applied by action of muscles in humans and animals |
2.2.1 Friction
- Definition: Force opposing motion between surfaces in contact
- Cause: Arises from tiny irregularities on surfaces
- Direction: Acts opposite to direction of motion
- Surface effect: Rougher surfaces produce greater friction
- Fluid friction: Air and water produce drag or air resistance
- Streamlining: Reduces fluid friction
- Contact nature: Acts only when surfaces are in contact
2.3 Non-contact Forces
| Term | Definition |
|---|---|
| Magnetic force | Force exerted by a magnet on magnetic materials or another magnet at a distance |
| Magnetic poles | Like poles repel; unlike poles attract |
| Electrostatic force | Force exerted by a charged body on another charged or uncharged body |
| Static charges | Electrical charges produced on surfaces by rubbing certain materials |
| Charge interaction | Like charges repel; unlike charges attract |
| Electric current | Movement of charges that constitutes an electric current |
| Gravitational force | Force with which Earth attracts objects towards itself |
| Gravity property | Always attractive and acts between objects with mass |
| Vertical motion under gravity | Upward motion slows then reverses; downward motion accelerates |
3. Mass, Weight and Measurement
3.1 Mass versus Weight
| Term | Definition |
|---|---|
| Mass | Amount of matter in an object |
| Mass unit | Gram (g) or kilogram (kg) |
| Mass property | Remains the same at all places |
| Weight | Force with which Earth pulls an object |
| Weight unit | Newton (N) |
| Weight formula | W = m × g with g = 9.8 m/s² |
| Weight variability | Weight changes with location if gravitational acceleration changes |
3.2 Measuring Weight with a Spring Balance
- Spring balance: spring fixed at one end with hook at other
- Reading: pointer indicates force (weight) on scale
- Scale markings: often in newtons and in grams/kilograms for Earth
- Range: maximum weight measurable by the balance
- Least count: smallest difference measurable; equal to difference between two major marks divided by number of divisions
- Procedure: hang object from hook and read pointer without exceeding range
- Mass reading note: mass scale assumes Earth's gravitational acceleration
3.3 Variation of Weight on Different Planets (for mass = 1 kg)
| Planet | Weight for 1 kg (N) |
|---|---|
| Earth | 10 |
| Moon | 1.6 |
| Mars | 3.8 |
| Venus | 9 |
| Jupiter | 25.4 |
4. Buoyancy and Archimedes
4.1 Upthrust (Buoyant Force)
| Term | Definition |
|---|---|
| Upthrust | Upward force exerted by a liquid on an immersed object |
4.2 Floating and Sinking
- Sinks if weight of object is greater than buoyant force
- Floats if buoyant force equals weight of object
- Density of liquid affects buoyant force
4.3 Archimedes' Principle
| Term | Definition |
|---|---|
| Archimedes' Principle | Upthrust on an immersed object equals weight of liquid displaced by the object |
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