What is ‘Moment of a force about a point’? ‐ The turning effect it causes on the body about the pivot.
Principle of moments
- The sum of clockwise moments about a point is equal to the sum of anti‐clockwise moments about the same point,
- Moment = Force x Perpendicular distance from pivot.
Center of Gravity/Mass
‐ The point which the weight of the object appears to act.
Why constant temperature at melting?
- During melting, heat energy is used to weaken the attraction between the solid particles so that they can roll and slide over one another and become liquid particles.
- The heat energy is not used to increase the Kinetic energy of particles.
Note: Convection occurs when there is difference in density Radiation occurs when there is difference in surrounding temperatures.
When a thermometer is placed into hot water, why the reading drops first then increases. ‐ The glass bulb expands before the mercury expands.
Energy changes of a oscillating pendulum as it comes to rest. ‐ Due to friction and air resistance, kinetic energy will be changed to heat energy.
♂Precautions when handling radioactive substances
1) Radioactive substances should not be touched.
2) Do not point their rays at any person.
3) Wear a photosensitive badge to check whether a person is exposed to an overdose of radiation.
♂Operation of Transformer
1. Alternating current is supplied to primary coil.
2. Alternating current causes iron core to magnetise and magnetic field to change.
3. Magnetic fields follow iron core and link to secondary coil.
4. Hence emf is induced in secondary coil by electromagnetic induction.
1. Turn the wire.
2. The wire cuts through the magnetic field as it turns.
3. This produces an induced emf.
4. Because of the load, an induced current is produced.
Note: An image that is real is always inverted!
An image that is Virtual is always upright! (REAL – INVERTED) (VIRTUAL – UPRIGHT)
Note: If an object is moving at constant speed, the resultant force is zero!
If a car is driving around a circle at a constant speed, its velocity is not constant! *The Car is accelerating! (Because Velocity has magnitude and direction. When the car is turning around the circle, its direction changes constantly)
Remember: Speed has magnitude only, Velocity has magnitude and direction.
*Acceleration is the rate of change of velocity. �� A change in velocity is acceleration.
- If the metal sheet can be balanced on the tip of the finger directly below point C, then it is the center of gravity.
How to measure period of a pendulum?
- Measure the time ‘t’ for 20 oscillations with a stop‐watch
- Calculate the average time for 1 complete oscillation, T= t/20
Extra info: When the current in the electric circuit increases, it’s temperature increases, and then it will cause its resistance to increase.
Examples of Energy Changes
- Kinetic to Gravitational Potential Energy (GPE) ‐ A man running up a flight of stairs.
- Chemical to Heat ‐ A lighted Candle
- Gavitational Potential Energy (GPE) to Kinetic Energy ‐ A car speeding down a hill.
Why no convection current if heated from top? ‐ The hot less dense water remains at the top while the cold denser water remains at the bottom. Hence no convection current is produced.
Water
Heater
