We have covered the truth tables in class for:
AND OR and NOT gates
Here they are to refresh your memory
Look at this. Can you write a truth table for this circuit?
See me for the solution
A woman goes back to work after forty years. Here’s a change from kicking balls off cliffs – for a four-second laugh – no physics, I promise, check this out…
By way of explanation, old-fashioned typewriters had something called a ‘carriage return’ which you pushed when you wanted a new line of text…
It really is. Various international educators are in agreement that testing is a great way not just to learn new stuff but to apply what you know. Year 10 did their CATS tests recently, I took a few of these, just for fun. Why not try a few of them to see how you match up.
Sisyphus was famed as the craftiest of men. He was nasty, deceitful and greedy. As a punishment from the gods for his trickery, Sisyphus was made to push a huge rock up a steep hill, but before he could reach the top of the hill, the rock would always roll back down, forcing him to begin again – an endless cycle of energy conservation. The gravitational potential energy gained by pushing the rock up the hill was converted to kinetic energy as the rock rolled down again.
Notice that Sisyphus had to push the stone up a hill. The VERTICAL height of the hill is used to calculate the gain in gravitational potential energy, not the length of the slope of the hill.
Next: conserving E, cancelling m gives the expression to show how fast the stone would be
going if it was allowed to fall freely (forget about rolling)
You might think about how much higher the hill would have to be to cause the stone to be going twice as fast at the bottom – this kind of thing crops up in MC all the time.
My ‘hill’ in the diagram is a straight line. In terms of energy conservation, would the shape of the hill matter? The answer is ‘no’, but can you explain why?
The original track to the hilltop fortress of Masada in Israel is shaped like this.
You might like to reflect on this…
Work done = energy transformed (against gravity) = applied force x distance moved in the direction of the force.
All life on earth gets its energy from the sun. Plants and animals can store energy and some of this energy remains with them when they die. It is the remains of these ancient animals and plants that make up fossil fuels.
Fossil fuels are non-renewable energy resources and will one day run out and we can’t replace them. Burning fossil fuels generates polluting greenhouse gases and we mustn’t continue to rely on them to make the energy we need.
Renewable or infinite energy resources are sources of energy that can be used again and again.
Some resources can be thought of as both renewable and non-renewable.
Over the last 200 years most of our energy has come from non-renewable sources such as oil and coal.
Non-renewable energy resources
|Type of fuel||Where it is from|
|Coal (fossil fuel)||
|Oil (fossil fuel)||
|Natural gas (fossil fuel)||
How long will fossil fuels last?
If we all continue to burn fuels “like there’s no tomorrow”, oil and gas reserves may run out within our lifetimes. Coal is expected to last a little bit longer.
Estimated length of time left for fossil fuels
|Fossil fuel||Time left|
|Natural gas||70 years|
The centre of mass or is the mean or average location of all the mass in a system. In the case of a rigid body the position of the centre of mass is fixed in relation to the body. For example the centre of mass of a pool ball is exactly in the middle of it. In the case of a loose distribution of masses in free space such as pellets scattered from a shotgun or the planets of the solar system the position of the centre of mass is a point in space among them that may or may not correspond to the position of any individual mass.
The concept is useful since sometimes we want to know where exactly to apply a force on a body.
The centre of gravity of a body corresponds to the point where all the gravitational force acts, in other words the single point through which the resultant of the gravitational forces on the component particles of the body acts.
Homogeneous objects. Think about where the c.g of objects like these might lie
Non- homogeneous objects – a reminder about stable equilibrium and toppling (IGCSE)
For practical purposes, the centre of gravity corresponds exactly to the centre of mass because the gravitational field of the Earth (the gravitational force exerted on every kilogram of mass) is the same at 9.81N kg-1 close to the Earth’s surface
Resistance is a property of a DEVICE or a COMPONENT. like a lamp, a resistor, a thermistor, a diode and so on. Its value depends on four things.
1. What it’s made of. Metals are good conductors so have lower resistance than an insulator of the same dimensions.
2. Length – the longer, the greater resistance.
3. Area of cross section – the larger the lower.
4. Temperature (metals: higher temperature = higher resistance because the vibration of the metal lattice impedes the drift of the electrons, semiconductors: like thermistors , resistance decreases with temperature)
Variable resistors or potentiometers control the length of wire we introduce into a circuit – the volume control on a hi-fi
Thermistors have a high resistance in the cold and a low resistance in the warm. They are used in logic circuits to turn heaters on and off.
LDR’s (light dependent resistors have a high resistance in the dark but a low resistance in the light. As darkness falls, they are used in logic circuits to turn on street lights.
A diode is an electrical one way street, which is useful sometimes in electronic circuits. Current is allowed through a low resistance pathway in one direction, the resistance is very high in the reverse direction so no current flows.