Im trying to revise for higher physics so im doing past papers, currently the 2008 paper. most of the question are okay but some Im not sure how to do, even after looking at the answers page. I need worked answers please!

So if anyone has done it and knows how to get the answers to:

-Q4

-Q21c)ii)

-Q22

-Q25b)ii)&iii)

I don't know why Im having trouble with them, Im usually quite good at physics... honest!

Thanks

## Physics Paper 2008. I need help! ...please.

### Cognitive

Posted 08 April 2009 - 03:20 PM

Question 4:

You can apply the conservation of energy. Assume ALL the

Question 21C (ii):

What I would do is work out the current in the circuit using P=IV and the values for the LED. You know 5V of the supply is dropped across the LED, so the voltage across the resistor must be 12-5=7V. You can use this voltage and the current you got earlier to work out the resistance using V=IR.

Question 22:

a) i) The easiest way to do this is to remember that the component of weight down a slope is mgsinθ where θ is the angle the slope makes with the ground (30 degrees in this case). Our teacher taught us "slidey siney" to remember it,lol, it's kinda crap and cheesy, but it works! So basically, you need to work out the weight of the mass using W=mg, put this value into mgsinθ et voila!

ii) You (should) now know the component of weight down the slope, and you also know the force the crate is being pulled with. Note the question says the crate is moving at a constant speed - the forces are balanced. So you need to find the difference between the pulling force, and the component of weight (i.e. 240-196=44 N).

b) i) Can't really say much for this, other than look at what the answers says - the 6ms^-2 comes from the gradient of the v/t graph from 0 to 0.5 seconds.

ii) You need to find the gradient of the v/t graph from from 0-0.5s and from 0.5-1s. This gives you the magnitude of the acceleration. You should remember that your lines on the a/t graph should be below the x-axis as the gradients are negative.

iii) Rereading the answers is probably best for this.

Question 25b:

ii) Look at the current/time graph on the previous page - find the current at 0.4 secs. Use the value for p.d. from part (i) and this current in V=IR to calculate the resistance.

iii) You need to know that when the capacitor is fully charged, the entire supply voltage is dropped across it, 12V. You can then use the formula E = 1/2CV^2 along with the energy given in the question to calculate the capacitance.

I hope that's all right sorry if it's not!

You can apply the conservation of energy. Assume ALL the

**kinetic**energy of the system is converted into heat. How do you work out kinetic energy?Question 21C (ii):

What I would do is work out the current in the circuit using P=IV and the values for the LED. You know 5V of the supply is dropped across the LED, so the voltage across the resistor must be 12-5=7V. You can use this voltage and the current you got earlier to work out the resistance using V=IR.

Question 22:

a) i) The easiest way to do this is to remember that the component of weight down a slope is mgsinθ where θ is the angle the slope makes with the ground (30 degrees in this case). Our teacher taught us "slidey siney" to remember it,lol, it's kinda crap and cheesy, but it works! So basically, you need to work out the weight of the mass using W=mg, put this value into mgsinθ et voila!

ii) You (should) now know the component of weight down the slope, and you also know the force the crate is being pulled with. Note the question says the crate is moving at a constant speed - the forces are balanced. So you need to find the difference between the pulling force, and the component of weight (i.e. 240-196=44 N).

b) i) Can't really say much for this, other than look at what the answers says - the 6ms^-2 comes from the gradient of the v/t graph from 0 to 0.5 seconds.

ii) You need to find the gradient of the v/t graph from from 0-0.5s and from 0.5-1s. This gives you the magnitude of the acceleration. You should remember that your lines on the a/t graph should be below the x-axis as the gradients are negative.

iii) Rereading the answers is probably best for this.

Question 25b:

ii) Look at the current/time graph on the previous page - find the current at 0.4 secs. Use the value for p.d. from part (i) and this current in V=IR to calculate the resistance.

iii) You need to know that when the capacitor is fully charged, the entire supply voltage is dropped across it, 12V. You can then use the formula E = 1/2CV^2 along with the energy given in the question to calculate the capacitance.

I hope that's all right sorry if it's not!