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# AQA Physics PHYA4 - Thursday 11th June 2015 [Exam Discussion Thread] watch

1. (Original post by gcsestuff)
Is the definition of time constant simply- the time take for the voltage to reduce to 37% or 1/e^1 its original value

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Yep or just T=CR
2. (Original post by Sbarron)
Yep or just T=CR
Could also replace voltage with current or charge!!
Remember if there is a variable resistor keeping current constant then the circuit is NOT EXPONENTIAL! As I got caught out with this calculating something on a past question!
3. (Original post by Plasmapause)
Yes because it's an "elastic" collision, it's like throwing a tennis ball at a wall, it will collide with the wall and change direction and so change momentum. Also kinetic energy is conserved so the change in momentum must be equal to 2mv (2xmassxvelocity).
At least I'm sure thats right.
Can you show/explain why it's 2mv
I always get confused with this... I think it's just rearranging it all but can you explain a bit more please
4. Hey guys could anyone explain question 17 on this paper please: http://filestore.aqa.org.uk/subjects...1-QP-JUN14.PDF the answer is A but I'm just confused as to why the potential energy is positive...
5. (Original post by Sbarron)
Can you show/explain why it's 2mv
I always get confused with this... I think it's just rearranging it all but can you explain a bit more please
I think of it like change of momentum = mv-mu

because it is bouncing back and it is an elastic collision. One of the velocities can be said to be negative as it is in the opposite direction.

So the change in momentum would be mv-(-mv)
which is 2mv
6. (Original post by Sbarron)
Can you show/explain why it's 2mv
I always get confused with this... I think it's just rearranging it all but can you explain a bit more please
Just prove it to yourself. Change in momentum = difference in velocity x mass. If you had a tennis ball that had a mass of M and when you throw it against the wall, it's velocity before colliding with the wall is V. Because it's an elastic collision, and the mass doesn't change and it's moving in the opposite direction at the same speed (so it's moving at a velocity of -V ). Difference in velocity is the velocity before collision minus the velocity after collision so ( V - -V) so basically V+V, hence 2V and so the momentum is 2MV (mass of the tennis ball did not change).

7. Why is 8 not c.

I'm imaging a ball on a turntable. When it slides of how does it come of I a vertical plane?

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8. (Original post by gcsestuff)

Why is 8 not c.

I'm imaging a ball on a turntable. When it slides of how does it come of I a vertical plane?

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It's not C, because when the string snaps there is no force acting on it in the horizontal plane, however because gravity is acting on the string, it would be D.

"When released it goes off at a tangent to the circle... as if it had been thrown horizontally. Gravity affects its vertical velocity - acceleration in the vertical direction; it goes at constant velocity in the horizontal direction. It therefore follows a parabolic path when viewed in a verical plane."
9. (Original post by gcsestuff)

Why is 8 not c.

I'm imaging a ball on a turntable. When it slides of how does it come of I a vertical plane?

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The mass is whirled in a horizontal circle so by Newton I the path it follows when centripetal force is no longer applied is a straight line tangent to the circle at that point and at the same velocity (speed and direction) as it was at that point. The only force acting after this is the acceleration vertically downwards due to gravity. So the horizontal velocity of the mass is unchanged from when it was released (by Newton I as I said) but the vertical velocity increases downwards, hence it follows a parabolic curve in the vertical plane.
I think of it like change of momentum = mv-mu

because it is bouncing back and it is an elastic collision. One of the velocities can be said to be negative as it is in the opposite direction.

So the change in momentum would be mv-(-mv)
which is 2mv
Indeed. I would just like to point out here a common mistake that people make. When the initial velocity is u and the final velocity is v in a direction opposite to u, then taking the direction of v as positive we have that the change of velocity is v-(-u)=v+u. Therefore the impulse is m(u+v). People usually state this as m(v-u) and then forget to take account of u having a different sign to v.
11. (Original post by Plasmapause)
Just prove it to yourself. Change in momentum = difference in velocity x mass. If you had a tennis ball that had a mass of M and when you throw it against the wall, it's velocity before colliding with the wall is V. Because it's an elastic collision, and the mass doesn't change and it's moving in the opposite direction at the same speed (so it's moving at a velocity of -V ). Difference in velocity is the velocity before collision minus the velocity after collision so ( V - -V) so basically V+V, hence 2V and so the momentum is 2MV (mass of the tennis ball did not change).
Another way of showing that change in velocity equals final velocity minus initial velocity (and not vice versa) consider the boundary case where the ball stops immediately (perfectly inelastic collision). Initial velocity is u and final velocity is 0, so the change in velocity is either +u or -u. But clearly it has to be -u because the ball is losing velocity when it stops. So we can deduce that the formula for this particular situation is: change of velocity = 0 - u = final velocity - initial velocity. Now simply replace 0 with any value and it follows that change in velocity = v - u, where v = final velocity.
12. Whats the difference between a dynamo and a generator??
13. (Original post by AR_95)
What's the difference between AC and DC
Alternating current regularly reverses direction. So if you consider for example a capacitor you will see that it allows AC to flow but not DC. This is because with AC the capacitor repeatedly charges and discharges, but this DC it can only either charge or discharge, but not both. If you connected a bulb in series with the capacitor and an AC supply the bulb will stay on because it perceives that current is always flowing in one direction or the other but with DC it is bright for a bit and gradually shuts out when the capacitor is fully charged or fully discharged.
14. (Original post by noseypo)
Whats the difference between a dynamo and a generator??
They're the same (I think): The main function of both is to convert kinetic energy to electrical energy.
15. (Original post by NEWT0N)
They're the same (I think): The main function of both is to convert kinetic energy to electrical energy.
Are they both to do with generating emf?
16. In JAN10 4bii why does it use the input power of 3W rather than the secondary power of 2.7W? I thought the power wasted due to the standby mode is 2.7W (from 4bi).

Edit: Typo (now corrected in bold)
17. (Original post by noseypo)
Are they both to do with generating emf?
At least I think they're the same thing, yes. When a coil is caused to rotate in the magnetic field between two bar magnets the flux through its area changes so an induced current flows in the coil.
18. And Mehrdad and CD were nowhere to be seen ever again... the end.
19. Anyone able to explain why the periodic frequency being in phase with velocity makes it resonate? Why can't it be in phase with displacement

Can anyone give the aqa definition of resonance ?
20. can someone please definitively explain the different degrees of damping? My teacher and textbooks all say different things about heavy, critical and overdamping

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