Aqa as physics 2018 unofficial mark scheme

There is an easier way, that revolves around the fact that the sum of the two angles is 90 degrees. Because the three forces are in equilibrium and therefore sum to zero, they can be represented as the three sides of a vector triangle. And then;

Actually, yes that is very true. I do remember our mechanics tecaher saying that our method is fool-proof and doesn't go wrong, but in defence many of our problems are ones were there is no right angle. Frankly I just forgot about that method. Nice, thanks

I've also looked at the volume of the iron question. I even ran the numbers on wolfram alpha to check my arithemetic and I get my answer: 3.2*10^-8

How did you do it?

I got that as well, im sure that we have got it right

Ah ok thanks. I got that wrong in the exam then oh well....

I've changed the markscheme.

I got that as well, im sure that we have got it right

That question was easy. A lot of people were overthinking it.

The rain has mass. The rain collects onto the truck. The mass of the truck increases. p = mv, therefore momentum will increase.

I got that as well, im sure that we have got it right

Paper 1:
1.1: Strong nuclear force
Attractive for a short range (0.5-3 fm)
Exchange particle is a pion (3 marks)

1.2: alpha decay baryon number 1 on both sides (proton to neutron)
Beta decay Baryon number 1 on both sides (neutron to proton) (3 marks)

1.3: quark structure is uds (2 marks)

1.4: weak because strangeness isnt conserved (1 mark)

1.5: Need for collaboration so can carry out experiments and make results Reproducible and repeatable (2 marks)

2.1: speed of truck is 12.5 m/s (2 marks)

2.2: velocity changes
acceleration changes constantly
Gravity no longer acts on the horizontal part unlike on the first slope. (3 marks)

2.3: speed of truck is the same as GPE at the top is the same in both. Since air resistance and friction are negligible kinetic energy at the bottom is the same in both (1 mark)

2.4: Momentum in the horizontal direction
No horizontal force from rain
No change in momentum
Mass increases so velocity decreases
(3 marks)

3.1: diagonal set of ticks starting from top right (1 mark)

3.2: the photon has to have energy= to e1-e2
Energy of the photon completely absorbed by the atom
Whereas an electron just transfers its kinetic energy (3 marks)

3.3: transtion C would result in photoelectrons being emitted as frequency of the photon is higher than threshold frequency. Transition A and B wouldn't cause emission (3 marks)

3.4: photoelectron speed is 1.87*10^6 m/s
Find kinetic energy of photoelectron then find speed (3 marks)

4.1: total anticlockwise moment must be = to total clockwise moment
Line of action of the weight force must be in the base/ No resultant force.
(2 marks)

4.2: point where the entire mass of an object is thought to be (1 mark)

4.3: centre of mass vertically below A because the beam has come to rest in that position.
The centre of mass is vertically below the point where a suspended object comes to rest (2 marks)

4.4: tension worked out by resolving forces forming 2 simultaneous and solving

T1- 7221.8 N
T2- 9583.6 N (4 marks)

4.5: using young modulus= stress/ strain work out strain then usr strain= change in length/ original length.

Extension is 4.07*10^-3 m (3 marks)

5.1: weight of ice cube is 0.036 N (2 mark)

5.2: vol of water dipslaced by ice cube is
(0.036÷9.81) ÷1000= 3.7×10^-6 (1 mark)

5.3: volume of iron was 3.2*10^-8 (3 marks)

6.1: AC (ratio question) (4 marks)

6.2: The brightness of the other 4 bulbs remains the same however bulb E doesn't light up. The PD across bulb E is 0 because 4.5V from either end cancel out. (3 marks)

6.3: Current is reduced. (3 marks)

7.1: there will be a central maxima composed of white light with a spectra of colour fringes on either side of the maxima all adjacent to eachother. (2 marks)

7.2: red light will have wider fringe spacing than green light. There will be alternate red and green bright fringes on the screen with dark fringes between them.
Use a calculation to show this (I put in arbitary values into the eqt) (4 marks)

7.3: w=(lambda*D)/s
Decreasing slit sparation means bigger fringe spacing so interference pattern more spread. It also means uncertainty in the length of the slit is increased as you need a measuring instrument with a higher resolution in order to accurately measure it. As a reseult uncertainty in the wavelength increases.

As you decrease D the reading decreases, and resolution stays the same, this means the uncertainty increases.
The fringe width decreases, which therefore means the reading for the fringe width decreases, resolution is the same so uncertainty increases. (6 marks)

2.4:Change of momentum qs as rain falls???? (3 marks)

Horizontal and Vertical Momentum vectors are independent of each other. The Rain falling vertically loses all it's momentum and gives it to the Truck/planet earth - so vertical momentum is conserved.

If the truck is moving initially at constant velocity with constant mass then it has a set momentum. Rain falling into it increases the total mass of the system but momentum HAS to be conserved therefore the velocity of the truck has to reduce in order to compensate for the increase in mass and keep the total horizontal momentum equal before and after the rainfall.

R
(Physics A level teacher who also found the papers a bit tough.)

OK so - This is what I did. increased mass of water displaced = 0.0002 Kg. (0.0039 - 0.0037)

volume = 0.0002/7800 = 2.6x10^-8

So I changed this so that the density is the density of the iron-the density of the water it's replacing ..

0.0002/6800 = 2.9x 10^8 metres cubed.

I still think I'm missing something and would appreciate seeing someone working a full solution.

This is making my BRANE HURT. R

Haven't really read through this, but the last bit is wrong
'Decreasing D means fringe spacing is reduced so pattern more close together. Uncertainty may be reduced as there are more fringes to measure so more accurate. (6 marks)'

Yes that's correct. Coming from a second year student

Do u mean he is correct (so It is wrong) or what I said is correct?

Your answer is not quite right. If you decrease s, W increases. If the value you wish to measure is greater then the percentage uncertainty is lower (assuming the resolution of the equipment is the same), therefore the uncertainty in your value of the wavelength is lower. Also, if you used equipment with increase resolution, your uncertainty would decrease also. You would only increase the resolution of your equipment if the value you wished to measure was lower (eg if you want to measure the thickness of a wire, you don't use a metre ruler, since the percentage uncertainty would be huge. You'd use a micrometer).

Paper 1:
1.1: Strong nuclear force
Attractive for a short range (0.5-3 fm)
Exchange particle is a pion (3 marks)

1.2: alpha decay baryon number 1 on both sides (proton to neutron)
Beta decay Baryon number 1 on both sides (neutron to proton) (3 marks)

1.3: quark structure is uds (2 marks)

1.4: weak because strangeness isnt conserved (1 mark)

1.5: Need for collaboration so can carry out experiments and make results Reproducible and repeatable (2 marks)

2.1: speed of truck is 12.5 m/s (2 marks)

2.2: velocity changes
acceleration changes constantly
Gravity no longer acts on the horizontal part unlike on the first slope. (3 marks)

2.3: speed of truck is the same as GPE at the top is the same in both. Since air resistance and friction are negligible kinetic energy at the bottom is the same in both (1 mark)

2.4:Change of momentum qs as rain falls:
Horizontal and Vertical Momentum vectors are independent of each other. The Rain falling vertically loses all it's momentum and gives it to the Truck/planet earth - so vertical momentum is conserved.

If the truck is moving initially at constant velocity with constant mass then it has a set momentum. Rain falling into it increases the total mass of the system but momentum HAS to be conserved therefore the velocity of the truck has to reduce in order to compensate for the increase in mass and keep the total horizontal momentum equal before and after the rainfall.3 marks)

3.1: diagonal set of ticks starting from top right (1 mark)

3.2: the photon has to have energy= to e1-e2
Energy of the photon completely absorbed by the atom
Whereas an electron just transfers its kinetic energy (3 marks)

3.3: transtion C would result in photoelectrons being emitted as frequency of the photon is higher than threshold frequency. Transition A and B wouldn't cause emission (3 marks)

3.4: photoelectron speed is 1.87*10^6 m/s
Find kinetic energy of photoelectron then find speed (3 marks)

4.1: total anticlockwise moment must be = to total clockwise moment
Line of action of the weight force must be in the base/ No resultant force.
(2 marks)

4.2: point where the entire mass of an object is thought to be (1 mark)

4.3: centre of mass vertically below A because the beam has come to rest in that position.
The centre of mass is vertically below the point where a suspended object comes to rest (2 marks)

4.4: tension worked out by resolving forces forming 2 simultaneous and solving

T1- 7221.8 N
T2- 9583.6 N (4 marks)

4.5: using young modulus= stress/ strain work out strain then usr strain= change in length/ original length.

Extension is 4.07*10^-3 m (3 marks)

5.1: weight of ice cube is 0.036 N (2 mark)

5.2: vol of water dipslaced by ice cube is
(0.036÷9.81) ÷1000= 3.7×10^-6 (1 mark)

5.3: volume of iron was 3.2*10^-8 (3 marks)

6.1:which lamps are at normal brightness??? (4 marks)

6.2: The brightness of the other 4 bulbs remains the same however bulb E doesn't light up. The PD across bulb E is 0 because 4.5V from either end cancel out. (3 marks)

6.3: lamp b fails deduce effect of current??? (3 marks)

7.1: there will be a central maxima composed of white light with a spectra of colour fringes on either side of the maxima all adjacent to eachother. (2 marks)

7.2: red light will have wider fringe spacing than green light. There will be alternate red and green bright fringes on the screen with dark fringes between them.
Use a calculation to show this (I put in arbitary values into the eqt) (4 marks)

7.3: w=(lambda*D)/s
Decreasing slit sparation means bigger fringe spacing so interference pattern more spread. It also means uncertainty in the length of the slit is increased as you need a measuring instrument with a higher resolution in order to accurately measure it. As a reseult uncertainty in the wavelength increases.

As you decrease D the reading decreases, and resolution stays the same, this means the uncertainty increases.
The fringe width decreases, which therefore means the reading for the fringe width decreases, resolution is the same so uncertainty increases. (6 marks)