# Mechanics question helpWatch

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#1
https://filestore.aqa.org.uk/sample-...2-QP-JUN18.PDF

1. I don't understand 17(b).
At first I wrote the buggy, driver and roller skater are a particle, and there is no air resistance.
But this is not in the mark scheme and the mark scheme says mentioning air resistance or friction is WRONG - why is this the case??

2. For 17(c)(ii),
I wrote the tension pulling the buggy has been removed ( 1 mark)
so the driver will have noticed an increase in driving force.
But driving force is wrong and the mark scheme says I should've put
'so there is a higher resultant force and thus the driver will notice an increase in ACCELERATION'.
I understand that the tension removed results in a higher resistance force,
but I don't understand why driver having a higher DRIVING FORCE is wrong and driver having a higher ACCELERATION is right.

the first link was the question paper,
and this is the link for mark scheme:
https://filestore.aqa.org.uk/sample-...W-MS-JUN18.PDF
0
#2
(Original post by ChloeYeo)
https://filestore.aqa.org.uk/sample-...2-QP-JUN18.PDF

1. I don't understand 17(b).
At first I wrote the buggy, driver and roller skater are a particle, and there is no air resistance.
But this is not in the mark scheme and the mark scheme says mentioning air resistance or friction is WRONG - why is this the case??

2. For 17(c)(ii),
I wrote the tension pulling the buggy has been removed ( 1 mark)
so the driver will have noticed an increase in driving force.
But driving force is wrong and the mark scheme says I should've put
'so there is a higher resultant force and thus the driver will notice an increase in ACCELERATION'.
I understand that the tension removed results in a higher resistance force,
but I don't understand why driver having a higher DRIVING FORCE is wrong and driver having a higher ACCELERATION is right.

the first link was the question paper,
and this is the link for mark scheme:
https://filestore.aqa.org.uk/sample-...W-MS-JUN18.PDF
I understand my second question now.
F = ma
so F(driving force) - T(tension) = ma
when tension is removed,
it doesn't actually INCREASE the number/quantity of F(driving force).
So I cannot mention change in driving force in my answer
because driving force/F stays the same after and before tension is removed.
Mass obviously stays the same as well.
So it's the a(acceleration) that changes.
"F-T" is the resultant force.
If tension is removed, resultant force increases and thus acceleration increases.
(F-T)/m = a.

I still don't understand why it's wrong to mention that it assumes that there's no air resistance.
0
1 week ago
#3
Air resistance is incorrect because in the question it already stated that there were 140N of resistive forces, which would include air resistance. It isn't an assumption because they have already considered it.

Hope this helps
1
#4
(Original post by quo11)
Air resistance is incorrect because in the question it already stated that there were 140N of resistive forces, which would include air resistance. It isn't an assumption because they have already considered it.

Hope this helps
ohhhh I didn't think that total resistance force would include air resistance as well!!
omg thank you so much
ohh that's why only rope is considered!!
Does the total resistance force include friction as well?
is that why the answer is saying no friction either? Thank you!!
0
1 week ago
#5
(Original post by ChloeYeo)
Does the total resistance force include friction as well?
is that why the answer is saying no friction either? Thank you!!
Yep, the resistive force includes friction, in fact, it should include any force that opposes motion so friction, air resistance, viscous drag etc. Other questions may be more specific though so just be careful when reading each individual question
0
#6
(Original post by quo11)
Yep, the resistive force includes friction, in fact, it should include any force that opposes motion so friction, air resistance, viscous drag etc. Other questions may be more specific though so just be careful when reading each individual question T
Thank you!!!
0
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