crazycrazymech
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Not sure how I would start these/use the information given to answer the questions:

1)A motorist driving towards the debris notices it when he is 80m away, travelling at 20ms^-1 and with constant acceleration of 2ms^-2. As soon as he engages the brakes, he will decelerate at a constant 4ms^-2.
a) if the motorist reacts immediately, how far will he stop from the debris?
b)if he takes two seconds to react, at what speed will he be driving when he crashes into the debris?

And

2)Particles A and B are at rest at either end of a 100m track. They set off instantaneously towards eachother with constant accelerations of 6ms^-2 and 2ms^-2 respectively
a)where will they meet?
b)when will they meet
c)at what speed will each be travelling when they meet?

i'm assuming q2 will require simultaneous equations but q1 i'm not sure how you would use the info given to answer the following qs.

Would appreciate any guidance. Thanks so much!
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mqb2766
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(Original post by crazycrazymech)
Not sure how I would start these/use the information given to answer the questions:

1)A motorist driving towards the debris notices it when he is 80m away, travelling at 20ms^-1 and with constant acceleration of 2ms^-2. As soon as he engages the brakes, he will decelerate at a constant 4ms^-2.
a) if the motorist reacts immediately, how far will he stop from the debris?
b)if he takes two seconds to react, at what speed will he be driving when he crashes into the debris?

And

2)Particles A and B are at rest at either end of a 100m track. They set off instantaneously towards eachother with constant accelerations of 6ms^-2 and 2ms^-2 respectively
a)where will they meet?
b)when will they meet
c)at what speed will each be travelling when they meet?

i'm assuming q2 will require simultaneous equations but q1 i'm not sure how you would use the info given to answer the following qs.

Would appreciate any guidance. Thanks so much!
What are you confused about with q1?
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crazycrazymech
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(Original post by mqb2766)
What are you confused about with q1?
when it says react immediately how would you use the numbers? and then how far away will he stop, you're working out the displacement but the wording is slightly confusing for me
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Hellllpppp
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For 1a you have to look at the information given as it provides you with u, v and a and you are looking for s so you need to use the equation that excludes t.
For 1b think about what will happen in the first 2 seconds. You have u, a, t and using them you can work out s (I’ll call this s1) and v after 2 seconds as that’ll become u for the next equation. Use s1 to calculate the distance to the debris (I’ll call this s2) then use an equation with s2, u (v after 2 seconds) and a to find t.

Hope this helps
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Hellllpppp
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(Original post by crazycrazymech)
when it says react immediately how would you use the numbers? and then how far away will he stop, you're working out the displacement but the wording is slightly confusing for me
What do you think u, v and a could be?
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mqb2766
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(Original post by crazycrazymech)
when it says react immediately how would you use the numbers? and then how far away will he stop, you're working out the displacement but the wording is slightly confusing for me
What suvat equations do you know / which is the most appropriate here, based on the variables given?
Last edited by mqb2766; 5 months ago
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crazycrazymech
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(Original post by Hellllpppp)
What do you think u, v and a could be?
But there are two values for a? So what would the initial and final velocity be if this is the case?
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mqb2766
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(Original post by crazycrazymech)
But there are two values for a? So what would the initial and final velocity be if this is the case?
If a switches value, you have to do two separate suvat calculations, one for each of the seperate time intervals.
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crazycrazymech
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(Original post by mqb2766)
What suvat equations do you know / which is the most appropriate here, based on the variables given?
v^2=u^2+2as to work out the displacement and v=u+at to work out the time?
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mqb2766
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(Original post by crazycrazymech)
v^2=u^2+2as to work out the displacement and v=u+at to work out the time?
Sounds ok. Just do it?
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crazycrazymech
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(Original post by mqb2766)
If a switches value, you have to do two separate suvat calculations, one for each of the seperate time intervals.
ok so so far i've go s=(what we're working out) u=nothing v=2 and a=20. But i'm not sure what u would be as you need 4 values - EDIT wait no would u=2 and v=0 because there's no final velocity
Last edited by crazycrazymech; 5 months ago
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mqb2766
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(Original post by crazycrazymech)
ok so so far i've go s=(what we're working out) u=nothing v=2 and a=20. But i'm not sure what u would be as you need 4 values - EDIT wait no would u=2 and v=0 because there's no final velocity
Is this part a)?
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crazycrazymech
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(Original post by mqb2766)
Is this part a)?
yes 1a
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mqb2766
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(Original post by crazycrazymech)
yes 1a
So v=0 is right. What is u and a (note it's deceleration)?
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crazycrazymech
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(Original post by mqb2766)
So v=0 is right. What is u and a (note it's deceleration)?
so it would be u= -2 and a= -20

EDIT ok got s=10m
Last edited by crazycrazymech; 5 months ago
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mqb2766
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(Original post by crazycrazymech)
so it would be u= -2 and a= -20
What does u represent? Similarly for a? You're told them directly in the question.
You're only concerned with braking for this phase.
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crazycrazymech
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(Original post by mqb2766)
What does u represent? Similarly for a? You're told them directly in the question.
You're only concerned with braking for this phase.
u is initial velocity which is 20 and a is constant acceleration which is 2 but since it's stopping it would be 4
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mqb2766
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(Original post by crazycrazymech)
u is initial velocity which is 20 and a is constant acceleration which is 2 but since it's stopping it would be 4
u is now right. However it's decelerating so a=..?
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crazycrazymech
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(Original post by mqb2766)
u is now right. However it's decelerating so a=..?
a= -4?
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mqb2766
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(Original post by crazycrazymech)
a= -4?
Yes
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