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Suvat questions help!

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! :smile:

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Reply 1
Avatar for mqb2766
mqb2766
21
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! :smile:

What are you confused about with q1?
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
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
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?
Reply 5
Avatar for mqb2766
mqb2766
21
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?
(edited 3 years ago)
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?
Reply 7
Avatar for mqb2766
mqb2766
21
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.
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?
Reply 9
Avatar for mqb2766
mqb2766
21
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?
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
(edited 3 years ago)
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)?
Original post by mqb2766
Is this part a)?

yes 1a
Original post by crazycrazymech
yes 1a

So v=0 is right. What is u and a (note it's deceleration)?
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
(edited 3 years ago)
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.
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
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=..?
Original post by mqb2766
u is now right. However it's decelerating so a=..?

a= -4?
Original post by crazycrazymech
a= -4?

Yes

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