# Suburu WRX and a Honda Accord have collided at an intesection

so i add 1200 and 1400 then x 9.8 = 25480N
then divide .6 = 15288N

? im not doin this right am i?

andrevdh
Homework Helper
Seems fine. Now calculate their acceleration and their initial speed, that is their combined speed before the skidding started.

a = F/m
= 15288/2600
= 5.88m/s

as for initial speed how do i calculate that?

would that be using v2=u2 +2as somehow?

andrevdh
Homework Helper
That is correct.

do i have to assume the distance of the suburu seeing as it hasn't been given

andrevdh
Homework Helper
The cars are mangled together and are sliding as a unit.

still wont help me decide whether the suburu was going faster than 80 km/h would it?

andrevdh
Homework Helper
The answer need to be appoached in several steps. This is one of them before you will be able to answer that question.

The combined wrecks start with some speed v and are decelerated to zero by the frictional force. What do you get their initial speed?

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i would use the equation i stated above to find the combined wreck speed in the 12.8 m.
i tried that and got 12.26 m/s ---> 44.136km/h

that right?

andrevdh
Homework Helper
I get the same.

Now you can calculate the momentum of the combined wrecks after the collision. What will the direction and magnitude of this vector be?

so it would be p=mv
= 2600 x 12.26
= 31876
??

andrevdh
Homework Helper
Units of the calculated momentum? What would the direction of the vector be?

? ok now im lost here im gussin 7 deg

andrevdh
Homework Helper
Yes that is seven degrees west of north. The direction of the momentum vector is determined by the direction in which the object is moving. In this case the direction in which the wrecks are sliding.

What will the components of this momentum vector be in the north-south and east-west directions?

would this by any chance be wher sin tan cos would be used? if not then i dont really know

andrevdh
Homework Helper
Yes. The components of a vector is calculated with the angle that it makes with one of the chosen perpendicular directions, seven degrees, and the magnitude (size or length) of the vector, that is 31786 kgm/s in this case.

maybe i should sleep on it a bit its 1:10 am here so sorry for bothering u . ill try tacle this in the morning ( dont know how successful ill be)

actually ill stay a little longer i really need this done

andrevdh
Homework Helper
It is time for me to go home also. I might be online in about 16 hours time.

What you need to do is calculate the components of this vector and compare it with the momentum vectors before the collision. Since momentum is conserved the components need to be the same in both directions before and after collision.

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how did u work that out?

ok thats ok itl be too late i have class tomorrow
thank u for your help anyhow
i appreciate it very much

andrevdh
Homework Helper
You need to calculate the components of the momentum after the collision, $p_a$. These two components are such vectors that when added together will produce the vector $p_a$. They also form the sides of a right angle (its base and perpendicular) as indicated in the attachment, while the vector $p_a$ are the hypotenuse of the triangle.

#### Attachments

• components of momentum.gif
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