# Simple HW Junior Psychics Q

Ok here it is: A sports car is advertised to be able to stop in a distance of 50m from a speed of 90 km/h. What is its acceleration in m/s^2?

I turned 90km/hr into 25m/s but where do I go from there... I'm pretty bad at physics...

Ok I need to know what formula to plug these numbers into. I can't use the acceleration formula because there's no time that can be plugged in. Help please :)

A walkthrough on acceleration would be nice too :)

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Tom Mattson
Staff Emeritus
Gold Member
Ok I need to know what formula to plug these numbers into.
You're going to have to figure that out for yourself, but we will help you.

Go through the problem and list things that you know. For instance you know that the distance traveled by the car is 50 m. Since you can define the "zero" of position to be anywhere you want, you can say:

xi=0 m
xf=50 m

Do that for all of the given information. Then determine what is being asked for ("the unknown").

Then select the equation that relates what you've been given to what is being asked for.

You're going to have to figure that out for yourself, but we will help you.

Go through the problem and list things that you know. For instance you know that the distance traveled by the car is 50 m. Since you can define the "zero" of position to be anywhere you want, you can say:

xi=0 m
xf=50 m

Do that for all of the given information. Then determine what is being asked for ("the unknown").

Then select the equation that relates what you've been given to what is being asked for.
I've been looking at this problem for 45 minutes now and its not coming to me

Is it 1 of these 4 formulas?

v = v0 + at

x = x0 +v0t +0m5 at^2

v^2 = v0^2 +2a(x-x0)

v = (v +v0)/2

All the "0" are supposed to be in subscripts. I can't figure out which formula to use in this occasion. You're starting at 0 ok. you go to 50. The car is going at 90 m/km. The car on average, needs to be slowing 0.5 km/m of the 50 meters to stop from 25 m/s to 0 m/s... So what next?

Ok here it is: A sports car is advertised to be able to stop in a distance of 50m from a speed of 90 km/h. What is its acceleration in m/s^2?

Acceleration = -1/2m/s^2?

Tom Mattson
Staff Emeritus
Gold Member
reliquator said:
I've been looking at this problem for 45 minutes now and its not coming to me

Is it 1 of these 4 formulas?

v = v0 + at

x = x0 +v0t +0m5 at^2

v^2 = v0^2 +2a(x-x0)

v = (v +v0)/2
Yes, it's one of those. But you're putting the cart before the horse here. You shouldn't be scanning the list of equations until after you've identified the given information and the unknown.

All the "0" are supposed to be in subscripts.
OK, so in that case let x0=0 m and x=50 m.

I can't figure out which formula to use in this occasion. You're starting at 0 ok. you go to 50. The car is going at 90 m/km. The car on average, needs to be slowing 0.5 km/m of the 50 meters to stop from 25 m/s to 0 m/s... So what next?
Try assigning these numbers to the letters that appear in the equations. You've got x0 and x already. Now try to complete this table:

v0=______________
v=____________
a=____________
t=____________

Only then should you attempt to find the equation.

List your known quantities and then list your unknowns. After that, look at the equations you have to see if you can solve for the unknown you want.

v0=______________
v=____________
a=____________
t=____________
So would I be right in saying v0 = 90000 m/s and v = 0m/s?

so what does a and t stand for? My textbook really doesn't say anything about it.

t is time I think but how would i find that?

reliquator said:
So would I be right in saying v0 = 90000 m/s and v = 0m/s?

so what does a and t stand for? My textbook really doesn't say anything about it.

t is time I think but how would i find that?
a is acceleration, you should probably know that.

What I meant in my post was to list the quantities from your problem that you know and figure out what you are solving for.

Tom Mattson
Staff Emeritus
Gold Member
reliquator said:
So would I be right in saying v0 = 90000 m/s and v = 0m/s?
v0 is not 90000 m/s. It is 25 m/s, as you yourself stated in the opening post. You are correct about v.

so what does a and t stand for? My textbook really doesn't say anything about it.
a=acceleration
t=time

t is time I think but how would i find that?
Here's the real question: Do you even need it?

The question is asking for acceleration, right? So you have:

x0=0 m
x=50 m
v0=25 m/s
v=0 m/s
a=?

You weren't given t, and you weren't asked for it. So, choose the one equation that has all the symbols in the list above and that doesn't have t. Then plug in all the known quantities and solve for the one unknown.

v^2 = v0^2 +2a(x-x0)

^That equation then?

so it would be:

0^2 = 25^2 + 2a(0-50)
-625 = -100a

ACCELERATION = 6.25 m/s???

Tom Mattson
Staff Emeritus
Gold Member
Looks good to me. Looks good to me.
YESSSSSSSSSS

it's 12:33 eastern time right now and I can finally move on. Well this is been a monumental effort.

Having SAT prep class from 6 PM - 9PM really cuts out homework time and after 11 PM you really can't think. Thank you all that helped.