# Equation Help.

Hello!

in Physics we just started acceleration

We were givin 3 equations

v=V(not) + at

x=V(not)t + 1/2at(Squared)

V(Squared) =V(not) + 2ax

Can someone please explain the variables and how to figure them out.

With those equations I need to figure problems like this

The speed limit in a school zone is 44 km/h. A driver traveling at this speed sees a child run onto the road 11 m ahead of his car. He applies the brakes, and the car decelerates at a uniform rate of 10.5 m/s2. If the driver's reaction time is 0.65 s, will the car stop before hitting the child?

YES OR NO
What is the stopping distance of the vehicle in m?

And

A boy throws a stone straight upward with an initial speed of 18 m/s. What maximum height will the stone reach before falling back down

I really don't know where to start. I normally start listing the varables like
V=
a=
t=
x=

But I do not understand what numbers to put in which areas, Thank you very much

x is a distance in meters
t is a time in seconds
V is a velocity in meters/second
a is an acceleration in meters/second squared.

Also, V(not) for this problem is the initial velocity given.

Think it through, this is a very simple problem.

Thanks
Matt

Thank you for your help Matt.

How do I figure out time, if I only get 3/5 or 2/4 varibles?

You are given the time. t = 0.65 seconds. I think you are misreading the problem. You are not calculating a time, you are calculating a distance. You need to determine if the driver stops within 11 meters. Not the time required for the driver to stop.

Make sense?

Thanks
Matt

You are given the time. t = 0.65 seconds. I think you are misreading the problem. You are not calculating a time, you are calculating a distance. You need to determine if the driver stops within 11 meters. Not the time required for the driver to stop.

Make sense?

Thanks
Matt

I put the following

x=11
T=0.65
a=10.5 m/s(squared)
v=44 km/h

Now I would have to plug the above into one of these

v=V(not) + at

x=V(not)t + 1/2at(Squared)

V(Squared) =V(not) + 2ax

This is where I get confused.

I do not know which equation to use. I am thinking I would use
V(Squared) =V(not) + 2ax

Because it is the only one with an X

So if I plug in the variables I would get

44=0 + 2(10.5x)

44 = 21x

x=2

So the distance would be 2m?

Gosh I am confused lol

Thanks again !

You are solving for a distance, x, so use the equation that produces the distance, x, as the result.

Thanks
Matt

You are solving for a distance, x, so use the equation that produces the distance, x, as the result.

Thanks
Matt

So that would be x= v(not)t+1/2at(squared)

?

Yes, that is the one. Also, as a check, make sure all of your units cancel to produce meters.

Thanks
Matt

drizzle
Gold Member
the driver's reaction time is not the time taken till the velocity reaches zero, you can find the time using the first equation:
v=vo + at

[don’t forget to subtract the driver's reaction time from the calculated time]

Yes. That is correct.

Thanks
Matt