Solve Acceleration Problem: Car Starts at Rest & Travels 200m

[Nickles]

Homework Statement

hey everyone, i am new here and just a beginner in physics. i am in love with physics although i get stuck at some of the most SIMPLEST things, i think that i over evaluate it too much. I just seriously begun to understand physics.Here is the question : A car starts from rest and accelerates at a constant rate. After traveling a distance of 200m, the speed of the car is 80 km/h. What was the acceleration of the car?

Homework Equations

a= (vf-vi)/ t...i think so

The Attempt at a Solution

i don't know what to do. i am thinking that since acceleration is depending on the time taken, i would convert the 200m and the 80 km/h into the same unit and divide them to get the time taken for it to reach the 80 km/h speed.
after that, i would use that time and the equation above to find the acceleration. i tried and tried and the back of my book says the answer is 1.2 meters per second to fit with the data as the answer to 2 significant figures. i tried over and over and still can't find a method to get it to that answer.
please help and please explain the steps and solutions step by step so that i can fully understand this problem and answer. thank you all i just really started getting into physics and i want to become a physicist but i know that i have a long way to go but i am not going to give up!

 

[G01]

Yes, the equation you posted is true, but you aren't given the time. So, it's not much help here.

However, you are given the distance over which you accelerate. Perhaps there is a kinematic equation involving acceleration, distance, but not time?

 

[Nickles]

so are you meaning the equation v squared= u squared + 2as??...let me try. sometimes i get so mixed up because i don't know which equation t use and i f i do figure which equation to use i sometimes mess it up and cause my answer to be wrong. any tips with this please? thank you...i tried that equation and i still can't get it right

 

[gneill]

Perhaps you could post how you carried out the calculation with that equation. Then we might be able to see where things are going wrong.

 

[G01]

As gneill said: you need to show your work. How else will we know where you are making a mistake?

 

[Nickles]

ALRIGHT

80 x 80= 0 x 0 + 2(a)(200)
6400 = 400 a
a= 16?

the book i has says that the answer is 1.2 metres/second...i don't see where i went wrong or how i went wrong. sorry if this seems so simple but please help. it would be greatly appreciated

 

[gneill]

ALRIGHT

80 x 80= 0 x 0 + 2(a)(200)
6400 = 400 a
a= 16?

the book i has says that the answer is 1.2 metres/second...i don't see where i went wrong or how i went wrong. sorry if this seems so simple but please help. it would be greatly appreciated

Ah. You're forgetting about the units. The velocity is given in km/hr, the distance in meters. You need to account for the different units. The easiest method might be to convert everything to meters and seconds as base units. What's 80km/hr in m/s ?

 

[Nickles]

ok so that would be something like this right?

metres would be 80X 1000= 80000m
seconds would be 60X60 = 3600s

dividing them would give you 22.2 m/s

when i use the equation above i would get

22.2 X 22.2 = 400a
492.84/ 400 = 1.2 metres per second! yea! thank you much ! i truly truly appreciate it, you have no idea how thankful i am

 

[gneill]

You're quite welcome. Glad to be of service.