How to calculate instantaneous acceleration?

[treyh]

This is the graph and question:
http://img193.imageshack.us/img193/3489/physicquestion.jpg

I cannot seem to find the instantaneous acceleration. I must be doing it wrong because I'm pretty sure to find the the inst. acceleration all i need to know is the tangent line equation.

I've tried a bunch of answers but can't seem to get it right. I need to express the answer in m/s^2 and not km/h but that is easy to convert.

I haven't had much physics experience. How do i find the instantaneous acceleration?

Thanks

 

[physicsisgrea]

Since the acceleration is uniform, instantaneous acceleration = average acceleration.

acceleration = slope of v-t graph

from the graph, the slope of the graph during time = 30s to 40s is a straight line (that means the slope is not varying), that means the acceleration uniform.

so the instantaneous acceleration at time = 35s :
= [(60/3.6) - 0 / (30 - 40)] = -1.667 m/s^2.

 

[PhanthomJay]

I'm pretty sure to find the the inst. acceleration at t = 35 seconds [/color] all i need to know is the tangent line equation.

That's correct, so what is the definition of instantaneous acceleration and what is the slope of that line at t = 35 s?