**1. The problem statement, all variables and given/known data**

I’m not sure if this is the right place to post this but I need help understanding this question…

Discounting air friction, approximately how far will the boulder have fallen in 2 seconds?

a. 20m

b. 45m

c. 30m

d. 90m

**2. Relevant equations**

x-x_0 = v_0t + 1/2at^2

**3. The attempt at a solution**

Now I know this is just a plug and chug equation and the answer is 45m (B). My question is more in understanding the physics behind using this equation….

Originally i used a=Δv/Δt then plugged my found velocity into v=d/t but that gives me 40m, which is not an option.

How do I know when to use which?

Thanks for any help in clarifying this for me! ðŸ˜€

http://ift.tt/1vUPkBL