1. Nov 6, 2013

### Qube

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

The position of an object traveling in a straight line at time t is given by s(t) = (2/3)t^2 + 4t^2 + 6t + 2.

What's the displacement after 3 seconds?

2. Relevant equations

Displacement is final position minus initial position.

3. The attempt at a solution

Displacement at 3 seconds is s(3) - s(0).

s(0) = 2. Note the constant term!

s(3) = 18 - 36 + 18 + 2.

s(3) - s(0) = 0.

Interestingly enough, my key claims displacement is simply s(3).

I'm guessing this is an error (and not on my part)!

Last edited: Nov 6, 2013
2. Nov 6, 2013

### AATroop

Can you explain how 2/3 * t^2 = 4*t^2 + 6*t + 2?

3. Nov 6, 2013

### Qube

Sorry, replace the second equal sign with a plus sign. I've corrected it in the OP.

Last edited: Nov 6, 2013
4. Nov 6, 2013

### AATroop

OK, can you also show how you're getting s(3)? Because, if the equation is s(t) = (2/3)t^2 + 4t^2 + 6t + 2, then at t = 3, the current position is 6 + 36 + 18 + 2. Which is equal to 62. If the equation is s(t) = (2/3)t^2 - 4t^2 + 6t + 2, then at t = 3, the current position is 6 - 36 + 18 + 2, which is equal to -10. Not sure if you're copying the equation wrong or what...

Last edited: Nov 7, 2013
5. Nov 6, 2013

### Qube

I think I did copy the problem wrong, but I know there was a constant at the end, which makes the answer key unequivocally wrong, correct?

6. Nov 7, 2013

### Qube

Okay, I found it!

http://i.minus.com/jW4oBtV2mWi3O.png [Broken]

http://i.minus.com/jbrSQnX1kxCiGG.png [Broken]

Last edited by a moderator: May 6, 2017
7. Nov 7, 2013

### AATroop

Well, it definitely seems like now there is an error in the answer key. So, yes, I would not follow it.