AAMC test 10 PS #17

[kabtq9s]

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This might be an easy question but I just want to double check it

The question is asking about how h is represented mathematically and the passage deals with projectiles so I immediately thought of the projectile formula for height h = 1/2 gt^2. I see how they got their answer but I don't see how they expect us to figure to not use the projectile equation and instead use the kinetic/ potential energy equations

http://mysowar.files.wordpress.com/2011/07/mathematical-h.png

Thanks in advance

 

[SephirothXR]

That equation (h = 1/2 gt^2) is better used when you drop an object from a certain height.

There's another equation: Vf^2 = Vi ^2 + 2*a*delta h (displacement). Since Vo is 0, and if you solve for h, it's h = Vf^2/2g.

Also, when I did this question, I didn't even do the math. All those expressions have mass in it. Mass won't play a factor in acceleration. So choose the only one that doesn't have "m" in it.

 

[MT Headed]

Use x = (1/2) at^2 when you do care about the time.

Use v^2 = 2ax when nobody cares about the time. While you can derive this multiple ways, it's just such a useful formula that it is worth memorizing.

And if you can solve a multiple choice problem via unit analysis alone, realizing that an answer with kg can't possible resolve to meters, 👍

 

[salim271]

And if you can solve a multiple choice problem via unit analysis alone, realizing that an answer with kg can't possible resolve to meters, 👍

👍👍👍 thats exactly what i thought lol. Unit analysis makes problems that take a lot of thinking go to a quick check. Even if you only narrow it down to two its a lot easier to rationalize between two than four.

 

[kabtq9s]

Thanks guys for all your help

MT, I didn't quite get ur last sentence are you refering to the (g) in the answer choices ?

 

[salim271]

Thanks guys for all your help

MT, I didn't quite get ur last sentence are you refering to the (g) in the answer choices ?

He's talking about how all the answer choices except for the right one have mass (kg) in them, and how there's nothing to cancel those units so you can't possibly end up with meters alone as your answer for all but the correct answer choice, the one that doesnt have mass.