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# Freefall for AP Physics B & C

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By McGraw-Hill Professional
Updated on Feb 12, 2011

Practice problems for these concepts can be found at:

Kinematics Practice Problems for AP Physics B & C

Problems that involve something being thrown off a cliff 3 are great, because vertical acceleration in these problems equals g in just about every case.

Falling-object problems should be solved using the method we outlined above. However, you have to be really careful about choosing a positive direction and sticking to it. That is, figure out before you solve the problem whether you want "up" to be positive (in which case a equals –10 m/s2) or "down" to be positive (where a would therefore equal +10 m/s2).

Here's a practice problem:

Begin by defining the positive direction. We will call "up" positive. Then use the four-step method to solve the problem.

Step 1: Table of variables.

Remember that displacement is a vector quantity. Even though the melon goes up before coming back down, the displacement is simply equal to the height at which the melon ends its journey (0 m) minus its initial height (30 m). Another way to think about displacement: In total, the melon ended up 30 m BELOW where it started. Because down is the negative direction, the displacement is –30 m.

Step 2: Count variables.

Three! We can solve the problem.

Step 3: Solve.

The rest of this problem is just algebra. Yes, you have to do it right, but setting up the problem correctly and coming up with an answer that's reasonable is more important than getting the exact right answer. Really! If this part of an AP free-response problem is worth 5 points, you might earn 4 of those points just for setting up the equation and plugging in values correctly, even if your final answer is wrong.

But which equation do you use? We have enough information to use **(xx0 = v0t 1/2at2) to solve for t. Note that using **means that we'll have to solve a quadratic equation; you can do this with the help of the quadratic formula.4 Or, if you have a graphing calculator and you're working on the free-response portion of the exam (where calculators are allowed), you can use your calculator to solve the equation.

Practice problems for these concepts can be found at:

Kinematics Practice Problems for AP Physics B & C

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