Now that we know t , we can use either ** or * to solve for displacement. Let’s use :
Step 4 : Units.
Always remember units! And make sure that your units are sensible—if you find that an object travels a
distance of 8 m/s, you’ve done something screwy. In our case, the answers we found have sensible units.
Also, our answers seem reasonable based on the initial estimates we made: It makes sense that the car
should travel a bit more than 6 s, and it makes sense that it should go several hundred meters (about half a
mile) in that time.
Freefall
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.
g : The acceleration due to gravity near the Earth’s surface; about 10 m/s^2Falling-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/s^2 ) or “down” to be
positive (where a would therefore equal +10 m/s^2 ).
Exam tip from an AP Physics veteran:
You may remember that a more precise value for g is 9.80 m/s^2 . That’s correct. But estimating g as 10
m/s^2 is encouraged by the AP readers to make calculation quicker.—Jake, high school juniorHere’s a practice problem:
You are standing on a cliff, 30 m above the valley floor. You throw a watermelon vertically upward at a
velocity of 3 m/s. How long does it take until the watermelon hits the valley floor?