AP Physics

Remember, when approaching conceptual questions to use physics facts, equations, or calculations to defend and support your answer. Don't just write the physics fact (or equation). Apply that fact to the question asked and answer all parts. The more time and effort you put into these now, the more you will remember the material and the better you will do on the test. If you practice bad habits now, they will show up on the test as well. 

For all problems using forces, use the force toolkit.

Bookwork Assignment #2 Key 

(Note: The answer to problem #2 on page 127 is incorrect. The correct answer is 0.31 m/s2, west. I accidentally put one of my eastward forces pointing west). 

• The key to doing #82 (crate and truck problem) is although the box is not accelerating according to the truck, it is accelerating according to the reference frame of the earth. Therefore, static friction is equal to mass*acceleration.
• The key to doing #104 (car on hill problem) is that the FBD you are drawing is for the tire. As the car moves up the hill, your tire actually rotates backwards down the hill, therefore the static friction is opposing that backwards motion thus, it is up the hill when you are going up the hill. When going down the hill, static friction is also down the hill for the same reason. The reason it is static friction is because the tire is rotating so that part of the tire is only in contact with the surface for a brief second so it is not considered "moving" (opposed to a tire skidding on a surface).