Here's the second of the daily fundamentals checks. My answers will be in the comments either Sunday or Monday. Enjoy!
11.
Give
a specific example of a situation in which a moving object’s acceleration is
NOT constant.
12. Object A moves left with momentum 2.0 N·s; object B moves right with momentum 3.0 N·s. What is the total momentum of the two-object system?
14.
Write
the formula for gravitational potential energy.
15. An object moves downward and slows down. Which is correctly set equal to ma? Choose one.
(A A) (up
forces) – (down forces)
(B B) (down forces) – (up forces)
1616. Define power.
1617. Write the definition of torque. (Not Newton’s second law for rotation, the equation that defines torque.)
Questions 18 and 19: A ball is thrown up and to the right with speed v at an angle of 30o above the horizontal.
18. What is the vertical component of the ball’s acceleration immediately after it’s released?
19. What is the horizontal component of the ball’s velocity as it reaches the ground?
20.
A car speeds up while moving east. The two horizontal forces acting on the car
are the force of the engine and the force of friction. What is the direction of the car’s
acceleration?
Thank you very much for sharing these. This will be really helpful for the review process in my AP classes!
ReplyDeleteMore followups:
ReplyDelete14b) Now write the *other* formula for gravitational potential energy. When should you use one vs. the other?
19b) What's the slowest speed the ball will reach while in midair, and at what exact point in the flight will the ball have that speed?
19c) What's the ball's acceleration at the moment in 19b), and is the ball speeding up, slowing down, or neither at that moment?
Only one I don't like so far is #20, because I've been really strict with my class about nothing internal (like the engine within a car) being able to exert a force that makes the system accelerate. Might just get rid of the middle sentence.
14b) U_G = Gm1m2/r . Use U_g = mgh when the system is the Earth and a small object on or near the surface of the Earth. Use U_G = Gm1m2/r in all other cases.
Delete19b) v_min = v*cos(30), which is the x-component of the initial velocity. The ball will have this speed at the maximum height of its trajectory.
19c) At the max height of the trajectory a = -g = -9.8 m/s/s. The ball is speeding up downwards.
Correct except for 19c. According to the practice questions for AP Physics 1 (I'm referring to the 2015 guide), the ball is neither speeding up nor slowing down at that moment, because acceleration is perpendicular to velocity at that instant.
DeleteAh yes, I see now this is correct.
DeleteSolutions to Fundamentals check #2:
ReplyDelete11. object on a curved track; anything in simple harmonic motion; an object falling where air resistance is important; etc.
12. 1.0 Ns. (Momentums in opposite directions subtract, i.e. momentum is a vector.)
13. static.
14. mgh. (That's near Earth's surface. Physics C students, or P1 students as of 2025 or 2026, may choose to write negative GmM/d, which is valid everywhere as long as you take the zero of PE to be a very long distance away from a planet.)
15. A. (When an object slows, acceleration is opposite the direction of motion, so acceleration and unbalanced force are upward; I suggest never trust students with a negative sign, so always start N2L with the bigger force minus the smaller force.)
16. energy per time. (Equivalently, work per time, or energy used in one second.)
17. force times lever arm.
18. g, i.e. 10 m/s/s.
19. v cos (30)
20. east. (When an object speeds up, the acceleration is in the direction of motion.)