2.19 - Interactive checkpoint: penny drop
You drop a penny off Taiwan’s Taipei
101 tower, which is 509 meters tall.
How long does it take to hit the
ground? Ignore air resistance,
consider down as negative, and the
ground as having zero height.
Answer:t =s
2.20 - Gotchas
Some errors you might make, or that tests or teachers might try to tempt you to make:Switching the order in calculating displacement. Remember: It is the final position minus the initial position. If you start at a position of three
meters and move to one meter, your displacement is negative two meters. Be sure to subtract three from one, not vice versa.
Confusing distance traveled with displacement. Displacement is the shortest path between the beginning point and final point. It does not
matter how the object got there, whether in a straight line or wandering all over through a considerable net distance.
Forgetting the sign. Remember: displacement, velocity and acceleration all have direction. For one-dimensional motion, they require signs
indicating the direction. If a problem says that an object moves to the left or down, its displacement is typically negative. Be sure to note the
signs of displacement, velocity or acceleration if they are given to you in a problem. Make sure you are consistent with signs. If up is positive,
then upward displacement is positive, and the acceleration due to gravity is negative.
Confusing velocity and acceleration. Can an object with zero acceleration have velocity? Yes! A train barreling down the tracks at 150 km/h has
velocity. If that velocity is not changing, the train’s acceleration is zero.
Can an object with zero velocity have acceleration? Yes again: a ball thrown straight up has zero velocity at the top of its path, but its
acceleration at that instant is í9.80 m/s^2.
Confusing constant acceleration with constant velocity. If an object has constant acceleration, it has a constant velocity, right? Quite wrong
(unless the constant acceleration is zero). With a constant acceleration other than zero, the velocity is constantly changing.
Misunderstanding negative acceleration. Can something that is “speeding up” also have a negative acceleration? Yes. If something is moving
in the negative direction and moving increasingly quickly, it will have a negative velocity and a negative acceleration. When an object has
negative velocity and experiences negative acceleration, it will have increasing speed. In other words, negative acceleration is not just “slowing
something down.” It can also mean an object with negative velocity moving increasingly fast.(^42) Copyright 2000-2007 Kinetic Books Co. Chapter 02