Download 04-16 Applying Newton`s 2nd Law

04-16 Applying Newton’s 2nd Law
Name:________________________________
Warm Up: A student, standing on a scale in an elevator at rest on the ground floor, sees that his weight is 840 N.
 As the elevator rises, the scale reading increases to 1050 N, then returns to normal.
1. Which statement best describes the elevator’s motion?
(a) The elevator moves at a constant velocity the entire time
Explain:
(b) The elevator accelerates (speeds up) the entire time
(c) The elevator decelerates (slows down) the entire time
(d) The elevator accelerates at first, then moves at a constant velocity
(e) The elevator decelerates at first, then moves at a constant velocity

When the elevator slows to a stop at the 10th floor, the scale reading drops to 580 N, then returns to
normal.
2. Which statement best describes the elevator’s motion now?
(a) The elevator moves at a constant velocity the entire time
Explain:
(b) The elevator accelerates (speeds up) the entire time
(c) The elevator decelerates (slows down) the entire time
(d) The elevator accelerates at first, then comes to rest
(e) The elevator decelerates at first, then comes to rest
Scale reading increases (1050N)
a. Force Diagram
Scale reads normal (840N)
a. Force Diagram
Scale reading decreases (580N)
a. Force Diagram
b. Fnet = ______
b. Fnet = ______
b. Fnet = ______
c.
c.
c.
a = ______
a = ______
Force Equations :
F
a  net or Fnet  ma
m
Fg  mg
Fs  k x
a = _______
Kinematic Equations :
v  at  v0
x  12 at 2  v0t
A grocery bag is filled with 20 kg of groceries and lifted with an acceleration of 5.0 m/s 2.
The maximum force that a grocery bag can withstand
Make a quantitative force diagram for the grocery bag. without ripping is 250 N. Do the groceries stay in the
bag or does the bag rip? Explain using evidence and
reasoning from your force diagram and calculations.
1.
Fnet = __________
Suppose the 20kg bag of groceries is held in
equilibrium by a spring that is stretched 25cm. What is
the spring constant (k) of the spring?
2. A 1000kg car is traveling at 25m/s (55 mph). The driver suddenly sees an accident blocking the road 70 meters
ahead and slams on the brakes, which can apply a maximum force of 5000N.
How much time does it take the car to come to a full
Make a quantitative force diagram for the car while
stop?
braking.
Fnet = __________
a = ___________
Does the driver have enough time to stop before
reaching the accident that is 70 meters ahead? Use
calculations to justify your answer.