71. A 0.60 kg fishbowl is lifted from the ground. What is the
smallest force needed to lift it? Describe its resulting motion.
Fnet = Fscale + Fg = 0
 Fscale = - Fg
 Fscale = - (-mg) = mg
 Fscale = (0.60 kg)(9.8 m/s2)
= 5.88 N

If Fnet = 0, then a = 0
 If a = 0, then motion is constant velocity


Force exerted by fluid (air or water) on
object moving through fluid
• Ex: air resistance


Opposes motion
Increases with object’s velocity

Object in free fall
• No vi
• Accelerating downward, a = g
t=0s
v = 0 m/s
t=1s
v = 9.8 m/s
t=2s
v = 19.6 m/s
t=3s
v = 29.4 m/s


Reach a point where Fnet = 0
Fdrag = -Fg
Object stops accelerating
 Continues to move with constant
velocity


Terminal velocity
Hannah is going skydiving for her birthday. She has a mass of
68 kg, and jumps from rest. She reaches terminal velocity after
falling for 15 seconds.
(a) What drag force is exerted on her when she reaches
terminal velocity?
(b) What is her terminal velocity?



Newton’s 3rd Law:
FA on B = – FB on A
Two forces, in opposite directions, with
equal magnitude
Two free body diagrams
F boy on snowball
F snowball on boy
Two blocks, masses 4.3 kg and 5.4 kg are pushed across a
frictionless surface by a horizontal force of 22.5 N.
(a) What is the acceleration of the blocks?
(b) What is the force of the 4.3 kg block on the 5.4 kg block?
(c) What is the force of the 5.4 kg block on the 4.3 kg block?



force exerted by a string
or rope
FT
equal to weight of all
forces hanging from rope
A 45 kg child sits on a 3.2 kg tire swing. What is the
tension in the rope that hangs from a tree branch?
Fnet = Fscale + Fg
Fnet = Fupward + Fg
scale
tension
Snow White has been sent by the Queen to fetch a bucket of water
from a well, so that she can clean the castle. If the rope attached to
the bucket is guaranteed not to break as long as the tension does
not exceed 450 N and she fills the bucket until it has a mass of 42 kg,
what is the greatest acceleration that she can give the bucket as she
pulls it up from the bottom of the well?
Two blocks, one of mass 5.0 kg,
and the other of mass 3.0 kg,
are tied together with a
massless rope. This rope is
strung over a massless,
resistance-free pulley. The
blocks are released from rest.
Find (a) the tension in the rope,
and (b) the acceleration of the
blocks.
[Hint: you will need to solve a
system of two equations.]
5 kg
3 kg
contact force exerted by a surface on
another object
 perpendicular to surface, upward

F normal
F gravity
F normal puts system in equilibrium
Andy and his family are packing up their car, getting ready for him
to move to college. They put one 7 kg crate of books in the car, and
then put a 2 kg box of action figures on top of it.
(a) What is the normal force on the 2 kg box?
(b) What is the normal force on the 7 kg box?