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CPS Question
Which team wins a game of tug-of war?
A. The team that pulls hardest on the rope.
B. The team that pushes hardest on the floor.
C. The outcome depends on other factors.
Stubborn Mule Problem
Let’s make free-body diagrams for the mule
and the cart!
Source: Griffith, The Physics of Everyday Phenomena
Statics
Forces in equilibrium
§ 5.1–5.2
What’s the point?
• How do forces balance?
• What forces must structures withstand?
• What is the result of non-balancing forces?
CPS Question
weight
1m
F
8m
F
A hammock slung between trees 8 m apart sags
1 m when a person lies in it. The tension of each
rope holding the hammock is
A.
B.
C.
D.
Equal to the weight of the person.
Half the weight of the person.
More than the weight of the person.
Less than the weight of the person, but more than half.
Group Work
weight
1m
F
8m
F
A hammock slung between trees 8 m apart sags
1 m when a person lies in it.
Make a free-body diagram for the hammock.
Free-Body Diagram
tension
tension
weight
Acceleration = 0, so forces add to zero
Observe the magnitudes!
Example Problem 4.1001
A person pulls on a rope attached to a 25-kg box
on a frictionless plane inclined 30° to the
horizontal. She pulls with a force of 20 N parallel
to the plane of the ramp. Find the speed and
position of the box after 5 s.
30°
Group Work
Make a free-body diagram of the box.
30°
Example Problem 4.1002
A cart of mass M is attached to a free-hanging
weight of mass m over a frictionless pulley by a
massless string. Find the acceleration of the cart
and the tension in the rope.
M
m
Example Problem 4.49
Two blocks are connected by a
heavy uniform rope with a mass
of 4 kg. An upward force of
200 N is applied.
a)
b)
c)
d)
6 kg
5 kg
Draw the FBD for each object
What is the acceleration of the system?
What is the tension at the top of the rope
What is the tension at the bottom of the rope
4 kg