Happy Monday
• Today: Equilibrium Examples
• HW: POTW #5 (Due Friday in class), WebAssign (Due Friday at
Midnight)
• Tomorrow: Practice w/ Equilibrium Problems
Static Equilibrium
• Objects are in a state of equilibrium if Fnet = 0.
• Equilibrium: the state of motion with no
acceleration
• At rest
• At constant velocity
• Application of Newton’s 1st Law
• Forces in each direction add to zero
Steps to Solving Equilibrium Problems
• Free Body Diagram
• Label all REAL forces
• Break down any forces which are at angles into their x-
and y-components (do not do this on the diagram
itself)
• Fnet Equation
• “Sum of” the forces
• One for each axis
• Set equal to zero
We set the equation to
‘zero’ because the net
force is equal to zero.
Sample
A 3.2 kg paint bucket is hanging by a massless cord
from a 4.5 kg paint bucket attached to another
cord that is pulled up. If the buckets are rising at
1.2 m/sec, determine the tension in each rope.
Sample
Superhero and Trusty Sidekick hanging
motionless from a rope. Superhero’s mass is 90.0
kg, while Trusty Sidekick’s is 55.0 kg, and the
mass of the rope is negligible.
a) Draw a free-body diagram of the situation
showing all forces acting on Superhero and
Trusty Sidekick.
b) Find the tension in the rope above
Superhero.
c) Find the tension in the rope between
Superhero and Trusty Sidekick.
Sample
Find the tension in each cable supporting the 600 N cat
burglar.
Sample
A traffic light weighing 125 N hangs from a cable which is
tied to two other cables fastened to a support. The upper
cables make angles as shown below. Find the tension in
all three cables.
Good Wednesday to You!
• Trade N Grade Yesterday’s WS
• HW: WebAssign and POTW
• TONIGHT: Read Chapter 5
Quiz on reading tomorrow
• If you take notes you can use them on the quiz
•
• Newton’s 2nd Law!
Newton’s 2nd Law
• If a net external force acts upon an object, the object must
accelerate in the direction of the net force.
• If an object is accelerating, there must be a net external force
acting upon the object.
• m is mass (kg)
𝐹𝑛𝑒𝑡 = 𝑚𝑎
• a is acceleration (m/s2)
• Fnet is the net force in Newtons (N)
It is okay to define
directions based on
direction of acceleration
(i.e. let the direction of
acceleration be the
positive direction).
Net Forces
• No net force
• Net force
Steps to Solving Any Force Problem
• Since we now have an equation to set the net force
equal to (Fnet = ma) we follow the same steps as before
but set the sum of forces equal to 𝑚𝑎 instead of zero.
• Draw a FBD
• Create a Fnet equation for each axis
• Set equal to ma
• Remember, acceleration is a vector as well. So we are really
setting up the following equations:
• 𝐹𝑛𝑒𝑡𝑥
• 𝐹𝑛𝑒𝑡𝑦
= max
= 𝑚𝑎𝑦
One More Day…
• HW: Test Addendum Tonight
• Webassign #8 Due tomorrow at Midnight
• Today:
• Chapter 5 Quiz
• 2nd Law Examples
• Tomorrow: Force Table Lab!
Sample
The horizontal surface shown on which the block slides is
frictionless. If F = 30 N and M = 3.0 kg, what is the
magnitude of the resulting acceleration of the block?
Sample
A 10 kg box sits on the bed of a truck. If the truck
accelerates from rest to 10 m/sec in 3.0 sec, determine
the force of friction on the box.
Sample
A 0.14 kg baseball traveling at 35 m/sec strikes the
catcher’s mitt, which moves back 11 cm in bringing the
ball to rest. What is the average force of the glove on the
ball?
Sample
A 75 kg petty thief wants to escape from a third story
window. Unfortunately, the rope he makes can only hold
580 N without breaking. How fast must the thief
accelerate down the rope to avoid it breaking?