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.

SampleSuperhero 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)

•a is acceleration (m/s2)

•Fnet is the net force in Newtons (N)

𝐹 𝑛𝑒𝑡=𝑚𝑎It 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 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:

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?