Day 5

• “Physics is the only piece of magic I have ever seen. I am grateful for real magic.”

• Jim Gates, University of Maryland, on being asked what about physics he is most thankful for, PBS.org, November 22, 2011.

2nd Tuesday

• Is there anyone who still does not have a textbook?

• Texting the student(s)• Texting the instructor

Section 6-1, Forces of Friction

• Force of kinetic friction, fk = k N 6-1

• k = coefficient of kinetic friction

• N = normal force

• fs,max = sN 6-3

• s = coefficient of static friction

• fs = Force of static friction

Friction as a function of Force

Hockey Puck

• A hockey puck on a frozen pond is given an initial speed of 20.0 m/s. If the puck always remains on the ice and slides 115 m before coming to rest, determine the coefficient of kinetic friction between the puck and the ice.

• Assume k = constant. We always consider acceleration a constant in this course.

• ? Find k ?

Box on a SlopeExperimental determination

of s and k

Mooring a Ferry Boat

• First loop around bollard on pier• Then one or more additional loops

• ? How many loops does it take ?

Locomotive – Coal Cars

• Assume Locomotive = 220 x 103 kg• Assume Coal car = 120 x 103 kg.• Assume s = steel to steel for driver wheels

assume 0.50• Assume Rolling Resistance Coefficient, RRC,

for coal cars 0.0005.• ? How many coal cars can this locomotive pull

on level tracks ?

The Body Farm, by Cornwell

• Skid marks are associated with kinetic friction.• Antilock brakes assure the stopping force that

results from static friction.

Sheldon at the DMV

• Sheldon’s education of the DMV

Sheldon’s Excellent Explanation• Question 2: When are roadways most

slippery?• Sheldon’s answer: “When covered by a film of

liquid sufficient to reduce the coefficient of static friction between the tire and the road to essentially zero, but not so deep as to introduce a new source of friction.”

Strings and Ropes

• For this course, we will assume all ropes, strings, wires, and so on are practically massless – unless specifically stated otherwise – and, therefore, that the tension is the same throughout their length.

Springs and Hooke’s Law• F = kx gives magnitude of force• F = -kx gives direction of force• k = spring constant

Error on page 160

• See the illustration at the lower left on the spiral spring inside a watch.

• Book say is exerts enough force to power the delicate mechanism of the watch.

• In fact, the spring regulates the mechanism of the watch. A much larger spring, the mainspring, provides the watch’s power.

Running Blades are Springs

• Amy on Dancing With The Stars

Translational Equilibrium

• Also called “statics.”

• Sum of all forces Fx = 0, or Fx = 0

• Sum of all forces Fy = 0, or Fy = 0

• Object is not moving.

Traffic Light

• Suspended by cables (1) and (2) which are angled and (3) which is vertical.

• Angle for T1 is 37o.

• Angle for T2 is 53o.

• Weight of traffic light is 122 N

• ? Find T1 and T2 ?

Cat Burglar

• Mass of Burglar = 600 N• Suspended by horizontal cable (1) and angled

cable (2) which is 37o from horizontal

• ? Find T1 and T2 ?

6-4 Connected Objects

• We did experiment with connected objects in Lab #2 last Wednesday.

Circular Motion

• Centripetal force fcp = m vt2 / r

• Centripetal acceleration acp = vt2 / r

• fcp from string of David’s slingshot, sloped racetrack, chains on a child’s swing, …

• fcp is a force acting toward the center of rotation.

Incomplete words on page 172

• Under the illustrations at the bottom of the page, the caption says “airplanes bank when making turns to keep from ‘skidding’ sideways.”

• More correctly, as taught to me as a student pilot, airplanes bank to make turns. A “coordinated” turn uses both bank and rudder to turn the plane so that the sum of the forces on the passengers is always directed toward the floor of the cabin.

Grandson Kyle at Bele Chere

Grandson on “swings” ride. All seats, whether occupied or not, are standing out at the same angle from the horizontal. Why???

Banked Track

Work

• Work, W = (F cos ) x• M = 50 kg• x = 5.00 m• F = 1.20 x 102 N• For = 0, W = ?• For = 30o, W = ?

Sledding

• Eskimo sled m = 50.0 kg• k = 0.200

• F = 1.20 x 102 N• x = 5.00 m

• ? Net work done on sled for = 0o ?

Section 7-2

• Kinetic Energy = K = ½ m v2

• SI units: kg m2 / s2 = Joule, J 7-6

Baseball

• Mass of baseball m = 0.150 kg• Speed of toss = 40.0 m/s• = 30o above horizontal

• ? What is K at highest point of motion ?

Average Force

• A 2.0-g bullet leaves the barrel of a gun at a speed of 300 m/s.

• ? (a) Find its kinetic energy. ?• ? (b) Find the average force exerted by the

expanding gases on the bullet as it moves the length of the 0.50-m-long barrel. ?

Earthquake energy

• Richter: R = 2/3 log E/Eo, Eo = 104.8 J

• Google “earthquake power calculator” to get site which facilitates comparisons of earthquake energy and TNT energy in the familiar units of Joules!

Work done by a varying force

• Work = Area under Fx-x curve

• Force in N• Distance in m• Work in J

Spring Potential Energy

• Work to Stretch or Compress a Spring a Distance x from Equilibrium

• W = ½ k x2 7-8

• SI Unit: Joule, J• Spring constant, k, in N/m

Physics of a Elastic-Band Slingshot

• In years past at Tourists’ baseball games, t-shirts were shot into the stands with a slingshot.

• If pull length x is doubled• Eelastic = ½ k x2 is quadrupled

• Kinetic energy K = ½ m v2 is quadrupled (as the shirt leaves the sling shot)

Power

• Power, P = Work / t• P in watts, W• Work in J• t in seconds, s• 1 W = 1 J/s = 1.34 x 10-3 horsepower (hp)

Dave Barry Lift Station #16Grand Forks, ND

• Sewage raised 5.49 m• 1 890 000 liters of untreated sewage each day• Density of sewage is 1050 kg/m3

• ? Find the output power of the lift station ?• A continuously operating electric motor with

an average power of 5.90 kW runs the pump.• ? Find the efficiency of the lift station. ?