Dynamics – Ch. 6 – part 2

•  Friction

– Static – Kinetic – Drag

•  More Examples Using Newton’s Second Law

Fermi Problem #2

•  Explicitly state all your (hopefully reasonable!) assumptions. Show all your work, including units!

•  There is no “right” answer? Estimate everything within an order of magnitude.

•  Write down who you worked with, roughly how long you spent, and what numbers you looked up, if any.

•  Due Thursday 10/18.

Fleas can jump proportionately higher than any other animal. What is their acceleration as they jump?

Friction

Static Friction

The box is in static equilibrium, so the static friction must exactly balance the pushing force:

Static Friction There’s clearly a limit to how big fs can get. If you push hard enough, the object slips and starts to move. In other words, the static friction force has a maximum possible size fs max. •  An object remains at rest as long as fs < fs max. •  The object slips when fs = fs max. •  A static friction force fs > fs max is not physically possible.

where the proportionality constant µs is called the coefficient of static friction.

Kinetic Friction

The kinetic friction force is proportional to the magnitude of the normal force.

where the proportionality constant µk is called the coefficient of kinetic friction.

Relating Friction to the Normal

Both kinetic and static friction are perpendicular to the normal Use these formulas to calculate magnitude

Important Concepts

Applications

Think – Pair - Share Rank order, from largest to smallest, the size of the friction forces to in these five different situations. The box and the floor are made of the same materials in all situations.

A.  fc > fd > fe > fb > fa. B.  fb > fc = fd = fe > fa. C.  fb > fc > fd > fe > fa. D.  fa > fc = fd = fe > fb. E.  fa = fb > fc = fd = fe.

fa

fe

Rank order, from largest to smallest, the size of the friction forces to in these five different situations. The box and the floor are made of the same materials in all situations.

A.  fc > fd > fe > fb > fa. B.  fb > fc = fd = fe > fa. C.  fb > fc > fd > fe > fa. D.  fa > fc = fd = fe > fb. E.  fa = fb > fc = fd = fe.

fa

fe

The terminal speed of a Styrofoam ball is 15 m/s. Suppose a Styrofoam ball is shot straight down with an initial speed of 30 m/s. Which velocity graph is correct?

The terminal speed of a Styrofoam ball is 15 m/s. Suppose a Styrofoam ball is shot straight down with an initial speed of 30 m/s. Which velocity graph is correct?

Friction Activity

•  Determine the coefficient of static and kinetic friction

•  Equipment •  Board •  Wooden Block •  Protractor

Upcoming Assignments

•  Read Chapter 7 to 7.3 by Tuesday 10/16 •  Pre-class MasteringPhysics assignment due before

class on Tuesday 10/16 Thursday 10/18 •  Read Chapter 7.4 to end by •  Pre-class MasteringPhysics assignment due before

class on Thursday 10/18 •  MasteringPhysics online HW due, Thursday 10/18 by

11 pm