CBA Warm up

A cup slides off the table with an unknown velocity. The table is 1 m high and the cup hits the ground 2.5 m from the edge of the table.

What was the velocity of the cup as it flew off the table?

CBA Warm up A young kid pushes a cup with a mass 0.25 kg off the table; it slides off the table with an unknown velocity. The table is 1 m high and the cup hits the ground 2.5 m from the edge of the table.

What was the force applied to the cup by the kid as it flew off the table if he pushed it for 0.5s and the cup was sitting still on the table when he started pushing it?

Physics HonorsA/B –Day 10/27-10/28/14FREE BODY DIAGRAMS AND FRICTIONAL FORCES

Test

Period Class Average

Failing % A %

1 71 (-2) 35 (-3) 18 (+1)

3 63 (-1) 68 (+13) 16 (+16)

4 64 (-5) 50 (-3) 0 (-13)

6 85 (+6) 13 (-40) 40 (+0)

7 75 (-7) 33 (-6) 50 (+23)

8 78 (-2) 35 (-2) 20 (-6)5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100More

0

2

4

6

8

10

12

14

16

Test 2

Agenda

Announcement: Retest - Friction

Static Kinetic

Free Body Diagrams

Why do we care about friction in real world?

When do we use friction? Walking, driving, writing

When would we want to reduce friction? In moving mechanical part – like engine of car –

causes wear Wastes energy

New Technology in reducing friction Liquid Crystal Lubricants, (same crystals that are in

LCD screens)

What happens when we push something We lightly apply a force. This force applied is

canceled out by the frictional force.

What happens when we push something We lightly apply a force. This force applied is

canceled out by the frictional force.

What happens when we push something We lightly apply a force. This force applied is

canceled out by the frictional force.

What happens when we push something Until we hit a point where we overcome friction

and we start moving.

2 Regions of friction are being observed Static Friction – is the resistive force that

opposes the relative motion of two contacting surfaces that are at rest with respect to one another

The friction that keeps the object from moving is static friction.

2 Regions of friction are being observed

Once the object starts moving new type of friction Kinetic Friction – the resistive force that opposes

the relative motion of two contacting surfaces that are moving past one another

The friction that slows down the object moving is kinetic friction.

2 Regions of friction are being observed

What causes Friction

Static friction arises from microscopic roughness of the two surfacesElectrostatic forces Atomic Forces

Van der Waal Forces (in non metals)Metallic bonding (in metals)

What causes Friction

The surfaces determine the coefficients of static or kinetic friction So this leads to the fact that different materials

will have different coefficients of friction

Example ProblemA wooden box is on a wood floor and is trying to be

pushed across the room. If the box has a mass of 100 kg, what will be the Force at which the box will start to move?

Example ProblemA wooden box is on a wood floor and is pushed across

the room. If the box has a mass of 100 kg, what is the Force being exerted on the box if it is accelerating with a rate of 0.7 m/s?

Example ProblemA wooden box is on a wood floor and is being pushed

across the room. If the box has a mass of 100 kg, what is the Force being exerted on the box if it is moving with a constant velocity?

Example ProblemA box is on aincline plane and being held in place by

the Force due to Static Friction. Determine the coefficient of static friction between the box and the ramp.

Example ProblemA box slides down an incline plane with an

acceleration of 1.20 . Determine the coefficient of kinetic friction between the box and the ramp.