# 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.

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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 Honors A/B –Day 10/27-10/28/14 FREE BODY DIAGRAMS AND FRICTIONAL FORCES

Test PeriodClass Average Failing %A % 171 (-2)35 (-3)18 (+1) 363 (-1)68 (+13)16 (+16) 464 (-5)50 (-3)0 (-13) 685 (+6)13 (-40)40 (+0) 775 (-7)33 (-6)50 (+23) 878 (-2)35 (-2)20 (-6)

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

What causes Friction  Static friction arises from  microscopic roughness of the two surfaces  Electrostatic 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 Problem

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