UNIT 4Forces and

the Laws of Motion

ConcepTest 5.1a Gravity and Weight I

1) Fg is greater on the feather

2) Fg is greater on the stone

3) Fg is zero on both due to vacuum

4) Fg is equal on both always

5) Fg is zero on both always

What can you say

about the force of

gravity Fg acting on a

stone and a feather?

The force of gravity (weight) depends

on the mass of the object!! The stone

has more mass, therefore more weight.

ConcepTest 5.1a Gravity and Weight I

1) Fg is greater on the feather

2) Fg is greater on the stone

3) Fg is zero on both due to vacuum

4) Fg is equal on both always

5) Fg is zero on both always

What can you say

about the force of

gravity Fg acting on a

stone and a feather?

Force of Static Friction

FaFfs m

Fg = mg

FN

Everyday Forces

Force of Kinetic Friction

FaFfk m

Fg = mg

viFN

Everyday Forces

Steel on steel 0.74 0.57Aluminum on steel 0.61 0.47Copper on steel 0.53 0.36Rubber on concrete 1.0 0.8Wood on wood 0.25-0.5 0.2Glass on glass 0.94 0.4Waxed wood on wet snow 0.14 0.1Waxed wood on dry snow ------ 0.04Metal on metal (lubricated) 0.15 0.06Ice on ice 0.1 0.03Teflon on Teflon 0.04 0.04

Coefficients of Frictionμs μk

Everyday Forces

x

y

mg

mg cos(q)

mg sin(q)

Review of Similar Triangles

Everyday Forces

q

q

FN

mg sin(q)

mg

mg cos(q)

Find the acceleration on a smooth incline (NO friction)

ΣFy = 0

FN = mg cos(θ)

ΣFx = ma

Everyday Forces

a

mgsin() = ma

a = gsin()

mg sin(q)

y

x

9

END