calculating friction

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Force Calculating friction

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Slides for teaching the factors affecting the magnitude of friction force, and how to calculate this for static and kinetic friction on horizontal surfaces and inclined planes.

TRANSCRIPT

Page 1: Calculating friction

Force

Calculating friction

Page 2: Calculating friction

fs max & fk

are affected by

Page 3: Calculating friction

fs max & fk

are affected by

Smoothness

Page 4: Calculating friction

fs max & fk

are affected by

affected by

materials

Smoothness

Page 5: Calculating friction
Page 6: Calculating friction
Page 7: Calculating friction

fs max & fk

are affected by

Smoothness Normal force

Page 8: Calculating friction

fs max & fk

are affected by

Smoothness Normal force

Page 9: Calculating friction

fs max & fk

are affected by

Smoothness Normal forceµs or µk

Page 10: Calculating friction

Materials in contact µs µk

Steel on Teflon 0,04 0,04

Wood on snow 0,08 0,06

Metal on metal (lubricated) 0,10 0,07

Wood on wood 0,7 0,4

Steel on steel 0,75 0,57

Glass on glass 0,9 0,4

Rubber on concrete 0,9 0,7

Page 11: Calculating friction

fs max & fk

are affected by

Smoothness Normal forceµs or µk N

Page 12: Calculating friction

fs max & fk

are affected by

Smoothness Normal forceµs or µk N

Page 13: Calculating friction

fs max = µs N

Page 14: Calculating friction

fk = µk N

Page 15: Calculating friction

Maximum static friction

Fstatic friction max = µs N

coefficent of static friction;depends on the materials of the rubbing surfaces

normal force surface exerts on object;depends on object’s weight, downward forces on object and surface’s slope; if no slope and no extra forces, = weight

Page 16: Calculating friction

Kinetic friction

Fkinetic friction = µk N

coefficent of kinetic friction;depends on the materials of the rubbing surfaces

normal force surface exerts on object;depends on object’s weight, downward forces on object and surface’s slope; if no slope and no extra forces, = weight

Page 17: Calculating friction

Question 1

A 4kg concrete slab is pulled horizontally along a piece of rubber.

Calculate the frictional force:a) just before the slab starts to move(maximum static friction)b) as the slab moves along (kinetic friction)

Rubber-concrete:

µs = 0,9µd = 0,7

Page 18: Calculating friction

mass = 4 kg

Page 19: Calculating friction

mass = 4 kg weight = 40 N

Page 20: Calculating friction
Page 21: Calculating friction

Normal

Weight

Page 22: Calculating friction

40 N

40 N

Page 23: Calculating friction

fs max = µs N

fs max = 0,9 • 40 N

fs max = 36 N

a)

Page 24: Calculating friction

fk = µk N

fk = 0,7 • 40 N

fk = 28 N

b)

Page 25: Calculating friction

Question 2

A 4 kg wooden sled is pulled up a snow-covered slope which is at 20° to the horizontal.

Page 26: Calculating friction

Question 2

A 4 kg wooden sled is pulled up a snow-covered slope which is at 20° to the horizontal.

Calculate the frictional force:a) just before the sled starts to move(maximum static friction)b) as the sled moves along (kinetic friction)

Wood-snow:

µs = 0,08µd = 0,06

Page 27: Calculating friction

mass = 4 kg

20°

Page 28: Calculating friction

mass = 4 kg weight = 40 N

20°

Page 29: Calculating friction

Weight

Page 30: Calculating friction

WWy

W

Page 31: Calculating friction

WWy

W

N

Page 32: Calculating friction

WWy

20°20° W

N

Page 33: Calculating friction

WWy

20°20°

Wy = W • cos 20°W

N

Page 34: Calculating friction

WWy

20°20°

Wy = W • cos 20°Wy = 40 N • cos 20°

W

N

Page 35: Calculating friction

WWy

20°20°

Wy = W • cos 20°Wy = 40 N • cos 20°Wy = 37,59 N

W

N

Page 36: Calculating friction

W37,59 N

37,59 N

Page 37: Calculating friction

Materials in contact µs µk

Steel on Teflon 0,04 0,04

Wood on snow 0,08 0,06

Metal on metal (lubricated) 0,10 0,07

Wood on wood 0,7 0,4

Steel on steel 0,75 0,57

Glass on glass 0,9 0,4

Rubber on concrete 0,9 0,7

Page 38: Calculating friction

fs max = µs N

fs max = 0,08 • 37,59 N

fs max = 3 N

a)

Page 39: Calculating friction

fk = µk N

fk = 0,06 • 37,59 N

fk = 2,26 N

b)

Page 40: Calculating friction

fs max & fk

are affected by

Smoothness Normal forceµs or µk N

Page 41: Calculating friction

Question 2c

A 4 kg wooden sled is pulled up a snow-covered slope which is at 20° to the horizontal.

c) How hard must a person pull parallel to the slope upward for the sled to move at a constant velocity?

Page 42: Calculating friction

Question 2c

A 4 kg wooden sled is pulled up a snow-covered slope which is at 20° to the horizontal.

c) How hard must a person pull parallel to the slope upward for the sled to move at a constant velocity?

(to balance the forces of:- kinetic friction and - the component of the sled’s weight down the

slope)

Page 43: Calculating friction

Fdown slope

Fup slope

For CONSTANT VELOCITY:

Page 44: Calculating friction

Fdown slope

Fup slope

For CONSTANT VELOCITY:

Page 45: Calculating friction

WWy

N

Page 46: Calculating friction

W

Component of W down slope

Page 47: Calculating friction

W x

y

Page 48: Calculating friction

W

Wx

Page 49: Calculating friction

W

Wx

Wx

Page 50: Calculating friction

W

Wx

Wx

𝜃

Wx = W • sin 𝜃

Wy

Page 51: Calculating friction

W

Wx

Wx

20°

Wx = W • sin 20°20 °

Page 52: Calculating friction

40 N

Wx

Wx

20°

Wx = W • sin 20°Wx = 40 • sin 20°

Page 53: Calculating friction

40 N

Wx

Wx

20°

Wx = W • sin 20°Wx = 40 • sin 20°Wx = 13,68 N down the slope

Page 54: Calculating friction

40 N

13,68 N20°

Page 55: Calculating friction

13,68 N

Person pulls

Page 56: Calculating friction

13,68 N

f

Person pulls

Page 57: Calculating friction

13,68 N

f

Page 58: Calculating friction

fk = µk N

fk = 0,06 • 37,59 N

fk = 2,26 N

b)

Page 59: Calculating friction

13,68 N

fk = 2,26 N

Page 60: Calculating friction

13,68 N

fk = 2,26 N

Fdown slope = 15,94 N

Page 61: Calculating friction

13,68 N

fk = 2,26 N

Fdown slope = 15,94 N

Fup slope = 15,94 N

For CONSTANT VELOCITY:

Page 62: Calculating friction