101 PHYS CH3 Part 2 Section (12) understand Friction Force. Draw free-body diagrams for objects at rest and in motion with Friction Force. Apply your.

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Presentation transcript:

101 PHYS CH3 Part 2 Section (12)

understand Friction Force. Draw free-body diagrams for objects at rest and in motion with Friction Force. Apply your understanding of kinetic and static friction to solving problems.

Friction forces are parallel to the surfaces in contact and opposite the motion or opposite the net force. Friction is a force that resists motion when two objects are in contact. F F friction

If you look at the surfaces of all objects, there are tiny bumps and ridges. Those microscopic peaks and valleys catch on one another when two objects are moving past each other.

1- Static frictional force: is the frictional force that prevents two surfaces from sliding past each other.. 2- kinetic frictional force: when object is sliding. Static frictional forces always greater than kinetic frictional force.

Law of Frictional Forces F fsfs FNFNFNFN Where: f s : static friction force. F N  s : Coefficient of static friction force. F N : normal force.

2 N2 N2 N2 N 4 N The force required to overcome static or kinetic friction is proportional to the normal force, F N The force required to overcome static or kinetic friction is proportional to the normal force, F N. FNFNFNFN 12 N 6 N FNFNFNFN 8 N 4 N FNFNFNFN

Material on Material  s = static friction  k = kinetic friction steel / steel Synovial Joint in Humans metal / ice brake lining / iron tire / dry pavement tire / wet pavement

Law of Maximum Frictional Force When an attempt is made to move an object on a surface, static friction slowly increases to a Maximum value. Where: f s : static friction force. F N  s : Coefficient of static friction force. F N : normal force. In this module, when we use the following equation, we refer only to the maximum value of static friction and simply write: F N F s max =  s F N

When F is greater than the maximum f s the object move. F N f k =  k F N F fkfk f k : Kinetic friction force. F N  k : Coefficient of kinetic friction force. F N : normal force.

Material on Material  s = static friction  k = kinetic friction steel / steel Synovial Joint in Humans metal / ice brake lining / iron tire / dry pavement tire / wet pavement

If the total mass pulled is constant, the frictional force is independent of the contact area. 4 N4 N4 N4 N 4 N4 N4 N4 N

2 N 5 m/s 2 N 20 m/s The force of kinetic friction is the same at 5 m/s as it is for 20 m/s. So: kinetic friction forces are independent of speed.

F

For this case: F – f s = 0 f s max =  s N F N = ?  F y = 0 F N – F g = 0 F g = 250 N F N = 250 N

For this case f s max For this case: F– f s max = 0 Next we find f s max from : Next we find f s max from : f s max =  s F N = 0.5 (250 N) F = f s max = 0.5 (250 N) F = 125 N

F

 F y = m a y = 0 F N - F g = 0 F N = F g =250N b) f k =  k F N  k = f k / F N  F x = 0; F - f k = 0 f k = F=75N  k = f k / F N  k = f k / F N =75/250=0.3

F = 67 N

 F y = m a y F N + 67 sin 40 – 300 = 0 F N = 300 –44=256 N a) F N =?

f k - F cos 40 0 = m (0) f k - 67 cos 40 0 = 0 f k = 67 cos 40 0 = 51 N  F x = m a x b) f k = ?

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