Download Physics 30 2.6 - Static and Kinetic Friction

Physics 30 2.6 ­ Static and Kinetic Friction
Physics 30
Unit 2 ­ Mechanics and Force
Lesson 2.6 Outline
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Introduction to Friction
Static Friction
Kinetic Friction
Why friction happens
Example Problems
You will be able to: • solve problems involving both static and kinetic friction
• explain why friction happens
• create FBD's that take friction into account
• differentiate between static and kinetic friction
**Unit 2 Test ­ Thursday March 24th**
What is Friction?
Friction is the force that opposes motion. Friction prevents motion from starting and stops motion from continuing forever. Air resistance is one form of friction. Friction converts kinetic energy to thermal energy (heat).
Try this:
Take your bare hands and place them together. Rub them together quickly. What did you notice?
Physics 30 2.6 ­ Static and Kinetic Friction
Basic Friction Examples
The force of friction between a surface and a stationary 5.00 kg block is 7.00 N. What is the acceleration of the block when a 6.20 Newton force is applied to it?
Basic Friction Examples
If a 3.00 kg cart moves at a constant 4.00 m/s when being pushed for 5.00 s by a force of 6.00 N, what is the force of friction acting on the cart?
Physics 30 2.6 ­ Static and Kinetic Friction
Basic Friction Examples
A constant force of 15.0 N is applied to a block of mass 3.00 kg on a horizontal table. If in 6.00 s the block’s velocity increases from 2.00 m/s to 23.0 m/s, what is the force of friction acting on the block?
More Challenging Friction Problem
A 1.50 kg block is at rest on a horizontal surface. The force of friction between the block and the surface is 3.00 N. The block is pulled by a force of 7.50 N for a distance of 13.5 m and then it is released. How far will it be from its starting point 5.00 s after it started to move?
Physics 30 2.6 ­ Static and Kinetic Friction
Heavier objects experience a larger force of friction than lighter objects due to their inertia. It takes more force to slow down a large object in the same period of time. However, this means that the force of friction must be related to the weight of the object in some way. While not a direct calculation, the force of friction is related to the normal (support) force an object experiences.
Ff = μFN
Ff = Force of Friction
μ = Coefficient of Friction
FN = Normal (support) Force
Depends on the substances
Physics 30 2.6 ­ Static and Kinetic Friction
Important Notes
The Normal Force is equal to the Weight of the object on a level surface. (Fg = mg)
The coefficient of friction is usually between 0 and 1 and depends on the substance involved.
Friction ALWAYS acts in the opposite direction of motion. Two types of friction
Static friction is the force of friction that opposes motion from starting.
Kinetic friction is the force of friction that opposes motion once it has begun.
Note that regardless of the situation Static friction force should be greater than Kinetic friction force.
Static Friction > Kinetic Friction
Physics 30 2.6 ­ Static and Kinetic Friction
Why is there a difference?
• Object surfaces are never perfectly smooth
these imperfections enable friction
• Objects settle into these imperfections
initial force needed to move out of the imperfections (Static Friction)
• Objects grind over the top of these imperfections during motion
not all of the surface is involved during motion (Kinetic Friction)
Two equations
Fs = μsFN
When the object isn't moving
Fk = μkFN
When the object is moving
Fs > Fk because μs > μk for the same substance.
Physics 30 2.6 ­ Static and Kinetic Friction
A horizontal force of 400.0 N is required to pull a trunk that weighs 1760 N across the floor at constant speed. Find the coefficient of sliding friction.
Physics 30 2.6 ­ Static and Kinetic Friction
How much force must be applied to push a 1.35 kg book across the desk at constant speed if the coefficient of sliding friction is 0.30? If a 1500.0 N force is exerted on a 200.0 kg crate to move it across the floor. If the coefficient of friction is .250, what is the crate's acceleration? Physics 30 2.6 ­ Static and Kinetic Friction
A 65 N boy sits on a sled weighing 52 N on a horizontal surface. The coefficient of friction between the sled and the snow is 0.012. What is the magnitude of the frictional force? The sled is pulled at constant speed by a rope held horizontally. What is the tension (the pull) in the rope?