S P H 4 C – U N I T # 1 M O T I O N

FRICTION

LEARNING GOALS

After this topic, I will be able to…

• Define friction, both static and kinetic

• Describe how friction affects an object and ways to increase or decrease the affect on said object

SUCCESS CRITERIA

I am successful if I know how to…

• Define friction, both static and kinetic

• Describe how friction affects an object and ways to increase or decrease the affect on said object

THE EFFECTS OF FRICTION

Friction is a part of everyday life. Removing a fried egg from a frying pan is easier if the pan has a non-stick surface. You find running easier on a dry side-walk than on ice. Skiers use wax to reduce friction between their skis and the snow. Cars need friction to speed up, slow down, and go around corners.

FRICTION

Friction resists motion and acts in a direction opposite to the direction of motion. It occurs because of the electrical forces between the surfaces where the two objects are in contact.

STATIC VS. KINETIC FRICTION

Using a brick and a spring scale try the following:

Pull on the brick, slowly, using the spring scale paying close attention to the reading on the scale. Find the point where it is just about to start moving.

Repeat the same as the above but this time get the brick moving and note any change in the value reading on the spring scale.

Discuss!

STATIC AND KINETIC FRICTION

One type of friction, called static friction (Fs), is the force that tends to prevent a stationary object from starting to move. (“Static” comes from the Greek word statikos, which means “causing to stand.”) The maximum static friction is called the starting friction. It is the amount of force that must be overcome to start a stationary object moving. Typically, it takes more force to start an object moving than it does to keep it moving.

Static Friction (Fs):

• Force that prevents a stationary object from starting to move• Typically higher in magnitude than the frictional force when the

object is in motion

Note:In certain circumstances static friction is useful; in others, it is not.

STATIC AND KINETIC FRICTION

Once the force applied to an object overcomes the starting friction, the object begins moving. Then, moving or kinetic replaces static friction. Kinetic friction (Fk) is the force that acts against an object’s motion in a direction opposite to the direction of motion. Different types kinetic friction have different names including: sliding friction, rolling friction, fluid friction,…

Kinetic Friction (Fk):

• Force that acts against an object that is in motion

Note:For horizontal motion, if the applied force has the same magnitude as the kinetic friction, the moving object will maintain uniform velocity.

CONTROLLING FRICTION

About 4500 years ago, the Egyptians built enormous pyramids using huge stone blocks that were difficult to move my sliding. The Egyptians places logs beneath the blocks to push them and move them. By doing this, people were taking advantage of the fact that rolling friction is much less than sliding friction.

CONTROLLING FRICTION

Modern technology uses the same principles as the Egyptians, though in a more sophisticated way. For instance, all machines have moving parts that experience friction during operation. Friction can wear out the machines, reduce efficiency, and cause unwanted heat. Excess friction in machines can be overcome by making surfaces smooth, using materials with little friction, lubricating with grease or oil, and using bearings.

CONTROLLING FRICTION

Friction

• Can wear out machines, reduce efficiency, cause unwanted heat, …

• Can be controlled by:• Making surfaces smooth• Using materials with little friction• Lubricating with grease or oil• Using ball bearings

CONTROLLING FRICTION – DYK?

Ways of reducing undesirable friction in other situations are also common. The wax applied to skis reduces sliding friction. A layer of air between a hovercraft and the water reduces fluid friction. A human joint is lubricated by synovial fluid between the layers of cartilage lining the joint. The amount of lubrication provided by synovial fluid increases when a person moves. In fact, our lubricating systems work so well that is it difficult for technologists to design artificial joints that function to the same standard.

CONTROLLING FRICTION – DYK?

Although friction is often undesirable, it can be useful. For example, South America has many earthquake zones and buildings have a tendency to crumble during an earthquake. To help overcome this problem, the Inca stonemasons developed great skill in fitting building stones together very tightly so that a great deal of sliding friction would help hold their buildings together, even during an earthquake.

CONTROLLING FRICTION – DYK?

The material called Teflon has many uses when low friction is desired, such as in non-stick frying pans. Two research scientists created this chemical by chance in 1938, but its usefulness was not realized until 20 years later. Since Teflon does not stick to any materials, the process used to make it stick onto a frying pan is unique: the Teflon is blasted into tiny holes in the surface of the pan where the material sticks well enough for use.

FRICTION

Practice:

1. For each situation, determine if friction is helpful, makes the action more difficult, or both. Explain your reasoning.

a) Turning a door knobb) Streamlining in the transportation industryc) Pushing a heavy box across a rough surfaced) Tying a knote) A car going around a curve

ANALYZING MOTION WITH FRICTION

Most frictional forces are complex because they are affected by a number of factors, such as the nature of the materials involved and the size, shape, and speed of the moving object. Solving problems involving friction brings together many concepts including:• Velocity• Acceleration• Forces• Free-body diagrams• Newton’s law of motion• Weight (Fg)

ANALYZING MOTION WITH FRICTION

However, the most common mistakes that students make in solving these types of problems are:① No FBD

② Accounting for too many forces (i.e. including an applied force when there is none)

③ Not including directions in their solutions

FRICTION

The force of friction can be calculated using the following:

Force of Friction (Ff)

Where Ff is the force of friction (N)

μ is the coefficient of frictionFN is the normal force (N)

Note:μhas no units since it is a ratio of forces.

COEFFICIENT OF FRICTION

The coefficient of friction (μ) is a number that indicates the ratio of the magnitude of the force of friction, Ff, between two surfaces to the magnitude of the normal force, FN.

Coefficient of Friction (μ):• Ratio of the friction force to the normal force (μ= Ff/FN)

• Constant – only depends on nature of two surfaces in contact

Note:The only way that the coefficient of friction will change is if the two materials in contact change.

COEFFICIENT OF FRICTION – KINETIC AND STATIC

In almost all situations, the force needed to start the motion of an object initially at rest is greater than the force needed to keep it going at constant velocity. This means that the maximum static friction (Fs) is slightly greater than the kinetic friction (Fk), and the coefficients of friction for these situations are different. To account for the difference, two coefficients of friction can be determined – the coefficient of kinetic friction and the coefficient of static friction.

Note:Determining the coefficients of friction for various surfaces can only be done experimentally. Even with careful control of other variables, results obtained are often inconsistent.

COEFFICIENT OF FRICTION – KINETIC AND STATIC

Coefficient of Kinetic Friction (μk)

• Ratio of kinetic friction to normal force (Fk/FN)

Coefficient of Static Friction (μs)

• Ratio of static friction to normal force (Fs/FN)

Note:Since it is harder to get an object moving than it is to keep it moving then:

Ff, static ≥ Ff, kinetic because μstatic ≥ μkinetic

ANALYZING MOTION WITH FRICTION

Practice1. What type of road, asphalt or concrete, provides better traction

(friction of a tire on a road) for rubber tires under:a) Dry conditions? μk, asphalt = 1.07 and μk, concrete = 1.02

b) Wet conditions? μk, asphalt = 0.25 and μk, concrete = 0.45

a) Asphaltb) concrete

2. Determine the appropriate coefficient of friction in each case.

a) It takes 59 N of horizontal force to get a 22 kg leather suitcase just starting to move across the floor.

2. Determine the appropriate coefficient of friction in each case.

b) A horizontal force of 54 N keeps the suitcase in (a) moving at a constant velocity.

3. A 73 kg hockey player glides across the ice on skates with steel blades (μk = 0.010). What is the force of friction acting on the skater?

4. A 0.17 kg hockey puck slides along the ice at 19 m/s [E] when it hits a rough patch of ice and comes to rest after 5 s. Assume the coefficient of kinetic friction between the puck and the rough ice is 0.47.a) Draw a FBD of the puck moving on the rough ice.

4. A 0.17 kg hockey puck slides along the ice at 19 m/s [E] when it hits a rough patch of ice and comes to rest after 5 s. Assume the coefficient of kinetic friction between the puck and the rough ice is 0.47.b) Calculate the acceleration of the puck.

4. A 0.17 kg hockey puck slides along the ice at 19 m/s [E] when it hits a rough patch of ice and comes to rest after 5 s. Assume the coefficient of kinetic friction between the puck and the rough ice is 0.47.c) Calculate the kinetic friction acting on the puck.

HOMEWORK

• Read: Pg. 52 – 55, 58 – 61• Practice Problems: Pg. 54 #2-5

Pg. 55 #2-4