4.4 Everyday ForcesWeight, Normal Force and Friction

pp. 141 - 149

Mr. Richter

Agenda

Check and Review HW

Warm Up

Notes Today and Tomorrow: Weight Normal Force Friction

Static Kinetic

Overcoming Friction

Problem-Solving Practice

Upcoming Dates: Take Home Quiz Due

Tomorrow Methods Rough Draft

Wednesday This can be written

directly into your notebook.

Chapter 4 Test Friday

Objectives: We Will Be Able To… Explain the difference between mass and weight. Find the direction and magnitude of normal force. Describe air resistance as a form of friction.

(Tomorrow) Use coefficients of friction to calculate frictional force.

(Tomorrow)

Warm-Up:

A child pulls a wagon along behind him. Is the force the ground applies up on the wagon…a. greater than the force of gravity pulling down on the

wagon?b. less than the force of gravity pulling down on the wagon?c. equal to the force of gravity pulling down on the wagon?

Draw a FBD to help you consider the problem, then choose one answer and discuss at your table. We will discuss as a class momentarily.

WeightThe force due to the acceleration of gravity.

Weight

Weight is the magnitude of the force due to gravity acting on an object.

Because force is simply mass*acceleration, the formula to calculate weight is Fg=mg. g is 9.81 m/s2

the units of weight are Newtons

1 lb = 4.4 N

Weight

Is your weight different on the moon? What about your mass?

Weight changes depending on the acceleration of gravity. If gravity changes, weight changes.

Mass is the amount of molecules in an object. Mass is not dependent on gravity, so it will not change on the moon.

The Normal Force

The Normal Force

We know that when objects are in equilibrium vertically (Σfy = 0), there must be some force to balance out gravity.

This force is the normal force. More technically: the normal force

is a force exerted by one object on another in a direction perpendicular to the surface of contact.

Normal = perpendicular.

The Normal Force

The normal force is ALWAYS perpendicular to the

surface NOT ALWAYS in the opposite

direction of gravity

Gravity always pulls straight down, the normal force can be at an angle.

Warm-Up

Is it easier to start an object in motion or to keep an object in motion?

Choose one and then discuss at your table.

Friction

Friction

Friction is the force that opposes an applied force. Friction opposes motion.

Friction can come in many forms. For example: surfaces fluids (like air and water) electrical (called resistance)

Objects experiencing no applied force experience no friction.

Static Friction

When an object experiences an applied force and still does not move, it experiences static friction.

Static friction (Fs) is the force that opposes motion of two objects whose surfaces are at rest. Static = not moving.

The force of static friction is equal and opposite to the applied force. Fs = -Fapplied

Kinetic Friction

When an object is moving, it still experiences friction, but the force of friction is less than the applied force (hence acceleration!).

Kinetic friction (Fk) is the force that opposes the motion of two objects whose surfaces are moving past one another. Kinetic = moving.

Kinetic friction is always less than the maximum static friction.

Static and Kinetic Friction

Coefficients of Friction

Coefficients of Friction

The force of friction is proportional to the normal force. The amount of force keeping the surfaces in

contact will affect how hard it is to move an object.

Friction also depends on which surfaces are in contact with each other. A hockey puck sliding on ice works better than

trying to slide on sandpaper.

The coefficient of friction (μ) represents the ratio between the normal force and the force of friction between two surfaces.

Coefficients of Friction

Static Friction: Fs = μsFn

Kinetic Friction: Fk = μkFn

p. 144

Practice Problem

A 24-kg crate initially at rest on a horizontal floor requires 75 N horizontal force to set it in motion. Find the coefficient of static friction between the crate and the floor.

Warm-Up

It takes 450 N of force to slide a 120 kg refrigerator across a linoleum floor at a constant speed. Find the coefficient of friction between the friction

between the refrigerator and the floor. Is this static or kinetic friction?

Everyday Force Problems

Plan of Attack

1. Draw a diagram and list your given info.

2. Find sum of the forces in the y-direction. Often but not always zero.

Σy = ?

3. Find sum of the forces in the x-direction.1. Zero if the object is in equilibrium (at rest or constant

velocity)2. Non-zero if asked to find acceleration of an object.

4. Solve and check for reasonableness.

Practice Problem

A student moves a box of books by attaching a rope to the box and pulling with a force of 90.0 N at an angle of 30.0°. The box of books has a mass of 20.0 kg, and the coefficient of kinetic friction between the box and the sidewalk is 0.50. Find the acceleration of the box.

1. Draw a diagram. List given info. Fa = 90.0 N

θ=30.0° m = 20.0 kg

μk = 0.50

30.0°

Fa

Fn

Fg

Fk

2. Sum of the forces in the y-direction

Σfy = 0. The box does not rise off of nor sink into the sidewalk.

30.0°

Fa

Fn

Fg

FkFay

3. Sum of the forces in the x-direction. Non-zero, to find acceleration.

30.0°

Fa

Fn

Fg

Fk

Fax

4. Solve. Find Acceleration

a = 0.12 m/s2

Wrap-Up: Did we meet our objectives?

Explain the difference between mass and weight. Find the direction and magnitude of normal force. Describe air resistance as a form of friction. Use coefficients of friction to calculate frictional force.

Homework

p145 #1-3 p 147 #1,4