Everyday ForcesChapter 4 Section 4

What is Weight?

Weight – The magnitude of the force of gravity acting on an object. Weight is directly proportional to force

Weight ~ Force Since weight is a force, it can be related to

Newton’s 2nd Law.F = maW = mg (Where “g” is equal to 9.8m/s²)

FACT:A scale doesn’t give you your weight. It gives you your normal force!!!

Weight Depends on Gravity

Since weight is dependant on gravity, weight depends on location. Such as planets. Earth (g = 9.8m/s²) Moon (g = 1.6m/s²) Jupiter (g = 23.1m/s²)

An astronaut on Earth that weights 800N (180 lb) would weight 130 N (30 lb) on the moon.

Weight Depends on Elevation

Since weight depends on how far away the object is away from the center of the planet, the altitude will effect the value of g. At sea level the value of g is different then it

would be at the top of Mount Everest (elevation - 6200m)

About a 0.2% difference Person weighing 180 lbs at sea level would weight 179.6 lbs at the top of Mount Everest.

Normal Force

Normal Force – A contact force exerted by one object on another in a direction perpendicular to the surface of contact. If a book is sitting on a desk and gravity is

pulling down on the book with 10N, the normal force is going to be 10N pushing up on the book as long as the surface is horizontal.

Normal Force – nAlso Known as the “Support Force”

Normal Force on an InclineThe normal Force is always perpendicular to the surface the object is resting on, not necessarily opposite the force of gravity.As the incline increase, the normal force will decrease.

Normal Force on InclineFn = mg cosθ

**Where the normal force is the only upward force acting on the object in the vertical direction

Force of Friction

Friction occurs in all movement, just at different levels.From Newton’s 2nd Law it is stated that any applied force causes an acceleration, but if a small force is applied to a large mass object (when compared to the force) it usually doesn’t move. Why?

Friction Opposes the Applied Force

Friction forces always act in the direction opposite to the object’s motion or intended motion.If a car is driving down the road due north, the air resistance acting on the car is due south.

Static Friction

Static Friction – The force exerted on a motion-less body by its environment to resist an external force. The static friction force is equal to the applied force and opposite in direction

Fs = FAs the applied force increase, the static friction force increases. As the applied force decrease the static friction force decreases.

Maximum Static Friction

When the applied force is as great as it can be without causing the an object to move, the force of static friction reaches its maximum value (Fsmax).Once the object begins to move, the friction force becomes less then Fsmax

Kinetic Friction

Kinetic Friction – The force exerted on a moving object opposite to the direction of motion.The force of kinetic friction (Fk) is less then the force of the maximum static friction (Fsmax)How could an engineer use this principle of friction?

What Causes Kinetic Friction?

Frictional force arises from the interaction of two surfaces in contact with each other. The surface of all objects are rough and

uneven, which means contact is only made at a few points.

What causes Static Friction?

When an object is at rest, it is said that the contact between the two surfaces are in a “cold-weld”. A cold-weld, or surface adhesion, is caused

by the intermolecular forces exerted on the molecules of the object and the surface at the contact points.

While an object is moving, these bonds can’t be made, so that is why kinetic friction is less than static friction.

Graphing Friction

Force of Pull

Magnitude of Friction Forces Depends on The Normal Force

The magnitude of the friction force is determined by the normal force of the two surfaces in contact.If the normal force increase, the friction force will increase as well.

Ff ~ Fn

How could an engineer to yourself use this principle for everyday application?

Magnitude of Friction Forces Depends on The Coefficient of Friction

Coefficient of Friction – The ratio of the force of friction action between two objects.The symbol “µ” defines the coefficient of friction. (Has no units!) The lowercase Greek letter called, “mu”

µs = Fsmax/Fn

µk = Fk/Fn

The Coefficients are listed on page 144 for different surfaces.

Coefficients of Friction Chart

Force of Friction Equations

The forces of friction can be found using the equations below:

Kinetic Friction ForceFkf = µkFn

Static Friction ForceFsf = µsFn

Air Resistance

Air resistance is a form of frictionSpeed and Area

You experience the force due to air resistance when you stick your hand out of the window of a moving car.

• If the car moves faster, the force on your hand increases.

• If instead of just your hand, you hold your physics book out the window with the large side facing forward, the air resistance force is much larger than on your hand at the same speed.

Air Resistance Proportion

Air resistance force ~ speed × frontal area The expression shows that the air resistance force is

directly proportional to the speed and frontal area of an object.

Air Resistance and Mass

The mass of an object doesn’t effect the air resistance acting on the object, but it allows the object to push through the air easier then an object with less mass.The terminal velocity of a larger mass object will have a high terminal velocity then an object with less mass.

Terminal Velocity

Terminal Velocity – The velocity at which the acceleration of a falling object is zero because friction balances the weight.

Example Problem #1

A 25 kg chair initially at rest on a horizontal floor requires a 365 N horizontal force to set it in motion. Once the chair is in motion, a 327 N horizontal force keeps it moving at a constant velocity. Find the coefficient of static friction between

the chair and the floor. Find the coefficient of kinetic friction between

the chair and the floor.

Example Problem #1 Answer

Static friction coefficient 1.49Kinetic friction coefficient 1.33