Everyday Forces Weight Normal Force Friction Force

Weight is Force of gravity acting on mass. Mass is amount of matter. Weight = F g = mg. g = m/s 2 on Earth. The direction of weight is straight toward the center of Earth.

Inertial and Gravitational Mass What is the difference between the two? Nothing!!

Inertial mass - defined by F = ma. When a force F is applied to an object, it accelerates proportionally, the constant of proportion is the mass of that object. a = F/m.

Gravitational mass has to do with force of gravity pulling on an object.

Gravitational mass is measured by comparing the F g of an unknown mass to the F g of a known mass. This is done with a balance scale.

Physically, no difference between gravitational and inertial mass. Einstein used the fact that gravitational and inertial mass were equal to begin his Theory of General Relativity in which he postulated that gravitational mass was the same as inertial mass.

Normal Force F n. Perpendicular to surfaces in contact. FnFn

How does Normal Force Arise? How does a wall know to push back harder when I push with increasing force?

Objects on a surface are repelled by electromagnetism. Outer electrons in the object are electrically repelled by the electrons that at the surface. The electrons offer a stronger and stronger repulsive force the closer and closer the object is moved to the surface. The more massive, the greater gravity pulls them onto a surface, & the greater the surface repels the object.

The object will be at rest only if the surface can exert an equal & opposite force to sustain it, otherwise the object crashes through the surface.

Friction Between Surfaces

Friction Force F f Acts in direction opposite motion (or attempted motion). Which way is the box below moving? Left

Friction depends on the nature of the materials in contact and the smoothness of their surfaces.

A closer look at friction At the microscopic level even two smooth surfaces look bumpy  this is what produces friction Magnified view of surfaces

. Friction is independent of surface area.. F f is directly proportional the normal force.. F f is directly proportional the normal force. F f =  F n. Where  is a constant. The ratio of F f, to F n, is the coefficient of friction, .   = FfFfFfFf FnFnFnFn  has no units.

Look at reference table

File Cabinet Demo

  depends on two surfaces in contact the type of sliding motion – starting or moving. Does not depend on any other factor. It is a constant.

Types of Friction Static Friction use  st – Present for objects at rest. Need to overcome to start objects moving. Kinetic Friction or sliding use  k – Present for objects in motion. Need to overcome to keep objects in motion. Always less than static friction.

Static friction If we push on an object and it doesn’t move then the force we exert is less than the max F f. push, P friction, f This is the static friction force at work. If I push a little harder, the block may still not move  the F f can have any value up to some maximum.

Kinetic friction Keep increasing the F ap, at some point the block moves  this occurs when the push P exceeds the maximum static friction force. When the block is moving it experiences a smaller friction force called the kinetic F f. It takes more force to get something moving than to keep it moving.  for kinetic always less than static.

1. A 10-kg rubber box is at rest on a dry horizontal concrete surface. Find:. a. Sketch the free body diagram. b. the weight of the box. c. the normal force on the box. d. the force needed to start the box moving. e. the force to keep the box moving a constant velocity. a. b N. c N d. 88-N. e N.

2: A 60-kg sled is being dragged on a horizontal snow surface by a force of 20-N at a constant velocity. Sketch the free body diagram. What is the friction force on the sled?

3. A 1-kg plastic toy chest is dragged across a horizontal wooden floor at constant velocity by an applied force of 5-N. What is , the coefficient of friction? Would that be static or kinetic friction?

Frictional Force Resisting Motion Force Causing the Object to Move Kinetic Region Static Region Max

Constant Velocity with applied force. F ap F f

On a flat surface, as F g increases F f must increase. Push down on a book as you slide it across a table – F n increases so F f increases too. The coefficient of friction,  cannot change, it is a constant. More weight (F g ) = Higher F n. F f =  F n increases.

Friction Problems 1 wksht Hwk Txt Read 141 – 145 Do pg 145 #2,3 & pg 149 #4, 6 on separate sheet.

Friction Problems Inclines.

Inclined Planes Fn is perpendicular to surface. Where is friction force if box is at rest?

Friction opposes motion or attempted motion. Friction

What is causing box to want to slide??

What holds the box in place? Friction For a box at rest or constant velocity motion, F f = W ll.