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Holt Physics-Chapter 4: Forces and The Laws of MotionI. Section 4-1Force

A. Definition: Force is the cause of an acceleration or the change

in an objects velocity

B. A force can be a push, a pull, and pressure; force can actdirectly through physical contact or at a distance like a

magnetic field.

C. We measure force in NEWTONS.

A newton is the force needed to accelerate a 1

kilogram mass by 1 meter per second squared

1N = 1kg x 1m/s2

D. Newtons ARE NOT units of weight or mass!

E. Weight: What you weigh is a measure of your mass on the

planet earth. Your weight can change relative to your

environment! Mr. Price weighs 200 pounds on earth, but on the

moon he would weigh only 33 pounds!F. While your weight can change your mass is constant regardless

of your environment. Although Mr. Price would weigh 33 pounds

on the moon his MASS would still be 200 pounds on the moon,

Jupiter or in a black hole!

G. Newtons are a measure of forcewe can use newtons to

measure how much force the earth is exerting on your body

(your weight) but newtons are units of force!

H. Conversion Factors

1 pound = 4.448 newtons

1 newton = 0.225 pounds

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I. Two Types of Forces

1. Contact Forces: The force that arises from the physical

contact of two objects.

2. Field Forces: The force that can exist between objects

even in the absence of physical contact (like gravity andmagnetism)

3. The 4 Fundamental Forces of the Universe

a. Gravity

b. Electromagnetism

c. The Strong Nuclear Force

d. The Weak Nuclear Force

II. Force Diagrams

A. FORCE IS A VECTOR with both a magnitude and a direction

B. Forces are represented by arrows which indicate the directionof the force. The length of the arrow indicates the magnitude

of the force.

C. Diagrams are often used to analyze situations where more than

one force is acting on an object. These are known as force

diagrams

D. Simple force diagrams of single objects and the forces acting

on them are called free-body diagrams.

III. Section 4-2Newtons First Law

A. Inertia: An object at rest remains at rest, and an object in

motion continues in motion unless the object experiences a net

external force.

B. Acceleration is determined by net external force.

1. Newtons first law implies that the net external forces

on an object (with a constant velocity) must be equal to

zero.

2. net external forces means the sum of all of the

individual forcesadd those vectors!

3. The Greek letter sigma () is used to designate a sum.

C. Mass is a measurement of inertia

1. The inertia of an object is proportional to its mass

bigger objects are harder to move and harder to stop.

2. Dont confuse size, mass and density

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D. Equilibrium

1. Equilibrium exists when the net forc3es on an object

equal zero.

2. Equilibrium can occur when an object is at rest or

moving with a constant velocity.IV. Section 4-3--Newtons 2nd and 3rd Laws

A. Newtons 2nd Law: Force = Mass x Acceleration

F = ma

B. For objects at equilibrium a=0

C. Newtons 3rd Law: For every action there is an equal and

opposite reaction.

1. Newtons third law implies that forces always exist inpairs.

2. We often split these pairs up into the action forceand

the reaction force, which are always have the same

magnitude but opposite directions.

3. Action/reaction pairs can cancel each other out which

results in equilibrium.

4. Action/reaction pairs can also accelerate objects (see

action/reaction handout)

D. Field Forces also Exist in PairsV. Section 4-4Everyday Forces

A. Weight

1. Weight is defined in physics as the force of gravity

exerted on a mass. Weight has magnitude but no

direction making it a scalarquantity.

2. Remember the force of gravity is g = 9.81m/s2

3. This value changes inversely to the distance from the

center of the earth (you weigh less at higher altitudes!)

B. The Normal* Force1. The Normal Force: A force exerted by one object on

another in a direction perpendicular to the surface of

contact. (The Floor Force)

2. *The mathematical definition of normalis

perpendicular.

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3. The normal force is always perpendicular to the surface

of contactNOT always opposite the force of gravity.

(insert figure 4-20 here)

Fn = m gcos

C. The Force of Friction

1. Whenever two objects come into contact some energy

is lost to friction.2. Friction is defined as the resistive force that opposes

the relative motion of two contacting surfaces.

D. Two Types of Friction

1. Static Friction is the friction experienced by two or

more objects that are in contact and at rest.

2. Kinetic Friction is the friction experienced by two or

more objects that are in contact and are moving

relative to each other.

3. Kinetic friction is less than static friction4. Friction must always be considered in order to

exactly calculate net forces.

E. The force of friction is proportional to the normal force (i.e.

more massive objects experience more friction and a larger

normal force)

F. Friction depends on the surfaces in contact (i.e. smooth

surfaces experience less friction than rough surfaces.)

G. The Coefficient of Friction (the Greek letter mu)

1. The coefficient of static friction (s) is the ratio of themaximum value of the force of static friction to the

normal force.

2. The coefficient of kinetic friction (k) is the ratio of

the force of kinetic friction to the normal

= Friction force

Normal force