Chapter 4: Forces and The Laws of Motion
I.
Section 4-1—Force
A. Definition: Force is the cause of an acceleration or the change
in an object’s velocity
B. A force can be a push, a pull, and pressure; force can act
directly 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! Weight = force x gravity (W=f*g). If my pet
mouse, Jane Fonda, weighs 200 lbs on earth, she would only
weigh 33 lbs on the moon (because the moon’s gravity is 1/6th of
what we experience on earth.
F. While your weight can change, your mass is constant regardless
of your environment. Although Jane Fonda, the mouse, would
weigh 33 pounds on the moon her MASS would still be 200
pounds on the moon, Jupiter or in a black hole!
G. Newtons are a measure of force—we 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
(stick of butter)
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 and
magnetism)
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 direction
of 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”
Simple force diagrams of single objects and the forces acting
Fnormal
III.
Section 4-2—Newton’s 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. Newton’s 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 forces—add those vectors!
3. The Greek letter sigma (Σ) 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. Don’t confuse “size”, “mass” and “density”
IV.
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.
Section 4-3--Newton’s 2nd and 3rd Laws
A. Newton’s 2nd Law: Force = Mass x Acceleration
ΣF = ma
V.
B. For objects at equilibrium a=0
C. Newton’s 3rd Law: For every action there is an equal and
opposite reaction.
1. Newton’s third law implies that forces always exist in
pairs.
2. We often split these pairs up into the action force and
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 Pairs
Section 4-4—Everyday 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 scalar quantity.
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* Force
1. 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 normal is
“perpendicular”
3. The normal force is always perpendicular to the surface
of contact…NOT always opposite the force of gravity.
Ffriction
Fn = m g cosΘ
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 friction
4. 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 –μ)
1. The coefficient of static friction (μs) is the ratio of the
maximum value of the force of static friction to the
normal force.
2. The coefficient of kinetic friction (μk) is the ratio of
the force of kinetic friction to the normal
μ = Friction force
Normal force