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Physics: Chapter 4, Forces and the Laws of Motion
Lecture Notes
I. Changes in Motion
A. Force
1. Defined as the cause of an acceleration, or the change in an object’s motion, or
simply a push or pull exerted on some object
2. Forces cause change in motion
a. stopping a moving object
b. starting an object from rest
c. changing direction of an object
3. Unit of force is the newton (N)
a. newton is defined as the amount of force that, when acting on a 1 kg mass,
produces an acceleration of 1 m/s2 (1 N = 1 kg × 1 m/s2)
b. weight is actually a unit of force---why??
c. 1 lb = 4.448 N = 0.4536 kg; 1 N = 0.225 lb = 0.102 kg
4. Forces can act through contact or at a distance
a. contact forces-forces that arise from the physical contact of two objects
b. field forces-forces that can exist between objects, even in the absence of
physical contact between the objects
B. Force Diagrams
1. The effects of forces depend on their magnitude and direction, so force is a vector
quantity
2. Free body diagrams (see example, pg. 127)
a. vectors are used to show all the forces important in a situation
b. forces are drawn as if they act on a particular point
c. good idea to use simple shapes to illustrate different objects in the diagram
II. Newton’s First Law---Inertia
A. Stated---An object at rest remains at rest, and an object in motion continues in motion
with constant velocity unless it experiences a net external force
1. Inertia is defined as the tendency of an object not to accelerate
2. Newton’s first law is often referred to as the law of inertia since his law says that
when the net external force on an object is zero, its acceleration is zero
a. external force-a single force that acts on an object as a result of the interaction
between the object and its environment
b. net external force-the vector sum of all the forces acting on a body (the
resultant of multiple force vectors)
3. An object’s inertia is directly proportional to its mass; the greater the mass of an
object, the less it accelerates when a force is applied to it
B. Equilibrium is a term associated with Newton’s first law which describes things that
are either at rest, or in motion with a constant velocity. The net external force acting
on an object must be zero for it to be in equilibrium.
III. Newton’s Second and Third Laws
A. Newton’s Second Law---F = m  a
1. Stated---The acceleration of an object is directly proportional to the net external
force acting on the object and inversely proportional to the mass of the object
2. Equationally, net external force = mass x acceleration, F = m  a
3. Often, in problem solving, forces are broken into x and y components
B. Newton’s Third Law
Physics: Chapter 4, Forces and the Laws of Motion
Lecture Notes
1. Stated---If two bodies interact, the magnitude of the force exerted on object 1 by
object 2 is equal to the magnitude of the force simultaneously exerted on object 2
by object 1, and these two forces are opposite in direction
2. This law is more simply stated “for every action there is an equal and opposite
reaction
3. Out of this law, comes the idea of an action-reaction pair of forces
a. action-reaction pair-a pair of simultaneous equal but opposite forces resulting
from the interaction of two objects
b. action-reaction pairs don’t usually result in equilibrium; movement of an
object usually results—this can happen since the pair is acting on different
objects
4. Field forces also occur in pairs, so, according to Newton’s law, as an object
accelerates toward earth, earth also accelerates toward the object
IV. Everyday Forces
A. Weight
1. Defined as the magnitude of the force of gravity acting on an object
2. Weight can be determined from F=ma, where g is used for a
3. Weight varies with location, since the acceleration due to gravity varies with
altitude, depends on the “planet” you’re on
B. The Normal Force
1. Defined as a contact force exerted by one object on another in a direction
perpendicular to the surface of contact
2. The normal force in always perpendicular to the surface, not necessarily opposite
the force of gravity
3. The normal force is usually used in problem-solving and becomes very important
when determining forces for objects along an incline
C. The Force of Friction
1. Types of Friction
a. static friction-the force exerted on a motionless body by its environment to
resist an external force
b. kinetic friction-the force exerted on a moving object (kinetic is less than static
c. air resistance is also a force of friction, sometimes referred to as fluid
friction—the friction caused by air resistance is proportional to the speed of
the object in motion
2. The amount of the force of friction depends on the surfaces in contact; different
surfaces in contact have different ratios of friction force compared to the normal
force
a. these ratios are called coefficients of friction, and can be calculated for static
and kinetic friction situations
b. coefficient of friction () = force of friction (ff)
normal force (fn)