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Forces & Newton 1
What Is a Force?
• A Force is an interaction between
two bodies.
– Convention: Fa,b means
“the force acting on a due to b”.
• A Force is a push or a pull.
• A Force has magnitude & direction
(vector).
• Adding forces means adding vectors.
Forces - 1151
• Contact (fundamentally E+M)
– Normal: Perpendicular to surface
– Friction: Parallel to surface
– Anything touching the object
• Rope: Tension
• Spring F = -kx
• Person
Physics
• Non-Contact or Action at a Distance
(these are field forces)
– Gravity
g
• Force
Force & Mass
– Symbol: F or F
– Units:
• SI – Newton (N) (~0.22 lb)
• English – Pound (lb)
• cgs – dyne (10-5 N)
• Mass
– Symbol: m or M
– Units:
• SI – kilogram (kg)
• English – slug (~14.5 kg)
• cgs – gram (g)
Weight Compared to Mass
• Weight is a Force
On Earth
– Depends on the gravitational
W = (9.8N/kg) m
field strength at the location
• Mass is the amount of “stuff”
– Mass is independent of location! (although it
is often measured by comparing weights)
– Mass is also the inertia of the object
• Inertia is resistance to a change in
motion – See Newton 1
Remember
1. Every force must have an agent that
produces it;
2. Every contact force must act only at the
point of contact;
3. The normal force acts only perpendicular to
the surface in contact;
4. The friction force acts only parallel to the
surface in contact;
5. The tension force from a string or rope acts
only along the line of the string or rope.
Tension
• Spring scale reads the force exerted on
each end.
• Tension is force transmitted by rope and
is the force exerted by each end of rope.
Tension ACT
A pair of tug-of-war teams are pulling on the ends of a
rope, each team with a force of 1000 N. The tension in the
rope is:
A. 2000 N
B. 500 N
C. 1000 N
D. 0 N
E. 2000 kg
Tension ACT
a. T1>T2
b. T1=T2
c. T1<T2
d. depends on pulley radius
Free Body Diagrams
Drawing a FBD
 Identify all forces acting on the object.
 Draw a coordinate system. Use the axes
defined in your pictorial representation. If those
axes are tilted, for motion along an incline, then
the axes of the free-body diagram should be
similarly tilted.
 Represent the object as a dot at the origin of
the coordinate axes. This is the particle model.
 Draw vectors representing each of the
identified forces. Be sure to label each force
vector.
Examples of Force Vectors
Pull
(contact force)
Push
(contact force)
Gravity
(long-range force)
Identifying Forces
 Identify “the system” and “the environment.” The system is the
object whose motion you wish to study; the environment is everything
else.
 Draw a picture of the situation. Show the object—the system—and
everything in the environment that touches the system. Ropes,
springs, and surfaces are all parts of the environment.
 Draw a closed curve around the system. Only the object is inside
the curve; everything else is outside.
 Locate every point on the boundary of this curve where the
environment touches the system. These are the points where the
environment exerts contact forces on the object.
 Name and label each contact force acting on the object. There is
at least one force at each point of contact; there may be more than
one. When necessary, use subscripts to distinguish forces of the
same type.
 Name and label each long-range force acting on the object. For
now, the only long-range force is weight.
Forces on a Bungee Jumper
y
T
w
x
The Forces on a Skier
n
T
fk
w
The Forces on a Rocket
Fthrust
y
D w
x
Inertial Reference Frame
• The following statements can be thought
of as the definition of inertial reference
frames.
– An IRF is a reference frame that is not
accelerating (or rotating) with respect to the
“fixed stars”.
– If one IRF exists, infinitely many exist since
they are related by any arbitrary constant
velocity vector!
Newton 1
• Newton’s First Law
• An object subject to no external
forces is at rest or moves with a
constant velocity if viewed from an
inertial reference frame.
– If no net forces act, there is no
acceleration.
F net  0  a  0