Download Chapter 4 - My Haiku

Forces
Chapter 4
Force
• Symbol: F
• Metric Unit = Newton (N)
– 1 N = 1 kg X m/s2 (derived unit)
– Standard Unit = Pound (lbs)
• 1 lb = 4.44 N
• Definition: push or pull in any direction
– Causes changes in velocity
– Vector Quantity
2 Categories of Forces
1. Contact Forces - Physical touching of the
object




Applied
Tension
Normal
Friction
2. Field Forces – no touching required
 Gravitational
 Electrical
 Magnetic
Newton’s Laws
1. Law of Inertia
2. ΣF = ma or Fnet = ma
3. Every action has an equal and opposite
reaction (Action-Reaction)
#1: Law of Inertia
States that:
Objects in motion stay in motion,
and objects at rest tend to stay at
rest unless acted on by an outside
force.
• Ex. Table cloth trick
https://www.youtube.com/watch?v=o94Pm-Cty3M
#2: ΣF = ma
• Force is directly proportional to mass and
acceleration
• ΣF or net force α acceleration
– As acceleration ↑, force ↑
• ΣF or net force α mass
– As mass ↑, force ↑
• Mass α 1/acceleration
– As mass ↑, acceleration ↓
Therefore ΣF or Fnet = ma
#2:continued
• If Fnet = ma, then zero acceleration means
there is no net force.
• Does this mean there are no external forces?
– NO
• Object is in a state of Equilibrium in which all external
forces are balanced
#3: Every action has an equal and
opposite reaction force
• If I push an object, the object will push back
on me.
• Example: I push against a wall with a force the
wall pushes back on me with the same force.
• Example: I stand applying a force on the floor,
and the floor pushes back on me with the
same force.
How Objects Move
• If you believe (which you should) every force
has an equal and opposite force, then why
would anything ever move?
To understand why things move you must look
at the individual forces acting on an object.
The Individual Forces
(we will work with this unit)
1. Gravitational Force (Fg)
2. Normal Force (FN)
3. Applied Force (FA)
4. Tensional Force (FT)
5. Frictional Force (Ff, also Fs)
Gravitational Force, Fg
• Downward force from the acceleration of
gravity on a mass
• Also known as weight.
• This force is always down
• Explains why astronauts are considered
“weightless” in space.
VERY IMPORTANT
• You must understand there is a difference
between mass and weight.
• Mass = measure of inertia measured in kg
• Weight = measure of gravitational force
measured in Newton’s (N) or pounds (lbs)
– Weight = mass x gravity
Fg
Normal Force, FN
• Force perpendicular to the surface.
• The definition of normal is perpendicular.
• The support force of the resting surface.
Applied Force, FA
• Force any object uses on the object in
question.
• Can be in any direction.
• Example: I push a box along the floor.
• Example: I use a rope to pull a wagon.
Tensional Force, FT
• A force that is provided by a material
suspended between two objects
• Can be either:
– Normal or Support Force if the objects are static
– Applied Force is objects that are in motion
Frictional Force, Ff
• Force caused by two surfaces rubbing
together.
• Parallel to the surface.
• Perpendicular to the normal.
• Opposite the attempted motion.
Free Body Diagrams
• Free Body Diagrams (FBD) are used to
represent ONLY the forces acting on the object
in question.
– isolates the individual forces on the object.
– used to solve problems involving forces
Free Body Diagram
• Example: I push a box along a floor to the right
FN
Ff
FA
Fg
Only draw the forces that act on the BOX.
How Objects Move: Not a FBD
• Action-Reaction pair: must be equal in size and
opposite in direction
– Is the box moving?
FN
Ff
If the applied force pair
is larger than the
friction pair, the object
will move right
FA
Fg
Action Reaction Pairs
Remember:
• Each forces has an equal but opposite reaction
force
Examples:
– Fg is the attraction of the box to the earth
the reaction pair is the earth attracted to the
object.
– FN is the force of the floor on the box
the reaction force is the box pushing on the
floor.
Action-Reaction Pairs, Cont…
– FA is the force applied by my hand
the reaction force is the force the box applies
to my hand.
– Ff is the force of the floor rubbing on the box
 the reaction force is the force of the box
rubbing on the floor.
Action Reaction Pairs, example
Fperson on hammer
Fhammer on person
Fnail on hammer
Fhammer on nail
Fwood on nail
Fnail on wood
FA
Free Body Diagrams
• The free body diagram of an object is not
concerned with the reaction pair.
– only draw the forces acting on the object, but
ignore any forces the object supplies.
Rules For Adding Forces
1. Draw the free body diagram
2. Rotate the axis if necessary
(only rotate if a majority of the forces will
align on the axis if rotated)
3. Write sum of the forces equations and
substitute in the individual forces.
ΣF = max ΣF = may
4. Do you have acceleration?
ax = ? ay = ?
Finding Fnet or ΣF
• ΣF is the result of all forces acting on an
object.
F = 100 N
EX:
N
Ff = - 50 N
FA = 100 N
Fg = -100 N
a) Find the mass of the box.
b) Find the net force acting
on the box.
c) Find the acceleration of
the box.
Finding Fnet or ΣF
• ΣF is the result of all forces acting on an
object.
F = 100 N
EX:
N
FA = 100 N
Ff = - 50 N
Fg = -100 N
a) Fg = mg , 10.19kg
b) Σ Fy = FA + Ff = 0 N
Σ Fx = FN + Fg = +50 N, right
c) Σ Fx = ma, ax = 4.91 m/s2
Σ Fy = 0, ay = 0
Equilibrium
1. Equilibrium exists when the net
forces on an object equal zero.
ΣF = 0
2. Equilibrium can occur when an
object is at rest or moving with a
constant velocity.
Forces at an Angle
FN
FA
FAY
FAX
Ff
Fg
What about forces at an angle on a
ramp?
Why does this box stay on the ramp? Didn’t we say that an object at
rest is in equilibrium? How can three forces be balanced?
FNX
FN
FNY
FA
FAY
FAX
Fg
Lets look at some examples of
finding net force.
More on Friction
• There are two types of friction
– Static Friction – the resistive force that keeps an
object from moving.
– Kinetic Friction – the resistive force that acts while
an object is in motion.
– Static Friction is always larger than kinetic friction.
This is why it is harder to get objects moving than
it is to keep them moving.
Friction
• When an object is pushed static friction
pushes back.
• Eventually Fs max is reached. At that point the
object begins to accelerate or move.
• Friction is still acting, but now it is kinetic
friction.
• Friction depends on the surfaces in contact
and the normal force. The contact surfaces
each have a static and kinetic relationship.
The Coefficient of Friction