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Forces Of Motion
Ch2
Physics (A), Winter 2010-2011
1
What is a Force?
• Force: Push or Pull on a body
• Units (Metric): Newtons (N), kg∙m/s2
(English): Pounds (lbf)
• Vector Quantity: Has Magnitude & Direction
– Ex. F = 8 N upward
– Ex. F = 60 N westward
• Symbol: Fx x = letter of type of force it is
• To get familiar with units: It takes 3N to lift a
can of pop
2
4 Types of Forces
•
•
•
•
•
Gravitational Force, Weight, Fw or Fg
Tension/Compression Force, Ft/Fc
Normal Force, Support Force, FN
Frictional Force, Ff
So, the previous slide, replace Fx with the
proper Fletter above
3
Gravitational Force, Fw
• Fw = The attractive force of the Earth on any
mass on or near Earth
• We say, “Force due to gravity” or “Weight”
• We measure MASS, we calculate WEIGHT
• Fw = m ∙ g
(Eq.1)
g = 9.8 (m/s2)
m=mass (kg) MUST BE IN kg!!!!
Fw = Weight (N)
4
Tension/Compression Forces, Ft and Fc
• Tension = Pull on an Object
– Ex. Tie a boat down to the docks
– Ex. Big Cables on the Mackinac Bridge
• Compression = Push on an Object
– Ex. Pushing your grocery cart
– Ex. Pushing your car out of deep snow!
• These Forces always work OPPOSITE of EACH
OTHER, so be careful with your signs
• Ft = - Fc
(Eq. 2)
5
Normal Force, FN
• Normal Force: A support force that holds two
objects together
– “Normal” in Math/Science = Perpendicular
– Ex. Book resting on table:
• Downward force of book, weight = Fw
• Upward force of book = FN
• FN = - Fw
(Eq. 3)
• Normal Force opposite of Weight Force
• FN=-Fw
6
Frictional Force, Ff
• Ff : The force opposing sliding motion of two
surfaces in contact
• Depends on the materials in contact with each
other: surface finish, actual material, etc.
• Two Types of Frictional Force:
– Static Friction, (object to slide is initially at rest)
– Sliding Friction, (object is already sliding on surface)
• Ff OPPOSITE of moving Force (Ft, Fc, etc.)
7
Net Force
• We say: Net Force is the sum of all forces
acting on a body
• We write: ∑Fnet = Fw + -FN + …. (Eq.4)
• For Ch2, 1D only
• When ∑Fnet = 0,
(Eq.4)
or when all forces on a body cancel each
other, the body is in EQUILIBRIUM
• 2 Types of Equilibrium:
1. Static Equilibrium (object is not moving)
2. Dynamic Equilibrium (object is moving at
constant velocity)
8
How Do We Find Fnet?
• Draw a Free Body Diagram (FBD) with all
forces acting on object
• Using sign convention, add up forces!
Free Standing Object
Supported Object
9
Practice Problem #1
A painter stands on scaffolding that is
suspended at the ends by two vertical ropes.
The painter and the board are in static
equilibrium. The tension in each rope is
350(N), and the painter weighs 550 (N).
What is the weight of the scaffolding?
10
Practice Problem-Static Equilibrium
1. Draw FBD:
2. Identify Knowns:
Ft = 350 (N) ea rope
Fwgirl = 550 (N)
Static Equilibrium = No motion, & ∑Fnet = 0
3. Identify Unknown(s):
Fwboard = ?
4. Add Forces to FBD
5. Write Equation:
∑Fnet = 0
Fnet = 2∙Ft – Fwgirl –Fwboard
6. Plug In Values & Solve:
0 = 2∙350 (N) – 550 (N) –Fwboard
Fwboard = 150 (N) downward
11
Practice Problem #2
You hit a hockey puck with an applied force of
50 (N). The puck travels down the ice rink at
a constant speed of 2.60 m/s. If the puck is in
dynamic equilibrium, what is the frictional
force applied by the ice/puck contact?
12
Practice Problem-Dynamic Equilibrium
1. Draw FBD:
2. Identify Knowns:
Fapplied = 50 (N)
v = 2.6 m/s (constant)
Dynamic Equilibrium = In Motion, v=constant
& ∑Fnet = 0
3. Identify Unknown(s):
Ff = ?
4. Add Forces to FBD
5. Write Equation:
∑Fnet = 0
Fnet = Fapplied - Ff
6. Plug In Values & Solve:
0 = 50 (N) - Ff
Ff = 50 (N) to the left
Ff
Fapplied
13