Forces:
Maintaining Equilibrium or
Changing Motion
PSE4U
Mr. MacMillan
Something to Think About
O A gymnast maintains a position on one
foot during a balance beam routine. A
rock climber clings by her fingertips to
the face of a cliff.
O What are the forces that act on each
of these athletes?
O How do athletes manipulate these
forces in order to maintain balance?
What are Forces?
O A force is a push or pull
O They are exerted by objects on other objects
O A force is something that accelerates or
deforms an object.
O A force is measured is a Newton
O This is named after Sir Isaac Newton
O 1 N of force is defined as the force required
to accelerate 1 kg mass 1 m/s/s
O 1.0 N = (1.0 kg)(1.0 m/s/s)
Classifying Forces
O Internal Forces
O Are forces that act within the object or system
whose motion is being investigated
O Internal pulling forces – tensile forces
O Internal pushing forces – compressive forces
O Examples of internal forces
O Muscles
O External Forces
O Forces that act on an object as a result of it’s
interaction with the environment surrounding
it
O Examples of external forces
O The force of gravity
O Magnetic forces
O Electrical forces
O Weight
O The force of gravity acting on an object
O W = mass x acceleration due to gravity
= m x g (9.81 m/s/s)
O To estimate the weight of something you
multiply its mass by 9.81.
Contact Forces
O Forces that occur between objects in contact
with each other
O Examples: Air resistance, water, reaction forces
O Static Friction
O Dry friction between two surfaces that are not
moving relative to each other
O Also referred to as limiting friction
O Dynamic friction
O Dry friction between two surfaces that are
moving relative to each other
O Also known as sliding friction and kinetic friction
Self-Experiment
O Lets do some experimentation to learn more
about friction. Place a book on a flat horizontal
surface such as a desk or tabletop. Now push
sideways against the book and feel how much
force you can exert before the book begins to
move.
O What force resists the force that you exert on the
book and prevents the book from sliding?
O Static friction
O If the book doesn’t slide then the static friction
force acting on the book is the same size as the
force you exerted on the book.
O Now add another book on top of the original
and push again.
O Can you push with a greater force before the
book begins to move?
O Add another book. What happens?
O As you add books to the pile, the magnitude
of the force you exert before the books slide
becomes bigger and so does the static
friction force
O Mathematically we can express static and
dynamic friction as:
O Static: Fs = usR
O us = coefficient of static friction
O R = normal contact force
O Dynamic: Fd = ud R
O ud = coefficient of dynamic friction
O R = normal contact force
Adding of Forces: Force
Composition
O Forces are added using the process of
vector addition
O The result of vector addition of two or more
forces is called a resultant force
O The vector addition of all the external forces
acting on an object is the net force.
O Colinear Forces are forces that have the
same line of action.
Sample Problem
O A spotter assists a weightlifter who is
attempting to lift a 1000 N barbell. The
spotter exerts an 80 N upward force on the
barbell, while the weight lifter exerts a 980
N upward force on the barbell. What is the
net force exerted on the barbell?
O Total Force = 80 N + 980 N + (-1000 N)
= 60 N
O The net vertical force acting on the barbell is
a 60 N force acting upward
Concurrent Forces
O Do not act along the same line, but they do
act through the same point
O i.e. a gymnast on the high bar
Sample Problem
O A gymnast jumps up and grasps the high bar and his
coach stops his swinging by exerting forces on the
front and back of the gymnast’s torso. The external
forces acting on the gymnast are the force of gravity
acting on the mass of the gymnast, a horizontal force
of 20 N exerted by the coach pushing on the front of
the gymnast, a horizontal force of 30 N exerted by the
coach on the back of the gymnast, and an upward
vertical reaction force of 550 N exerted by the bar on
the gymnast’s hands. The gymnast’s mass is 50 kg.
O What is the net external force acting on the gymnast?
Free-body Diagram
30 N
20 N
W = 500 N
O First, how large is the force of gravity acting
on the gymnast?
O W = mg
O = (50 kg)(10 m/s/s) = 500 kg m/s/s = 500
N
Scale Drawing
O Using an appropriate scale, draw all forces
30 N
550 N
500 N
Resultant force 51 N
20 N
Resultant Force 51 N
50 N
10 N
Trigonometric Technique
O A2= B2 + C2
O Pythagorean theorem
O Sample Problem
O The vertical ground reaction force (normal
contact force) acting under a runners force
is 2000 N, while the frictional force is 600 N
acting forward. What is the resultant of
these two forces?
Draw the forces
2. Draw the resultant force
3. Compute the resultant force
1.
Weekend Assignment
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6.
Research a career in the field of biomechanics
Give a one paragraph description of the career you chose
Find out what qualifications you need for the job and
describe the program(s) you may need to take in
college/university and/or special training
List 3 schools where you can get these qualifications
Find the average salary and, if there is one, a pay scale
for the profession
What is the job prospects in the future? Are there any
available jobs? Find an advertisement online to support
your claims.