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Forces: Maintaining Equilibrium or Changing Motion. PSE4U Mr. MacMillan. Something to Think About. 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.

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something to think about
Something to Think About
  • 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.
  • What are the forces that act on each of these athletes?
  • How do athletes manipulate these forces in order to maintain balance?
what are forces
What are Forces?
  • A force is a push or pull
  • They are exerted by objects on other objects
  • A force is something that accelerates or deforms an object.
  • A force is measured is a Newton
    • This is named after Sir Isaac Newton
    • 1 N of force is defined as the force required to accelerate 1 kg mass 1 m/s/s
    • 1.0 N = (1.0 kg)(1.0 m/s/s)
classifying forces
Classifying Forces
  • Internal Forces
    • Are forces that act within the object or system whose motion is being investigated
      • Internal pulling forces – tensile forces
      • Internal pushing forces – compressive forces
    • Examples of internal forces
      • Muscles

External Forces

    • Forces that act on an object as a result of it’s interaction with the environment surrounding it
  • Examples of external forces
    • The force of gravity
    • Magnetic forces
    • Electrical forces


    • The force of gravity acting on an object
    • W = mass x acceleration due to gravity = m x g (9.81 m/s/s)
  • To estimate the weight of something you multiply its mass by 9.81.
contact forces
Contact Forces
  • Forces that occur between objects in contact with each other
    • Examples: Air resistance, water, reaction forces
  • Static Friction
    • Dry friction between two surfaces that are not moving relative to each other
    • Also referred to as limiting friction
  • Dynamic friction
    • Dry friction between two surfaces that are moving relative to each other
    • Also known as sliding friction and kinetic friction
self experiment
  • 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.
  • What force resists the force that you exert on the book and prevents the book from sliding?
    • Static friction
    • 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.

Now add another book on top of the original and push again.

    • Can you push with a greater force before the book begins to move?
    • Add another book. What happens?
  • 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

Mathematically we can express static and dynamic friction as:

  • Static: Fs= usR
    • us = coefficient of static friction
    • R = normal contact force
  • Dynamic: Fd = ud R
    • ud = coefficient of dynamic friction
    • R = normal contact force
adding of forces force composition
Adding of Forces: Force Composition
  • Forces are added using the process of vector addition
  • The result of vector addition of two or more forces is called a resultant force
  • The vector addition of all the external forces acting on an object is the net force.
  • Colinear Forces are forces that have the same line of action.
sample problem
Sample Problem
  • 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?

Total Force = 80 N + 980 N + (-1000 N) = 60 N

  • The net vertical force acting on the barbell is a 60 N force acting upward
concurrent forces
Concurrent Forces
  • Do not act along the same line, but they do act through the same point
    • i.e. a gymnast on the high bar
sample problem1
Sample Problem
  • 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.
  • What is the net external force acting on the gymnast?
free body diagram
Free-body Diagram

30 N

20 N

W = 500 N


First, how large is the force of gravity acting on the gymnast?

  • W = mg
  • = (50 kg)(10 m/s/s) = 500 kg m/s/s = 500 N
scale drawing
Scale Drawing

30 N

  • Using an appropriate scale, draw all forces

550 N

500 N

Resultant force 51 N

20 N

trigonometric technique
Trigonometric Technique
  • A2= B2 + C2
    • Pythagorean theorem
  • Sample Problem
  • 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

  • Draw the resultant force
  • Compute the resultant force
weekend assignment
Weekend Assignment
  • 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.