Newton's Laws

• A Force is:

  • A push or pull applied to an object
  • Direct force = direct contact required, for body to generate force & whatever is against it
  • Used to change the state of motion of an object or body
  • Non-contact forces relate to mass & magnitude of attraction bw/ bodies - ie gravity
  • Typically used/required to control movement once it is generated
  • Doesn’t necessarily generate movement
• Internal force = muscle fibres that generate force, relates to strength movement & power
• Why should we care about force?

  • Kinetics describes movement of the body or object (eg soccer ball)
  • Force explains why movement occurs the way it does
  • How forces cause injury & to help prevent them
• Newton’s 1st Law:

  • Galileo (1580’s)...
  • What is the natural state of an object, if left to itself?
  • A natural state of an object
  • Either moving continuously or stationary & stays in that state unless something outside of the body was to change this state
  • He introduced concept of inertia
• Newton's 1st Law:

  • Newton (1660’s)...
  • An object will remain at rest or continue to move with constant velocity as long as the net force = 0
  • Rule: Forces must be external to the system!
• Newtons 1st Law:

  • Brought Galileo’s concepts further
  • Galileo’s only worked for horizontal well Newton’s for both horizontal & vertical; as well as in earth & space (= broader concepts that became laws)
  • Put mathematics to the idea
  • In a way we can be used in broader context of movement & how that may occur
• Newton’s 1st Law - Inertia:

  • Have natural state & in order to change that something must happen/force applied
• What is important to know about inertia?

  • Inertia describes an object's resistance to change its state of motion
  • Gives concept of resistance of an object to change
  • Change state of movement, to deform, to bend etc
• What is important to know about inertia?

  • Any object with mass will have inertia
  • Inertia proportional to mass
  • Everything offers resistance to move & is proportional to mass
  • Gain mass = more resistance
• Newton’s 2nd Law - Acceleration:

  • But, how do we know our state of motion?
  • Recall from kinematics that a change in velocity causes an object to accelerate or decelerate
• Newton’s 2nd Law - Acceleration:

  • The magnitude of the acceleration or deceleration is proportional to the net force acting on the object & inversely to the mass…
  • F = m * a
  • Size of change in velocity proportional to forces acting on object
  • Inversely proportional to our mass/resistance
  • Whenever change resistance cause movement, must be force
  • Mass is thing trying to overcome
• Newton’s 2nd Law - Acceleration

  • What is important to know about the law of acceleration?
  • F = ma, Force = mass x acceleration
• Newton’s 2nd Law - Acceleration:

  • To cause large Acceleration to a given mass we need to apply large Force
  • For a given mass, force is required to cause movement but it must exceed inertia/mass of object
  • Larger force, larger acceleration when mass is fixed
• Newton’s 2nd Law - Acceleration:

  • For a given Force a lighter object will Accelerate faster than a heavier one
  • If force is fixed (given force that you can apply)
  • If alter mass eg reduce mass/resistance
  • Can move lighter mass faster
  • Manipulate acceleration by changing mass of object
• Newton’s 2nd Law - Acceleration:

  • For a given Acceleration the lighter object will require less Force
  • To get same change in movement than a heavy mass
• Newton’s Laws:

  • F = ma tells us that to accelerate faster we need to apply a larger force
  • Or manipulate mass in a way (reduce mass increase acceleration)
  • However… (What’s required to get this change?)
  • The force will only cause acceleration if it is external to the system
  • Can not generate force that is not external to a system
• Newton's Laws:

  • How do we apply this force to ourselves?
  • Internal = muscles that generate forces, cause joints & stuff to move but not body to propel itself
  • Generate force to move the body, is that you need an action & reaction to make 2nd Law work
  • = mechanism to apply forces, to describe how generate or change movement
  • eg can cause arm to move in space but won’t make chair move → need more that muscle force need to apply external to move chair etc
• Newtons's Laws:

  • Newton's 3rd Law of action & reaction
  • Need for 2nd Law to be applied/work
• Newton's Laws:

  • “For every action there is an equal & opposite reaction”
  • Muscle force has to be generated into ground at a particular angle, direction & magnitude to cause chair to move in direction you want to go
  • Magnitude/force applied to ground applied reaction towards you to cause movement
  • More force applied more chair move & direction depends on direction of reaction force with ground
• Newton’s 3rd Law - Action Reaction:

  • 1) To have the greatest force applied to us, we need to apply the greatest possible force against that object
  • More force applied more gonna get back
  • eg to ground
• Newton’s 3rd Law - Action Reaction:

  • 2) If we need force to accelerate us in a specific direction, we need to produce it in a very specific, & opposite, direction
  • Magnitude & direction important in generating a movement
• What can we take away from a successful high jump performance?

  • Goal: to jump as high as you can
  • Working with the same gravity
  • Have to work with: body mass, how apply forces to ground
• What can we take away from a successful high jump performance?

  • The best jumpers will have the following characteristics:

  1. Lower resistance to changing their state of motion (Law 1)
  2. Less mass means less force is required to accelerate the body upward (Law 2)
  3. They can direct force precisely to redirect the horizontal (hz) velocity into vertical (vt) velocity (Law 3)
• What can we take away from a successful high jump performance?

  1. Lower resistance to changing their state of motion (Law 1)

  • ie less mass = lower inertia
  • Less force required for more acceleration & to overcome inertia
• What can we take away from a successful high jump performance

  • 2) Less mass means less force is required to accelerate the body upward (Law 2)
  • More acceleration for the same force (trade-off)
• What can we take away from a successful high jump performance

  • 3) They can direct force precisely to redirect the horizontal (hz) velocity into vertical (vt) velocity (Law 3)
  • Changing directory hz to vt
  • To make sure don’t lose momentum when changing from hz to vt velocity
  • Good at applying action force to get precise reaction
• Relative Strength:

  • Relative strength is very important
  • Those that require speed have highest strength to mass ratio
  • Reduce mass, increase strength to get best performance outcome that they can
• KINETICS is the study of FORCES & their REACTIONS:

  • For human movement
  • Contact forces = motion generating forces
  • Magnitude of motion related to Force & Mass
  • In order to generate movement need contact force
• KINETICS is the study of FORCES & their REACTIONS

  • For human movement
  • Non-contact forces = motion controlling forces
  • eg gravitational inertial
  • Reaction forces
  • Doesn’t generate motion, used once generate motion