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# Newtons Law's (Unit 4).pdf

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McMaster University

Physics

PHYSICS 1L03

Reza Nejat

Fall

Description

Biomechanics
Sprinter
Biomechanics
Newton Laws
Concepts:
Newton’s Laws
Free body diagram
Application of Newton’s Laws
Weight and Apparent Weight
Chapter: 4
Pages: 77-103 •Suimolt.
PHYS 1L03, Fall 2013, McMaster University, R.Nejat PHYS 1L03, Fall 2013, McMaster University, R.Nejat
Biomechanics Biomechanics
Questions:
•Hwdeprqicai?n •owosertr?
•Dpxrd by Sprinter on the
starting block directly give him acceleration? •owwexcs?
•Hdee)r •Wornwhtsre
•Wadf? do?
PHYS 1L03, Fall 2013, McMaster University, R.Nejat PHYS 1L03, Fall 2013, McMaster University, R.Nejat
1 Force Biomechanics
What is force? (review) eitr
•ir push or a pull. between different parts?
•ir interaction between two objects •b,menlo
•otn object. and bone
•rin agent. •,illa
•ir vector. transferred to the bone via a
•ces acceleration. tendon.
•ir contact force or a contact-
free force (field force).
PHYS 1L03, Fall 2013, McMaster University, R.Nejat PHYS 1L03, Fall 2013, McMaster University, R.Nejat
Force Force ▯
F
•Whdfro? •o: ▯ ▯ const. ▯
a
•oae changes in velocity. F ▯ m
•hn:t a
–Force produces acceleration.
–Theamountofaccelerationproducedis •eitrl
proportional to the amount of force to acceleration is simply the mass of the
object being accelerated:
▯
▯ ▯ F ▯ ▯
F ▯ a ▯ ▯ ▯ const. F ▯ ma
a
• This is Newton’s second law
PHYS 1L03, Fall 2013, McMaster University, R.Nejat PHYS 1L03, Fall 2013, McMaster University, R.Nejat
2 Newton’s Laws
Newton’s Second Law:
•etnetfs
m , it produces an acceleration,
the object such that:
m
. t c e j b o e h t f o s s a m ) l a i t r e n i ( e h t s •i
PHYS 1L03, Fall 2013, McMaster University, R.Nejat
Newton’s Laws
Newton’s Second Law:
•hacliaoeto
directly proportional to the net force
acting on the object and inversely
proportional to its mass:
▯
F ▯ m a
▯ ▯
a ▯ F m
PHYS 1L03, Fall 2013, McMaster University, R.Nejat Newton’s Law Example 1
Force and Motion (Discussion Point) •Wedtlt
5 kg object at 3 m/s ?
•onotfeaiag
moving object, can you tell in which
direction the object is moving?
PHYS 1L03, Fall 2013, McMaster University, R.Nejat PHYS 1L03, Fall 2013, McMaster University, R.Nejat
Example 2 Example 2
•ltlla
is in his hand for 0.5 s while being thrown and
leaves his hand with a speed of 6.0 m/s.
Estimate the force exerted on the ball by the
child. Assume constant acceleration during
throw.
F ▯ m a
net
▯v vf▯ v 1
a ▯ avg▯ ▯
▯t ▯t
PHYS 1L03, Fall 2013, McMaster University, R.Nejat PHYS 1L03, Fall 2013, McMaster University, R.Nejat
4 Newton’s Laws
Newton’s Laws
Discussion Point: Newton’s First Law:
• While driving down the road, the driver •hnexenlcacioan
of the car you are riding in hits the object is zero, or in the absence of
brakes suddenly. As a result, your external forces, an object at rest
body moves toward the front of the remains at rest, and an object in
car. motion continues to move with a
constant velocity in a straight line
Why?
(uniform motion).
PHYS 1L03, Fall 2013, McMaster University, R.Nejat PHYS 1L03, Fall 2013, McMaster University, R.Nejat
Newton’s Laws Newton’s Laws
Newton’s First Law (Application):
• An isolated object, free from external
forces, will continue moving at constant
velocity, or remain at rest.
•rit( net external force)
also may move at constant velocity.
• Motion at constant velocity doesn’t require
any external causes (forces)
• Only changes in velocityrequire external
•Whfnawtr causes (forces).
from the hair?
PHYS 1L03, Fall 2013, McMaster University, R.Nejat PHYS 1L03, Fall 2013, McMaster University, R.Nejat
5 Newton’s Laws
Newton’s Laws
Newton’s First Law:
▯ ▯ Newton’s First Law:
F ▯ F ▯ 0
•I net ▯ ▯ F ▯ 0
▯ net,x
Fnet ▯ 0 ▯ ▯
•Teethstomir ▯ Fnet,y▯ 0
motion (constant velocity), or at rest.
▯ ▯ ▯ Static
Fnet ▯ 0 ▯ a ▯ 0 ▯ v ▯ const •Eqim:
Dynamic
PHYS 1L03, Fall 2013, McMaster University, R.Nejat PHYS 1L03, Fall 2013, McMaster University, R.Nejat
Newton’s Law Example
Drag and terminal velocity orlrtAla
in still air with a terminal speed of 0.3 mm/s. The
•hmnf( the resistive force to the particle has a mass of 1g and a drag force of
motion of an object in air) depends on the the form bv. What is the value of b?
speed of the object. 2
R ▯ v or ▯ v
•Whie)
object become zero, the object reaches
equilibrium, and moves with constant velocity
(Terminal velocity.
Fnte R ▯▯ Fi▯ 0 v ▯ const. ▯ terminal velocity
Other
PHYS 1L03, Fall 2013, McMaster University, R.Nejat PHYS 1L03, Fall 2013, McMaster University, R.Nejat
6 Example Newton’s Law
dhifeys
of 180 km/h with her arms and legs outspread. Inertial Reference Frame
What is the magnitude of the upward drag force F
d •ThereferenceframeinwhichNewton’s
on the sky diver? first law is valid is called an inertial
frame.
•Allreferenceframesthatarestationary
or have a uniform motion (ie: constant
velocity) are inertial frames .
PHYS 1L03, Fall 2013, McMaster University, R.Nejat PHYS 1L03, Fall 2013, McMaster University, R.Nejat
Newton’s Law Newton’s Law
Discussion Point: Newton’s Third Law (interaction pair):
•natatapcwal •ihaofir
discovers that his jet pack no longer tIojtthtce
works, leaving him stranded 50 m from
the spacecraft. Assume the jet pack is exerted by object 1 on object 2 is
equal in magnitude to and opposite in
removable.
How the astronaut can still use it to direction to the force exerted by object
return to the ship? 2 on object 1.
▯ ▯
Explain! F ▯ ▯ F
on 1by 2 on 2by1
PHYS 1L03, Fall 2013, McMaster University, R.Nejat PHYS 1L03, Fall 2013, McMaster University, R.Nejat
7 Newton’s Law Newton’s Third Law
Newton’s Third Law (Action-Reaction): •eihy,tl
•fAtsfooet, there is a trick: just flip the sentence!
–Action:XexertsaforceonY
object B exerts an equal, opposite force –Reaction:YexertsaforceonX
back on A.
▯ ▯ –Ifthe exerts a force on the , the
F ▯ ▯ F reaction pair for this action is the force of the
on B by A on A by B on the
▯ ▯
– DO NOT get confused by jumping ahead: i.e.
F BA ▯ ▯ F AB the action is gravity and the reaction is the
normal force (wrong!)
•F AB and F BA are called “interaction pair”. (wrong!)
PHYS 1L03, Fall 2013, McMaster University, R.Nejat PHYS 1L03, Fall 2013, McMaster University, R.Nejat
Newton’s third Law Newton’s third Law
Example on Newton’s third law:
Example on Newton’s third law:
What is the “reaction” to the following forces?
▯ F What is the “reaction” to the following forces?
FB T
▯ Earth pulls block ▯ Block pulls earth
▯Fg down (Gravity) Fg up (Gravity)
Block pushes down on tableable pushes up on block
You push on a block The block push back on you Balloon pushes Air outside pushes
on air outside on balloon
PHYS 1L03, Fall 2013, McMaster University, R.Nejat PHYS 1L03, Fall 2013, McMaster University, R.Nejat
8 Newton’s Laws
Newton’s Laws Discussion Point
•Ail,’s
pulls on the wagon, wagon pulls back on the
horse with an equal force. Clearly, then, the
net force is zero and the wagon will stay put.
•haal!oyu •WclNfodedgse
explain this? to shake water from its coat?
PHYS 1L03, Fall 2013, McMaster University, R.NejatPHYS 1L03, Fall 2013, McMaster University, R.Nejat
Newton’s Laws Newton’s Laws
Two skaters of
different masses push
against one another.
•Which one will push
harder?

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