Introductory Lesson on Forces

1. Lesson 1
Forces and Motion Lesson 1
(Dynamics)
 Weight and MassWeight and Mass
 Galileo’s Thought Experiment (& Inertia)Galileo’s Thought Experiment (& Inertia)
 Common Forces (FCommon Forces (Fgg))
 Nelson TB Reference Pages:
114-116, 123-124, 164 - 165114-116, 123-124, 164 - 165

2. HistoryHistory
 Throughout history, people have tried toThroughout history, people have tried to
understand what keeps some objectsunderstand what keeps some objects
moving and some objects at rest.moving and some objects at rest. Aristotle
(384-322 BC) believed that a constant(384-322 BC) believed that a constant
“force” was required for constant “speed”.“force” was required for constant “speed”.
This idea went unchallenged for almostThis idea went unchallenged for almost
2000 years.2000 years.

3. Galileo’s Thought Experiment
 Galileo believed that a constant force was needed toGalileo believed that a constant force was needed to
keep an object moving on a level surface because therekeep an object moving on a level surface because there
was a force of friction which acted in a direction oppositewas a force of friction which acted in a direction opposite
to the direction of motion of the object.to the direction of motion of the object.
 If friction was eliminated, Galileo believed that a constantIf friction was eliminated, Galileo believed that a constant
force wouldforce would not be required to keep an object moving.be required to keep an object moving.
He used the following thought experiment to justify thisHe used the following thought experiment to justify this
conclusion.conclusion.
UP
B a l l r i s e s t o t h e s a m e f i n a l h e i g h t
B a l l r o l l s i n a s t r a i g h t l i n e f o r e v e r .
( A v e r y l o n g t i m e ! ! ! ! )
DOM
G a l i l e o ' s T h o u g h t E x p e r i m e n t
( T h e r e m u s t n o t b e a n y f r i c t i o n a l f o r c e s a c t i n g )

4. InertiaInertia
 Galilean InertiaGalilean Inertia was the conclusion of Galileo’swas the conclusion of Galileo’s
thought experiment. Which states:thought experiment. Which states: Objects atObjects at
rest will remain at rest, objects movingrest will remain at rest, objects moving (with(with
uniform motion)uniform motion) willwill continue to movecontinue to move (with(with
uniform motion). For this to occur there must beuniform motion). For this to occur there must be
no unbalanced forces acting (ie friction)no unbalanced forces acting (ie friction)
 It is best to think of an object’s inertia as its
ability to resist a change in motion.
 Which physical quantity, possessed by allWhich physical quantity, possessed by all
objects, gives us a measure of an objectsobjects, gives us a measure of an objects
inertia?inertia?
 Ans. MassAns. Mass

5. Examples of Inertia
1.1. Sitting in an airplane as it starts to take off.Sitting in an airplane as it starts to take off.
What happens to your body?What happens to your body?
 It gets pushed into the seat – it tries to resistIt gets pushed into the seat – it tries to resist
a change in motion.a change in motion.
1.1. What do seat belts in a car prevent?What do seat belts in a car prevent?
 Your body from moving forward when theYour body from moving forward when the
car stops suddenly.car stops suddenly.
1.1. A concussion is caused by…?A concussion is caused by…?
 Your brain continuing to move after your skull hasYour brain continuing to move after your skull has
stopped moving.stopped moving.

6. What is Force?
 In the simplest definition a force is just aIn the simplest definition a force is just a pushpush oror pullpull onon
an object.an object.
 Force is also a vector which has both a magnitude and aForce is also a vector which has both a magnitude and a
direction.direction.
How do we measure force?How do we measure force?
 Consider the force of gravity (FConsider the force of gravity (Fgg). Does it exist in deep). Does it exist in deep
space far from any planet? What quantity is not presentspace far from any planet? What quantity is not present
in deep space?in deep space?
 Gravity. This is one component of force.Gravity. This is one component of force.
 Now consider a 5 kg mass placed on your toe. This mayNow consider a 5 kg mass placed on your toe. This may
hurt, but not as much as a 10 kg mass. Each mass willhurt, but not as much as a 10 kg mass. Each mass will
exert a force of gravity on your toe. So, the otherexert a force of gravity on your toe. So, the other
component of Fcomponent of Fgg must be…?must be…?
 MassMass

7. Calculating FCalculating Fgg
 Force of gravity is a vector so it needs a direction.Force of gravity is a vector so it needs a direction.
However, since we know that FHowever, since we know that Fgg always acts downward,always acts downward,
we usually do not have give its direction or place a vectorwe usually do not have give its direction or place a vector
arrow above Farrow above Fgg
Units of ForceUnits of Force
 From the prior slide, we determined that FFrom the prior slide, we determined that Fgg was directlywas directly
proportional to both mass (m) and acceleration due toproportional to both mass (m) and acceleration due to
gravity (g), orgravity (g), or FFgg ∝∝ massmass andand FFgg ∝∝ acceleration,acceleration, thusthus
FFgg ∝∝ mg.mg.
 Here, the equation isHere, the equation is FFgg = mg= mg ,, units are (m/sunits are (m/s22
) x kg and we) x kg and we
call this a Newton (N)call this a Newton (N)
 1 N = 1 kgm/s1 N = 1 kgm/s22

8. Notes onNotes on gg
 We use the letterWe use the letter gg toto represent the acceleration atrepresent the acceleration at
the surface of earththe surface of earth g= 9.81 m/sg= 9.81 m/s22
(a constant)(a constant)
 The acceleration due to gravity isThe acceleration due to gravity is notnot a constanta constant
value. As you move away from the centre of earthvalue. As you move away from the centre of earth gg
decreases (decreases (gg∝∝ 1/r1/r22
),), wherewhere rr is the radial distance fromis the radial distance from
the centre of earth.the centre of earth.
 gg North PoleNorth Pole = 9.8322 m/s= 9.8322 m/s22
 gg EquatorEquator = 9.7805 m/s= 9.7805 m/s22
 On the surface of the moonOn the surface of the moon ggMoonMoon is much less thanis much less than
earth and equals 1.64 m/searth and equals 1.64 m/s22
. Other values (page. Other values (page
141) are141) are ggMarsMars = 3.72 m/s= 3.72 m/s22
,, ggJupiterJupiter = 25.9 m/s= 25.9 m/s22

9. Weight & Mass
 Weight and mass are not the sameWeight and mass are not the same
quantity in the SI system.quantity in the SI system.
 Weight is a force measured in Newtons, it isis a force measured in Newtons, it is
not a constant value because it dependsnot a constant value because it depends
on…?on…?
 Acceleration due to gravityAcceleration due to gravity
 Mass is a constant value – it does not change!is a constant value – it does not change!
For all force calculations we will use mass measuredFor all force calculations we will use mass measured
in kg.in kg.

10. Practice Questions
Nelson TB:
 Page 129 #1-5, 7, 12 { Inertia }Page 129 #1-5, 7, 12 { Inertia }
 Page 167 #6, 7, 9, 10 { Weight & Mass }Page 167 #6, 7, 9, 10 { Weight & Mass }