Unit 2
ForcesNo Push No GoIf there is nothing pushing on an object, it
will not move.If there is nothing pushing on an object, it will not
stop.
The List:No Push No GoNo Push No StopNo Push No Speed UpNo Push
No Slow DownNo Push No TurnIn Short:No Push No AccelerateForce push
or pulla force always acts in a certain directionex. if you push
something, the force is in the direction of the pushTypes of
ForcesWhat different types of forces are there?Force of
GravityForce of FrictionAir ResistanceNormal Force
Who Discovered this?
The first person to realize the relationship was Sir Isaac
Newton
1st Law of Motion (Law of Inertia)
An object at rest will stay at rest, and an object in motion
will stay in motion at constant velocity, unless acted upon by an
unbalanced force.1st LawInertia is the tendency of an object to
resist changes in its velocity: whether in motion or
motionless.
These pumpkins will not move unless acted on by an unbalanced
force.1st Law
Once airborne, unless acted on by an unbalanced force (gravity
and air fluid friction), it would never stop! 1st Law
Unless acted upon by an unbalanced force, this golf ball would
sit on the tee forever. NFLAnother way to think about it:If you are
in a car at a stop light, and the driver speeds up really fast,
where do you tend to go?If you are driving and come to a quick
stop, which way do you tend to go?If you spin a bucket on a rope,
in what direction will the bucket go if the line is cut?Some
ClarificationWhat is a force?This is the push or pull on an object
that either gets the object moving, makes it stop, or makes it
turn.It is measured in the units of Newtons (N)What does unbalanced
mean?This means that, if you add all the pushes and pulls together,
there is a overall push/pull in one directionBalanced ForcesWhen we
look at forces, we will break them into forces working in the same
plane.This means all vertical forces can be added and all
horizontal forces can be addedWhich forces can be added together in
the diagram?A) 10 NB) 10 NC) 10 ND) 10 NUnbalanced ForcesWhat
happens if the forces do not add to 0?These are unbalanced forces,
and this creates an acceleration in the object.What is the size of
the unbalanced force?C) 10 ND) 10 NA) 10 NB) 40
NREMEMBER!!!!!!!!Forces are vectors and need a direction!!!Free
Body DiagramsWhen you are performing calculations, you should
always look at a drawing of the situation.We do this using an FBD
or Free body diagramAn FBD uses arrows to describe the forces
acting on the body and in which direction.FBDs10 N10 N5 N10 NThis
is an FBD of a situation.You need to resolve the forces into their
x and y componentsIf there are forces in the same plane, they can
be added and subtracted from each otherIn this example, Fy = 0, and
Fx = -5N or 5N [W]FBDs ContdWhat happens if the forces are not in
just the x or y?Break each force into an x and a y component, then
find the sum.This is done using trigonometry (sine and cosine).
18Net ForcesNet force (resultant force): a vector produced when
2 or more forces act on a single object.Calculated by adding the
force vectors together.Also described as the overall force acting
on an objectWhat affects acceleration?What different things can you
think of that will affect how fast an object will accelerate?How
hard the object is pushed (force) the bigger the force the bigger
the accelerationHow much mass the object has NOT ITS WEIGHT20Mass
vs. WeightMassThe amount of matter that is in an object. This
amount never changes, no matter where you are.It is measured in
kilograms (kg)WeightThis value changes depending on where you
are.It is a force, so it is measured in Newtons (N)Newtonss 1st Law
and You
Dont let this be you. Wear seat belts.Because of inertia,
objects (including you) resist changes in their motion. When the
car going 80 km/hour is stopped by the brick wall, your body keeps
moving at 80 m/hour.Is this all Newton could do?No, he also came up
with the following The rate of change in an objects velocity
(acceleration) in directly proportional to the force applied, and
inversely proportional to the mass of the object.This is called
Newtons Second Law.2nd LawThe net force of an object is equal to
the product of its mass and acceleration, or F=ma.Newtons Second
LawNewton also gave us a formula that allows us to calculate the
force being applied to an objectF = maWhere:F is the forcem is the
mass of the objecta is the acceleration of the object
2nd LawWhen mass is in kilograms and acceleration is in m/s/s,
the unit of force is in newtons (N).One newton is equal to the
force required to accelerate one kilogram of mass at one
meter/second/second.2nd Law (F = m x a)How much force is needed to
accelerate a 1400 kilogram car 2 meters per second/per second?Write
the formulaF = m x aFill in given numbers and unitsF = 1400 kg x 2
meters per second/secondSolve for the unknown2800
kg-meters/second/second or 2800 N
If mass remains constant, doubling the acceleration, doubles the
force. If force remains constant, doubling the mass, halves the
acceleration.Newtons 2nd Law proves that different masses
accelerate to the earth at the same rate, but with different
forces.We know that objects with different masses accelerate to the
ground at the same rate.However, because of the 2nd Law we know
that they dont hit the ground with the same force.
F = ma98 N = 10 kg x 9.8 m/s/sF = ma9.8 N = 1 kg x 9.8
m/s/sCheck Your Understanding1. What acceleration will result when
a 12 N net force applied to a 3 kg object? A 6 kg object?2. A net
force of 16 N causes a mass to accelerate at a rate of 5 m/s2.
Determine the mass.
3. How much force is needed to accelerate a 66 kg skier 1
m/sec/sec?
4. What is the force on a 1000 kg elevator that is falling
freely at 9.8 m/sec/sec?
Check Your Understanding1. What acceleration will result when a
12 N net force applied to a 3 kg object? 12 N = 3 kg x 4 m/s/s 2. A
net force of 16 N causes a mass to accelerate at a rate of 5 m/s2.
Determine the mass. 16 N = 3.2 kg x 5 m/s/s3. How much force is
needed to accelerate a 66 kg skier 1 m/sec/sec?
66 kg-m/sec/sec or 66 N
4. What is the force on a 1000 kg elevator that is falling
freely at 9.8 m/sec/sec?
9800 kg-m/sec/sec or 9800 N
ExamplesCalculate the force on a 12 kg object if it accelerates
at 3.4 m/s2?F = m aF = (12 kg)(3.4 m/s2)F = 40.8 NF = 41 N (sig
figs)Calculate the acceleration of a 1.79x103 N force acting on a
743 kg object.F = m aa = F/ma = 1.79x103 N/743 kgA = 2.41 m/s2333rd
LawFor every action, there is an equal and opposite reaction.3rd
LawAccording to Newton, whenever objects A and B interact with each
other, they exert forces upon each other. When you sit in your
chair, your body exerts a downward force on the chair and the chair
exerts an upward force on your body.
3rd LawThere are two forces resulting from this interaction - a
force on the chair and a force on your body. These two forces are
called action and reaction forces.
Newtons 3rd Law in NatureConsider the propulsion of a fish
through the water. A fish uses its fins to push water backwards. In
turn, the water reacts by pushing the fish forwards, propelling the
fish through the water. The size of the force on the water equals
the size of the force on the fish; the direction of the force on
the water (backwards) is opposite the direction of the force on the
fish (forwards).
3rd Law
Flying gracefully through the air, birds depend on Newtons third
law of motion. As the birds push down on the air with their wings,
the air pushes their wings up and gives them lift.Consider the
flying motion of birds. A bird flies by use of its wings. The wings
of a bird push air downwards. In turn, the air reacts by pushing
the bird upwards. The size of the force on the air equals the size
of the force on the bird; the direction of the force on the air
(downwards) is opposite the direction of the force on the bird
(upwards).Action-reaction force pairs make it possible for birds to
fly.Other examples of Newtons Third LawThe baseball forces the bat
to the left (an action); the bat forces the ball to the right (the
reaction).
3rd LawConsider the motion of a car on the way to school. A car
is equipped with wheels which spin backwards. As the wheels spin
backwards, they grip the road and push the road backwards.
3rd Law
The reaction of a rocket is an application of the third law of
motion. Various fuels are burned in the engine, producing hot
gases. The hot gases push against the inside tube of the rocket and
escape out the bottom of the tube. As the gases move downward, the
rocket moves in the opposite direction.The MisconceptionSo long as
the forces are balanced on an object, is there motion?The answer is
YES, but why is this true?N2L says that there needs to be an
unbalanced force to have an acceleration, NOT motionBe careful how
you interpret the law!!43Gravityforce of attraction between all
objects in the universethe larger an objects mass, the greater the
gravitational force
Gravitythe greater the distance between two objects, the smaller
the gravitational forcethe force of gravity decreases by the amount
equal to one divided by the distance (d) squared1/d2Newtons Law of
GravitationUsing his own laws, Newton created a Universal Law of
Gravitation that applies to everything in the Universe.Every
particle attracts every other particle with a force that is
proportional to the square of the distance between them.
NULGIf you perform unit analysis on the previous formula, you
will find that they do not match, something is missing, they are
not the units of force.It wasnt until Cavendish worked on it, that
the constant was found, and the equation can be used.The constant
was called the Gravitational Constant, G.NULGThis lead the formula
to change to the following:
Example:Calculate the force of gravitational attraction between
a 2.00 kg mass and a 5.00 kg mass separated by a distance of 3.00
m.
ElevatorsEver had butterflies in your stomach if you moved
really fast up or down on an elevator?What do you think happens to
the weight the floor of the elevator sees as you move up or
down?Does the weight stay the same, get bigger or smaller?Elevators
ContdIf you stand on an scale that measures Newtons in an elevator
the following will happenStopped or constant velocity: you will
read your weightMoving up with an acceleration: the scale will read
a larger weight (you feel heavier)Moving down with an acceleration:
the scale will read a smaller weight (you feel lighter)Elevator
CalculationsHow do we prove these using calculations?Moving up: W =
Fg + FaMoving down: W = Fg Fa
Why is this so?Moving up: your weight seems to shift down and
you feel heavier. You are experiencing added weight due to the
acceleration of the elevator upwardsMoving down: you feel a little
lighter because you are experiencing a loss of weight due to the
downward acceleration.Are there any exceptions you can think of?The
ExceptionWhat happens to your weight on the scale if the cable is
cut?Are you accelerating at the same rate as that of gravity?
Why then, do we observe every day objects in motion slowing down
and becoming motionless seemingly without an outside force?Its a
force we sometimes cannot see friction.Objects on earth, unlike the
frictionless space the moon travels through, are under the
influence of friction.There are four main types of friction:Sliding
friction: ice skatingRolling friction: bowlingFluid friction (air
or liquid): air or water resistanceStatic friction: initial
friction when moving an objectFriction!What is this unbalanced
force that acts on an object in motion?Slide a book across a table
and watch it slide to a rest position. The book comes to a rest
because of the presence of a force - that force being the force of
friction - which brings the book to a rest position.
In the absence of a force of friction, the book would continue
in motion with the same speed and direction - forever! (Or at least
to the end of the table top.) What is friction?Frictionforce that
opposes the motion of an objectthe force of friction works in the
opposite direction of the force of motionTypes of FrictionSliding
Frictionthe source of friction is the contact between two surfaces,
at least one of which is in motionTypes of FrictionAir
Resistancefriction results from air pushing on an object as it is
movingTypes of FrictionRolling Frictionfriction between two
surfaces that are not in constant contactex. wheelsFrictionfriction
makes motion possible
friction also makes it hard to move objects
reducing friction makes it easier to move objectsHow can
friction be reduced?by changing sliding friction into rolling
friction
by using lubricantsLubricants substances that reduce
frictionFrictionWe have all talked about friction, but what is it
really?Friction is the force between objects in contact and
parallel to contact surfacesThere are 2 types of frictionStatic the
friction where surfaces are stationary (book on a desk).Kinetic the
friction between 2 surfaces that are in motion (tires on
pavement)Different Types of Kinetic FrictionThere are different
types of kinetic friction, and each have their own name depending
on the situationSliding friction affects sledsRolling friction
affects cars, bicycles etc.Fluid friction affects a boat moving
through the water or a plane through the airSurface area does not
affect frictionCalculating FrictionWe can calculate the force of
friction acting on an object.Ff = mFNWhere:Ff = Force of Frictionms
or mk = Coefficient of Friction (static or Kinetic)FN = Normal
Force (the force applied at a right angle to a surface)67
