Transcript Mechanics I basic forces FBD

a push or a pull
-we could name hundreds…
Stretch, squeeze,press,twist,crumple,bend,punch, etc etc
Net Force
•
The cause of an acceleration or the change
in an object’s velocity.
• Net Forces can cause objects to…
1. Start moving
2. Stop moving
3. Change their direction
For years physicists have been
grouping forces. Eventually they
would like to have all forces in
one unified theory
GUT’s (Grand Unification Theory)
TOE’s (Theory Of Everything)
• For now– 4 groups of forces
–Gravitational : exists between
all things with mass
–Electromagnetic : forces giving
materials strength, bending
squeezing, shattering. (object
interaction)
–Strong nuclear: holds particles
of atomic nucleus together
**strongest of four forces!
–Weak Nuclear : involved in
decay of atomic
nuclei
*
Forces
– a force is a push or a pull (bend stretch squeeze, accelerate)
– a force is a vector quantity (magnitude & direction)
– and there are 4 basic forces
» gravitational
» electromagnetic
» strong nuclear
» weak nuclear
Aristotle, Copernicus & Galileo all explained force and
motion.
Isaac Newton (1642 – 1727) explained what we study
today
Einstein extended our knowledge of gravity and forces –
You need to take a higher level of Physics to learn about it.
• Forces can act through contact or at a
distance
• Contact Forces – forces resulting from
physical contact between 2 objects.
• Field Forces – do not involve physical
contact. An example could be gravity, the
earth exerts a force even though its not
necessarily contacting the object. Also
magnets and electricity have forces without
touching.
Unit of force is named
after Isaac Newton
Newton
Symbol = N
N= Kg m/s2
Pounds and Newtons measure
Force
• 1 pound = 4.448 Newtons
• 1 Newton = 0.225 pounds
• Remember that a Newton is a small thing.
If you weigh 150lbs, you also weigh 667N
Free Body Diagram; FBD
• Free Body Diagrams isolate an object and
the forces acting on it.
• We draw a simple picture of the object and
then draw arrows (vectors) to represent the
forces on the object. Be sure to label them.
• Force diagrams are similar but they have
multiple objects.
• The Net Force causes acceleration
• There are many forces that act on objects….
–
–
–
–
–
–
Weight
Normal
Friction
Applied
Tension
Centripetal
On page 127 in the Text book
• Lets look at the picture of this red car
getting towed and do a sample FBD
together.
Do you see 4 forces acting on the car?
Weight 14700N, Ground 13690N,
Truck hitch 5800N, Friction 775N
Draw a FBD for this scene.
Holt worksheet FBD
Chapter Packet p. 160 I & J
• Let’s do some of these together, in pairs,
and on our own.
Section Review p. 128
• Answer questions 1-6.
• Work in groups of 4. Everyone completes
the assignment on their own paper, turn in
the best one of four with ALL four names
on it.
1. List three examples of each of the following:
a. a force causing an object to start moving
b. a force causing an object to stop moving
c. a force causing an object to change direction
2. Give two examples of field forces described in this section and two
examples of contact forces you observe in everyday life. Explain how
you know that these are forces.
3. Draw a free-body diagram of a football being kicked. Assume that the
only forces acting on the ball are the force of gravity and the force
exerted by the kicker.
4. Physics in Action Draw a force diagram of a crash-test dummy in
at once.
a car at the moment of collision. For this problem, assume that the forces
acting on the car are 19 600 N downward, 17 800 N forward, and 25 000 N
backward. The forces acting on the dummy are 585 N downward, 175 N
backward, and 585 N upward.
5. Physics in Action Use the information given above to draw a freebody
diagram showing only the forces acting on the car in item 4. Label
all forces.
6. Physics in Action Use the information given above to draw a freebody
diagram showing only the forces acting on the dummy in item 4.
Label all forces.
4, 5, and 6 can be done together
Weight
p.141 in text
• Is the magnitude of the force of gravity
acting on an object.
• Weight depends on location – if you
change the value of g, your weight will
change. (I weigh 220lbs on Earth : I
would weigh 37lbs on the moon: 80lbs
on Mars).
we must know the
difference
Mass
- is measured on
a balance scale
- is the amount of
Weight
- is measured on
a spring scale
matter in a body
- is the force of gravity
pulling on the mass
SCALAR
- VECTOR
- is the same
wherever you go
- Has the units of…
Kg
metric
slug
imperial
- Is a fundamental unit
- Depends on where
you are (planets, levels)
- Has the units of…
Newton
Metric
Pound
imperial
- derived
unit N= kg* m/s2
•
however, mass and weight are
related
Force = mass • acceleration
Weight(force) = mass • gravity
N= kg • m/s2
W= mg
g= W/m
m= W/g
Look at units
•Mass = Weight/ Gravity
•Mass = N/ m/s2
•M= kg m/s2 / m/s2
•M= Kg
The Normal Force
p.141 in text
• A force exerted by one object on another in
a direction perpendicular to the surface of
contact.
• Many times the normal force will be the
contact force an object has with the ground.
• If the object is on a slope(ramp), the normal
will be perpendicular with the slope.
Normal Force
Draw a FBD for this scene, the object is the t.v.
t.v.
table
Normal Force on a ramp
Normal Force can be
calculated. It’s the
perpendicular
component of the
weight. The angle
with the weight vector
and perpendicular
component is the same
as the ramp angle.
p. 142
text
• any force opposing motion
• occurs between any two surfaces that
are touching
– No surface is perfectly smooth
2 Kinds of Friction
- Static: occurs when no motion is present
between two surfaces
- Kinetic: (dynamic) occurs when objects on
surfaces slide past one another (sliding)
STATIC is the greater of the two
 Coefficient of friction 
 is a unit less number used to
describe how much friction is
present for certain material
 lesser the friction, lower the
number
 greater the friction, greater the
number
hot car tire
car tire
sand paper
tile floor
soapy water
ice
Teflon
• µ is the symbol for the coefficient of
friction (mu or mew)
• Normal force
– this is an equal and opposite force to the weight of an
object that sits on a level floor. On an incline the
normal force will be different than the weight.
Fn
W
Friction, µ, and normal force
are related this way
µ = Ff
Fn
Fn= Ff
Ff= µ Fn
µ
Ff= µ W
Ff= µ m g
Practice Problems 4C p.145
• 1 0.23
• 2 a. 1.5
b. 1.3
• 3 a. 870N , 670N
b. 110N , 84N
c. 1000N , 500N
d. 5N , 2N
many forces act on a
moving object
Fnormal
Fapplied
Ffriction
Fweight
Balanced and Unbalanced Forces
• Sometimes all the forces on an object will
cancel out. We say that object is in
equilibrium or a state of balance
• Sometimes all the forces will not cancel out
and we say there is a Net Force acting on
the object.
• The Net Force is the Force that causes
acceleration.
Consider the Forces on this box,
are they balanced or not?
**over head bucket problem
•sometimes Fnet = Zero
•when fnet = 0 -- No acceleration will
occur
•Remember, even without
acceleration, you can still have
velocity
•When Fnet= 0 for a moving object,
that object is moving at CONSTANT
VELOCITY
• for the case of constant velocity
friction force = applied force Ffr = Fapp
this occurs when you are on cruise control at highway speed, going in a straight path.
-- no acceleration but you still have velocity
Free Fall
• all objects fall at the same
accelerating rate, with out air
resistance
• This is ONLY true when air
resistance is not present
• Galileo showed this is true, but
he couldn’t explain why
• Here is why its true:
NEWTON”S 2nd LAW
A= F/m
F/ m
=
F/m
• lets compare a 1 kg ball to a 10 kg ball
Weight = 10N
= 100N
10 N/ 1 KG = 10 m/s2 100N / 10 Kg= 10 m/s2
You cannot just look at mass or weight alone,
you must consider them together
A big mass has a big force
A small mass has a small force
Falling With Air
• air resistance is a frictional force,
air resistance causes objects to
slow, even to where no acceleration
occurs.
When Fnet = 0 No Acceleration
When air resistance = weight of
object
Fnet= 0 N
http://www.youtube.com/watch?v=jybIWhW7BNM
•terminal velocity occurs when
–Acceleration = 0
–Fnet = 0
–Weight - resistance = 0
A = Fnet/ mass
= Weight – Resistance/ Mass
= 0 / Mass
= 0 m/s2
Falling ants
Sky Divers
Flying squirrels
Terminal Velocity
• As an object moves through the air, it
collides w/ air molecules that exert a
force on it. (drag, Ffr, air resistance)
The force depends on the size and
shape of the object, the density of air,
and the speed of motion.
• The faster you go, the more air
resistance you get, at some point
drag force will equal the force of
gravity
** overhead plane model**
Study for
the test!!