Transcript Forces and Motion

Forces and Motion
Three Incorrect laws of motion.
• http://www.youtube.com/watch?v=Yf0B
N0kq7OU&list=EC772556F1EFC4D01
C&safety_mode=true&persist_safety_m
ode=1
• Why does the Earth Spin?
– http://www.youtube.com/watch?v=TQxeutcYP6I
&list=EC772556F1EFC4D01C&safety_mode=tr
ue&persist_safety_mode=1
• Second clip?
– http://www.youtube.com/watch?v=9zso7ChaQX
Q&list=EC772556F1EFC4D01C&safety_mode=t
rue&persist_safety_mode=1
Vocabulary
•Force - A push or pull on an object.
•Net Force – vector sum of all forces
•Equilibrium:
– the state in which the net force on an
object is zero or balanced
– no change in velocity
Sir Isaac Newton
•
1643 - 1727
2.2.4 State Newton’s first law of motion.
2.2.5 Describe examples of Newton’s first
law.
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 Law of Motion
(Law of Inertia)
Translation: Objects keep doing
what they are doing, in a
straight line.
Newton’s 1st Law –
Inertia
• Inertia is a measurement of
RESISTANCE TO CHANGE
______________
mass The
• It is measured in terms of ______.
mass the greater the
greater the _____,
inertia.
What is Mass?
• Mass is the amount of material a
body/object contains
• You are what you eat
• Bigger does not equal more mass
2.2.2 - Identify the forces acting on an object
and draw free-body diagrams representing the
forces acting
Free Body Diagrams
•FBD’s are a neat way to show the forces
acting on an object.
•The object is drawn as a dot or a box,
and then vectors are drawn to show the
forces acting on it.
5N
•Ex:
10 N
6N
FBD’s
• Can you draw the FBD for you sitting on
your chair?
• Draw the FBD for your friend pushing
you in a rolling chair.
FBD’s
• Solve the net force for the following
FBD.
10 N
5N
6N
FBD
• The picture at the bottom shows a
snowman pulling a sled. Draw a freebody diagram for this sled. The
magnitudes of the forces acting on the
sled are 60N by the string, 130N from
gravity, and 90N upward by the ground.
FBD
• Now try doing some of the free-body
diagrams on
• pg 124 Practice A
• Pg 124 Section Review # 2-5
2.2.6 - State the condition for translational
equilibrium.
2.2.7 - Solve problems involving translational
equilibrium.
• Equilibrium in a straight line.
• Fnet = 0 so there is constant velocity
2.2.6 - State the condition for translational
equilibrium.
2.2.7 - Solve problems involving translational
equilibrium.
• Derek leaves his physics
book on top of a drafting
table that is inclined at 35º
angle. The FBD to the right
shows the forces acting on
the book. Find the net force
acting on the book.
Vocabulary
• Force - A push or pull on an object.
• Net Force – vector sum of all forces
• Equilibrium:
– the state in which the net force on an
object is zero or balanced
– no change in velocity
2.2.8 State Newton’s second law of
motion.
2nd Law
The net force of an object is
equal to the product of its
mass and acceleration, or
F=ma.
Newton’s 2nd Law – Simplified
• So, a little easier to read would be:
a = ΣF
OR
ΣF = ma
m
• So… what are the units for force
• Forces are measured in newtons
• N = (kg)(m/s2).
What does F = ma mean?
• Force is directly proportional to
mass and acceleration.
Imagine a ball of a certain mass
moving at a certain
acceleration. This
ball has a certain
force.
What does F = ma mean?
Now imagine we make the ball twice as big
(double the mass) but keep the
acceleration constant. F = ma says that
this new ball has twice the force of the old
ball.
What does F = ma mean?
Now imagine the original ball moving at
twice the original acceleration. F = ma
says that the ball will again have twice the
force of the ball at the original
acceleration.
More about F = ma
• If you double the mass, you double the force. If you
double the acceleration, you double the force.
What if you double the mass and the acceleration?
(2m)(2a) = 4F
Doubling the mass and the acceleration quadruples the
force.
So . . . what if you decrease the mass by half? How
much force would the object have now?
What does F = ma say?
• F = ma basically means that the force of an object
comes from its mass and its acceleration.
Something very massive (high mass)
that’s changing speed very slowly
(low acceleration), like a glacier, can
still have great force.
Something very small (low mass) that’s
changing speed very quickly (high
acceleration), like a bullet, can still
have a great force. Something very
small changing speed very slowly will
have a very weak force.
Newton’s 2nd Law – Vectors!
• Remember, acceleration is a vector
quantity, so Force will be too 
• Some equations to remember:
– Pythagorean Theorem
– Sin θ
– Cos θ
– Tan θ
2.2.1 Calculate the weight of a body using
the expression W = mg.
Gravity causes weight
• The weight of an object is caused by the force
of attraction between the earth and objects on
the surface of the earth.
Gravitation force is equal to weight, therefore…
How much does a person with
a mass of 70.0 kg weigh on
the earth?
Weight = mass X 9.8 m/s2
Weight = 70.0 kg X 9.8 m/s2
Weight = 686 N
Remember that 1 N = (1 kg) (1m/s2)
Mass vs. Weight
Your MASS is the amount of material in
your body.
Your MASS doesn’t depend on where you
are.
• Your WEIGHT is how much your body
pushes down on a scale.
• Your WEIGHT depends on how much
MASS you have and where you are.
• Weight is caused by gravity.
Marty
m = 68 kg
What does
Marty weigh?
•
G(sun)= 275 m/s2
18,700 N
G(pluto)= 0.654 m/s2
44 N
2.2.9 Solve problems involving
Newton’s second law.
1. Determine the accelerations that result
when a 12-N net force is applied to a 3kg object.
2. A net force of 15 N is exerted on an
encyclopedia to cause it to accelerate
at a rate of 5 m/s2. What is the mass of
the encyclopedia?
2.2.9 Solve problems involving
Newton’s second law.
•
Now try answering some problems on
pg 132 practice C # 1-4
• There is a net force of 12N acting on a
desk which has a gravitational force of
40N. How much is the desk
accelerating?
• There is a net force of 12N acting on a
desk which has a gravitational force of
40N. How much is the desk
accelerating?
• A 50N crate experiences a horizontally
applied force of 10N from a student and
a frictional force of 4N. What is the
acceleration of the crate?
Newton’s 2nd Law –
Let’s practice
1. You have run out of gas and have to push
your car. Your car weighs 560 N and you
push with a force of 395 N, and your
friend helps with 275 N. What is the
acceleration of the car?
2. You and a friend push a 20kg rolling chair
horizontally. You push with a force of 17
N. The chair has a frictional force of 5N.
The chair accelerates at a rate of
1.35m/s2. How much force is your friend
contributing?
NET FORCE Practice
• Free-body diagrams for four situations are
shown below. The net force is known for each
situation. However, the magnitudes of a few
of the individual forces are not known.
Analyze each situation individually and
determine the magnitude of the unknown
forces. Then click the button to view the
answers.
Newton’s 2nd Law –
A little bit harder
1. You have run out of gas and have to
push your car. Your car weighs 560 N
and you push with a force of 395 N, and
your friend helps with 275 N. What is
the acceleration of the car?
2. You push a rolling chair (m=20kg) with
a force of 17 N. A friend pushes with
15 N at 30o. What is the net force?
What is the acceleration of the chair?
Quick Quiz
Consider the following free-body
diagram. Ignoring air resistance and
friction, can this object be moving in a
horizontal direction?
We’ll come back to this question.
HWA
1. What is the net force acting on an
object that has a force of 40N to the
right and another 30N downward?
2. What are the components of a 20N
force acting on a desk at an angle of
30º above horizontal?
2.2.14 State Newton’s third law of motion.
2.2.15 Discuss examples of Newton’s
third law.
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safety_mode=true&persist_safety_mode=
1
• For every action,
there
is
an
3rd Law
equal and opposite reaction.
What does this mean?
• For every force acting on an object, there is an
equal force acting in the opposite direction.
Right now, gravity is pulling you down in your seat,
but Newton’s Third Law says your seat is pushing
up against you with equal force. This is why you are
not moving.
• There is a balanced force acting on you– gravity
pulling down, your seat pushing up.
•Put really simply: F = F
Think about it . . .
What happens if you are standing on a
skateboard or a slippery floor and push against
a wall? You slide in the opposite direction
(away from the wall), because you pushed on
the wall but the wall pushed back on you with
equal and opposite force.
Why does it hurt so much when you stub
your toe? When your toe exerts a force on a
rock, the rock exerts an equal force back on
your toe. The harder you hit your toe against
it, the more force the rock exerts back on your
toe (and the more your toe hurts).
Newton’s Third Law
• It states:
Whenever one object exerts a force on a
second object, the second object exerts an
equal and opposite force on the first object
• It requires two forces:
Action
– _____________
Force
Reaction
– ______________
Force
Newton’s Third Law
• Does it matter which force we call the
action and which is the _________?
reaction
________
pairs
• Forces always occur in ________.
doesn’t matter which is
Therefore, it ________
the action and the reaction, as long as
you can identify both.
Review
Newton’s First Law:
Objects in motion tend to stay in motion
and objects at rest tend to stay at rest
unless acted upon by an unbalanced force.
Newton’s Second Law:
Force equals mass times acceleration
(F = ma).
Newton’s Third
Law:
For every action there is an equal and
opposite reaction.
Quick Quiz
Consider the following free-body
diagram. Ignoring air resistance and
friction, can this object be moving in a
horizontal direction?
YES!!!! IT CAN!!!
Quick Quiz
Think about this situation.
What are the forces on the
boy on the sled between
points B and C?
I hope you got
something that
looked a lot like
this.
Quick Quiz
This means a force is
not required to keep
a moving object in
motion.
Forces
• Types of forces
• Contact
• Non-Contact
Forces
• Contact
• Normal
• Frictional
• Tension
• Air Resistance
• Spring
Forces
• Non-Contact
• Electrical
• Magnetic
• Nuclear
Normal Force
• http://www.youtube.com/watch?v=aJc4
DEkSq4I&list=EC772556F1EFC4D01C
&safety_mode=true&persist_safety_mo
de=1
The Normal Force
• The normal force (FN)
is one component of
the force that a surface
exerts on an object
with which it is in
contact
• Namely, the
component that is
Perpendicular to the
____________
surface.
The Normal Force
• How do I find the normal force?
• Easy, it is equal and opposite to the
weight of an object if the object is on a
horizontal surface.
• The Fn is not always horizontal.
• Lets look at Fig 12 (pg136)
• The Fn can be found by
• Fn=mg cosΘ (where Θ is the angle between the
contact surface and horizontal)
The Normal Force
• Where is the normal
force?
What is Friction?
-is a force resisting the relative motion of two surfaces in
contact with each other.
-is the parallel component of the force on an object that
is in contact with a surface.
-is a force that prevents motion from occurring.
-Types
-Static
-Kinetic
-Fluid
What causes friction?
• There are two factors which affect
friction between two surfaces:
– Kind of surfaces in contact (rough or
smooth)
– Amount of force pressing the surfaces
together.
The rougher the surface and the stronger
the force between the surfaces, the
greater the amount of friction.
From Glencoe Physical Science
Static and Kinetic Friction
Static Friction- Friction force that acts on objects that
are not moving
Kinetic Friction - Friction force that opposes the
direction of motion of an object as it slides over a
surface.
I better be
safe Ump!!
Static Friction
• Static Friction is the force
that prevents the start of
motion.
• With no movement,
Fs = Fapplied
• Has a range of 0 – Fs,max
Kinetic Friction
• Once motion has begun, the static friction force is no
longer a concern.
• Instead, kinetic friction is important.
• Kinetic friction is resistance to movement of an
already moving object.
Fluid Friction
- Friction force that opposes the motion of an object
through a fluid.
Frictional Forces
• The frictional force will vary depending
on the surface interactions.
– Ex. Carpet vs tile
• Friction can be calculated by using the
coefficient of friction.
• Use the basic formula of:
– Ff = μFn
• It can be rearranged to:
– μk = (Fk/Fn) and μs = (Fs/Fn)
Frictional Forces
• Use the formula of:
– Ff = μFn
– Fk = μkFn
and Fs = μsFn
• A 24kg crate initially at rest on a horizontal
floor requires a 75N horizontal force to set
it in motion. Find the coefficient of static
friction between the crate and the floor.
Force of Tension
• Tension commonly
means the tendency of
a rope to be pulled
apart.
• Tension is typically the
same everywhere in a
rope because the rope
is treated as massless.
If it isn’t (and is vertical)
you must account for
the mass of the rope.
Finally, AIR RESISTANCE!!!
2.2.6 State the condition for translational
equilibrium
• Translational equilibrium is equilibrium
in a straight line.
• The net force acting on a free falling
object at terminal velocity is zero, so it
falls to the ground at constant speed.
Terminal Velocity
The velocity at which a free falling object ceases to accelerate.
Force of Gravity
Air Resistance
T.V. is the constant velocity of a falling object when the
force of gravity equals the force of air resistance
Spring Force
• We will dive deep into
spring force when we
get to elastic energy.
• Until then, play safely!!
- forces that act between charged objects or
particles such as protons and electrons
Electric Forces
Magnetic Forces
-Forces that act on certain metals, poles of magnets,
and on moving charges
Nuclear Forces
- Nuclear forces act
over very short
distances and are
what hold the
particles of an atomic
nucleus together.
• Use the formula of:
Fk = μkFn
and Fs = μsFn
• A hockey puck has a coefficient of kinetic
friction of μk = .10. If the puck feels a
normal force (FN) of 5 N, what is the
frictional force that acts on the puck? What
is the mass of the puck?
• A desk has a mass of 71.25 kilograms. If
the coefficient of static friction between the
desk and the floor is 1.14, what force must
be used to move the desk from rest?
Quick Review (Net Force)
• There is a 15kg book sliding down a desk
that has an incline of 25º. If there is a
frictional force of 20N, what is the
acceleration of the book?
• Derek leaves his physics book on top of a
drafting table that is inclined at a 35° angle.
There is a frictional force of 11N, a normal
force of 18N and gravitational force of 22N.
What is the net force?
On your own!! (Net Force)
• A student is designing a support to keep a tree
upright. Two wires have been attached to the
tree and placed at right angles to each other.
One wire exerts a force of 30N on the tree; the
other wire exerts a 40N force. Determine where
the third wire should be placed and how much
force is should have so that the net force acting
on the tree is zero.
• A stationary, flying kite is acted on by a force of
9.8N downward. The wind exerts a force of 45N
at an angle of 50° above the horizontal. Find
the force that the string exerts on the kite.
Putting it all together!!
• A student attaches a rope to a 20kg box
of books. He pulls with a force of 90N at
an angle of 30° with the horizontal. The
coefficient of kinetic friction between the
box and the sidewalk is 0.50. Find the
acceleration of the box.
• Slow down, take a deep breath and lets
take this one step at a time.
Putting it all together!!
• Now lets try pg 141 Practice F, # 1-4
• Don’t get bogged down with #2
What is momentum?
• The momentum of an object is equal to
the product of its mass and its velocity.
The unit for momentum is kg m/s
How some things react
without forces…