Transcript Dynamics and Statics

Dynamics and Statics
{
What is the resultant velocity vector on
a boat that is traveling at 10 m/s north
and is being blow by the wind 0.5m/s
east?
 Include the magnitude and angle.

Bellringer
Learn more vocabulary that will help
you solve dynamics and statics
problems
 Practice identifying objects in a system
 Concurrent forces problems

Objectives
Kinematics Test
In order to get any points back you must
complete the following by next Friday
(10/17)
1. Add the correct equation and units when
substituting where you forgot to on the
Part Two to receive full points back.
2. Correct all problems you got wrong (Part
One and Part Two) to receive half points
back.
3. Hand them in to me in person during one
of my free mods.

You get to pick the two other teams forces.
 Pick some numbers between 0.1N and
1.962N
 Also pick the angle between them.



Pick some angle between 95 and 135 degrees
The plastic hangers have a mass of 5 grams
Lab #5
Bellringer – 2 mins to hand
in

Find the horizontal
component of a 15N
vector that is 15 degrees
above the horizon.
DO
STOP
WORK
Learn more vocabulary that will help
you solve dynamics and statics
problems
 Practice identifying objects in a system
 Concurrent forces problems

Objectives
Kinematics Test
In order to get any points back you must
complete the following by next Friday
(10/17)
1. Add the correct equation and units when
substituting where you forgot to on the
Part Two to receive full points back.
2. Correct all problems you got wrong (Part
One and Part Two) to receive half points
back.
3. Hand them in to me in person during one
of my free mods.

Robots?
Dynamics:


The study of forces and
motion
Statics:


The study of forces in static
equilibrium
Some words
Static:


Not moving.
Equilibrium:


The condition of a system in
which all competing
influences are balanced (Net
Force = 0)
More words

System:


A set of interacting or
interdependent pieces forming
one thing.
Net Force:

The overall force acting on a
system (all the forces added
together)
Even more words
1.
2.
3.
4.
If an object is moving at constant velocity is it in
static equilibrium?
What is the net force on an object that is moving
at constant velocity?
If an object is moving at constant velocity is it at
equilibrium?
If there is a non-zero net force on a system can it
be moving at constant velocity?
Checkpoint

Baseball in the catcher’s mitt.

Your dinner on a table.
Define all objects in each
system.

Baseball in the catcher’s mitt.

Your dinner on a table.
Free-Body Diagrams

1.
2.
3.
4.
For each of the following situations, specify the system
and draw a free-body diagram. Label all forces, and
indicate the direction of the acceleration and the net
force.
A skydiver falls downward through the air at constant
velocity. (The air exerts an upward force on the
person.)
You hold a softball in your hand and move it up.
Two people pushing equally on a table in opposite
directions.
Two equally strong huskies pulling a sled in the same
direction.
Practice

Demo
Force and mass

Two identical huskies are pulling a
sled through the snow. They are each
pulling with a force of 40 N in the
same direction. Neglecting friction
(snow) what is the total net force on
the sled?
Happy Huskies

Two identical huskies are pulling a sled
through the snow. They are each
pulling with a force of 40 N in the
opposite direction. Neglecting friction
(snow) what is the total net force on the
sled?
Hateful Huskies

Two identical huskies are pulling a sled
through the snow. They are each
pulling with a force of 40 N at an angle
of 90 degrees from one another.
Neglecting friction (snow) what is the
total net force on the sled?
Hungry Huskies

What have our little husky buddies taught us?

What is the maximum resultant of two forces
and the angle between them?

What is the minimum resultant of two forces
and the angle between them?
Humble Huskies

Concurrent Forces are the Regent’s
exams fancy way of saying forces that
act on a system at the same time.
Concurrent Forces

Which pair of concurrent forces may have a resultant of 40 N?





1N and 5N
20N and 10N
20N and 30N
50N and 5N
Which pair of concurrent forces may have a resultant of 20 N?




5N and 10N
20N and 20N
20N and 50N
30N and 5N
Concurrent Forces Examples

Two concurrent force of 50N and X Newtons have a resultant of
100N. Force X could be





10N
40N
90N
160N
Two concurrent force of 40N and X Newtons have a resultant of
100N. Force X could be




20N
40N
80N
150N
Concurrent Forces Examples

The resultant of two concurrent forces is minimum when the angle
between them is





0 degrees
45 degrees
90 degrees
180 degrees
As the angle between two concurrent forces is increased from 10 to 75
degrees, the magnitude of the resultant force



decreases
increases
Remains the same
Concurrent Forces Examples

Mary is holding a pillow with a mass of
0.3kg when Sarah decides that she wants it
and tries to pull it away from Mary. If Sarah
pulls horizontally on the pillow with a force
of 10N and Mary pulls with a horizontal
force of 11N, what is the horizontal
acceleration of the pillow?
Pillow Fight!

First identify the system and what is acting on it:
Pillow Fight

Then draw a free body diagram:
Pillow Fight

Then solve for the unknown:
Pillow Fight

Finally; as always, evaluate the answer:
Pillow Fight

Buddy is learning how to ice skate. He
wants his momma to pull him along so that
he has an acceleration of 0.8m/s^2. If
Buddy’s mass is 27.2kg, with what force does
his momma need to pull him? (Neglect any
resistance between the ice and Buddy’s
skates.)

Use the same steps as the pillow fight
problem.
Ice Skating
Bellringer – 2 mins to hand
in

What is the acceleration
of a baseball (0.145kg)
that is hit with a force of
150N
DO
STOP
WORK

Know the difference between weight
and mass, and be able to solve
problems.
Objectives
Vector Quiz
Kinematics Test
In order to get any points back you must
complete the following by next Friday
(10/17)
1. Add the correct equation and units when
substituting where you forgot to on the
Part Two to receive full points back.
2. Correct all problems you got wrong (Part
One and Part Two) to receive half points
back.
3. Hand them in to me in person during one
of my free mods.


Use inertia and Newton’s first law of motion to
explain this picture. (Write your response)

Car crash demo
Newton’s First Law

An object’s weight is the gravitational force
experienced by that object.

Weight = Mass x Gravitational Field
Gravitational Field on Earth is 9.81m/s^2
 So this is the same equation as F=ma

What is weight?
How do scales find your weight?????????
 Scales are not accelerating so the net force on
them must be zero.
 The scale pushes up with the same force that
gravity pushes you down.
 This action reaction pair causes calibrated
springs to stretch and turn a dial that displays
your weight.

Scales

Karan holds a golden puppy in each hand.
Puppy A has a mass of 4kg and puppy B has
a mass of 12kg. What upward forces do her
two hands exert to keep the puppies at rest?

If she then drops the two puppies, with
what acceleration do they fall? (Ignoring air
resistance.)
Comparing weights

Does an object’s mass change
depending on what gravitational field it
is in?

Does an objects weight change
depending on what gravitational field it
is in?
Changing mass

If you have a mass of 72kg what is your weight, in
Newtons, of Earth?
What is the weight, in Newtons, on the moon?
(The acceleration due to gravity there is 1.62m/s^2)
So what’s the easiest way to lose some
weight??


What is the weight in Newtons on Jupiter? (The
acceleration due to gravity there is 24.79m/s^s)
Changing Weight Checkpoint
Gravity map of the Moon

What is the gravitational acceleration on a
planet where a 4kg mass has a weight of 16N
on the planet’s surface?

What is the gravitational acceleration on a
planet where a 2kg mass has a weight of 8N
on the planet’s surface?
Weight Training
If your weight on Earth is 500N, what
is your weight on a planet where the
acceleration due to gravity is 6m/s^2

Weight Training

It is impossible to be massless, but is it impossible
to be weightless??

No you always have a weight too.

Sometimes it just doesn’t seem like you do.

This is called apparent weightlessness.
Weightlessness

Apparent Weightlessness means that there are no
contact forces acting to support the object, and the
object’s apparent weight is zero.

What are contact forces?


Any forces that need to be in contact with an object to
exert a force on it.
The feeling of weightlessness is most common
when you or an object is in free-fall.
Apparent Weightlessness

Big drops on rollercoasters

Dips in airplanes

Orbiting the Earth

Going down a big hill

Falling
Examples

Review sheet
Homework
Bellringer – 2 mins to hand
in
1.
2.
3.
What have you done to
make sure your grade is
what you want it to be?
What will you do to
improve your grade (or
keep it at a 100)?
What do you want your
grade to be at the end of
the quarter? (Can’t lower
it)
DO
STOP
WORK
Homework
Kinematics Test
In order to get any points back you must
complete the following by next Friday
(10/17)
1. Add the correct equation and units when
substituting where you forgot to on the
Part Two to receive full points back.
2. Correct all problems you got wrong (Part
One and Part Two) to receive half points
back.
3. Hand them in to me in person during one
of my free mods.


Like most Americans you weigh yourself
on a bathroom scale in an elevator.

If you have a mass of 75kg, what will the
scale read if the elevator is stationary?

735N
Elevator
What will the scale read if the
elevator is moving at a constant
velocity of 3m/s?

735N

Elevator

If the elevator accelerates upward at 2 m/s^2
for 2 seconds what does the scale read
during the 2 seconds of acceleration?

885N
Elevator

If the elevator accelerates downward at 2
m/s^2 for 2 seconds what does the scale read
during the 2 seconds of acceleration?

585N
Elevator

Review sheet
Homework
Bellringer – 2 mins to hand
in

What is the acceleration
of a 75kg person if the
net force acting on them
is -300N?
DO
STOP
WORK
Be able to solve stationary and moving
elevator problems.
 Be able to calculate the force of air
resistance.
 Know what tension is and use it to
solve problems.

Objectives
???

Vector Quiz
If you failed the last test and did not complete
corrections in order to raise your grade to a passing
one you are one of the newest members of the
science learning center!
 Can stay any day after school to catch up.
 Must stay Thursdays.
 You’re in the club until I think you’re good to go.

Science Learning Center
Why do skydivers have a
terminal velocity?

Why do they need a parachute?

Skydiving

What is a 80kg skydiver’s drag force if his net
acceleration is -2m/s^2.
Draw a free-body diagram of all the forces acting
on the skydiver.
 Solve for the missing forces.

Example

What is a 70kg skydiver’s drag force if his net
acceleration is -0.5m/s^2.
Draw a free-body diagram of all the forces acting
on the skydiver.
 Solve for the missing forces.

You Try

What is the minimum acceleration a skydiver
could have?

What is the strongest drag force a 65kg person
could have?

How does your velocity change the drag force?

How does air density change the drag force?
Maximum Drag Force
1.
What is the drag force on a 76kg
individual that is free-falling and has
no acceleration?
2.
What is the drag force of a 100kg
food supply crate that is falling with
zero net force?
Checkpoint

http://www.youtube.com/watch?v=raiFrxbHxV
0
Red Bull gives you wings
Lab Time
Parachute Lab

What is tension??

Tension is simply a specific name for the
force that a string or a rope exerts!!

How many tensions can one rope have?
Tension Force
Do tension forces push or pull? How
do you know?

Tension


Demos
Nails and scales demo
Tension and pulleys

You have a heavy crate of standard sized
envelopes that has a mass of 50kg. You decide
the easiest way to move it is to pull it with a rope.
If you put an acceleration on the box of 2.5m/s^2
what is the tension in the rope. (ignore friction)

Two people are playing tug of war. If they each
pull the rope with 50N force in opposite
directions what is the tension in the rope?
Tension Examples

If two 5kg masses hang from opposite sides of a
rope around a pulley. What is the tension force in
the rope?
Tension examples

Find the tension force between two 1kg boxes.
The box on the left is being pulled with a force of
50N and the box on the right is being pulled with
a force of 80N.
Draw a free body diagram.
 Define the system you want to study.
 Solve for the missing forces.

Back To Tension
When solving tension problems
always start by looking at a system
with the most known variables.
Remember if objects or different parts
of the system are physically connected
they must have the same acceleration.

Tension Problems

Find the tension force between two blocks that are
hanging around a single fixed pulley. One block
has a mass of 50kg, the other has a mass of 80kg.
Draw a free body diagram.
 Define the system you want to study.
 Solve for the missing forces.

Tension practice
Bellringer – 2 mins to hand
in

A 3,000kg Chevy is
having a tug of war with
a 2,800kg Ford. The Ford
pulls with a force of
8,000N and the Chevy
pulls with a force of
7,500N. What is the net
force and the
acceleration of the cars?
DO
STOP
WORK

Be able to calculate the acceleration of
a two mass system that is rotating
around a pulley.

Finish Lab
Objectives

Today after school in room 122.
Science Learning Center
The Atwood’s Machine:
 Invented by George Atwood in 1784 as a
way to verify Newton’s Second Law of
Motion


F=ma
Lab #7
Three blocks are each connected by separate
strings, and are pulled along a frictionless surface.
Box A is 4kg, box B is 2kg, and box c is 6 kg.
 If F is 36N what is the acceleration of each box?
 What are the tension forces in each of the strings?

Tension Example
Vector Quiz
Bellringer – 2 mins to hand
in

In your lab which mass
rotates the pulley in the
positive direction?
DO
STOP
WORK

Be able to correctly label and calculate
the normal force

Figure out what variables effect the
force of friction.

Collect lab data
Objectives
The Atwood’s Machine:
 Invented by George Atwood in 1784 as a
way to verify Newton’s Second Law of
Motion


F=ma
Lab #7
Vector Quiz
How would you label the force of:

A table
 The floor
 The ground
 A chair
 The road
 The rug

Labeling forces

The normal force is the perpendicular
contact force that a surface exerts on
another surface.
Use free body
diagrams to figure
out what is going on.

The Normal Force
The magnitude of the normal
force is enough force to keep a
system in equilibrium.

Adding weights example
The Normal Force

What is the normal force on a box that is just sitting on a
table?


What is the normal force on the same box if I’m pulling up
on it?


𝐹𝑁 = 𝑚𝑔
𝐹𝑁 < 𝑚𝑔
What is the normal force on the same box if I am pushing
down on it?

𝐹𝑁 > 𝑚𝑔
Examples

What is the normal force that the road exerts on a car that
has a mass of 1,234kg?


𝑚
𝑠2
= 12,105.54𝑁
What is the normal force on a 2kg bottle that is touching a
table but is being lifted with a 2N force at the same time?


𝐹𝑁 = 1,234𝑘𝑔 9.81
𝐹𝑁 = 17.62𝑁
What is the normal force exerted by the floor on a 15kg
desk if a 76kg student sits in it?

𝐹𝑁 =892.71N
More Examples
Friction can come from a number of
sources

Heat
 Sound
 Air resistance
 Any kind of resistance
 Any unwanted loss of energy

Friction Force

Though friction can come from a variety of
sources there are only two types of friction
that we are going to work with.

Kinetic Friction

Static Friction
Types of Friction

Kinetic Friction
Is exerted on one surface by another when
the two surfaces rub against each other
because one or both surfaces are moving
against each other.
 Rubbing Friction

Kinetic Friction
Static Friction

Is the frictional force exerted on one
surface by another when there is no
motion between the two surfaces.
 Stationary Friction


Increases to a point then becomes
kinetic friction
Static Friction

What factors do you think will change the
friction force between two objects?

Demo

Would this be important to an athletic
footwear manufacturer?

BOOM! The labs never end!
Friction Factors



http://www.youtube.com/watch?v=cPpOxPlCT
Es
https://www.youtube.com/watch?v=Ek5Zyzn3
Avw
http://www.youtube.com/watch?v=ibxtmAVLa
ag
Foot Friction
Bellringer – 2 mins to hand
in

What is the normal force
that a table exerts on a
5kg textbook?
DO
STOP
WORK

Figure out what variables control how
much friction there is between two
objects

Finish Foot Friction Lab
Objectives
The Atwood’s Machine:
 Staple it and hand it in

Lab #7

What factors have you identified in your lab
work that effect the magnitude of the friction
force?

Normal Force

Type of surfaces that are rubbing
Friction Factors

𝐹𝑓𝑘 = 𝜇𝑘 𝐹𝑁

𝐹𝑓𝑘 = The force of kinetic friction

𝜇𝑘 = The coefficient of kinetic friction

𝐹𝑁 = The normal force
Kinetic Friction

What is the coefficient of kinetic friction
between a 5kg laptop and a table if the force of
friction is 10N?

What is the kinetic friction force between
waxed skis and snow if a 75kg lady is skiing
on level snow at constant velocity?
Kinetic Friction example

𝐹𝑓𝑠 ≤ 𝜇𝑠 𝐹𝑁

𝐹𝑓𝑠 = The force of static friction

𝜇𝑠 = The coefficient of static friction

𝐹𝑁 = The normal force
Static Friction

What is the maximum static friction
force between a 500kg go kart with
rubber tires and dry asphalt?

What force is the minimum force
required to push a 75kg skier into
motion on snow with freshly waxed
skies?
Static Friction example
Dynamics and Statics Review
problems 1-15

Homework
Label which
part of the
graph is static
friction, kinetic
friction, and at
which point it
starts to move.
Bellringer

Be able to solve all kinds of friction
problems

Be able to solve force problems at an
angle
Objectives
Foot Friction Lab
Atwood’s Machine: Friction
and Air Resistance

Missing Labs…
Homework
Test next Monday and Tuesday
 Science Club on Thursday to catch-up/review
for the test
 Is anyone interested in sending something
into space?


http://space.1337arts.com/wpcontent/uploads/2009/09/1337artsGuide.pdf
Updates

You push a 25kg wooden box across a
wooden floor at a constant speed of 1m/s.
The coefficient of kinetic friction is 0.2. How
large is the force that you exert on the box?
Analyze and sketch
 Solve
 Evaluate

Friction Practice

Gwen exerts a 36N horizontal force as she pulls a
52N sled across a cement sidewalk at constant
speed. What is the coefficient of kinetic friction
between the sidewalk and the metal sled runners?
Ignore air resistance.
More Practice

Mr. Ames is dragging a box full of books from
his office to his car. The box and books
together have a combined weight of 134N. If
the coefficient of static friction between the
pavement and the box is 0.55, how hard must
Mr. Ames push horizontally on the box in
order to start it moving?
More Practice
Ricky Bobby is driving in his old Pontiac with
rubber tires on dry concrete road. If he and his
car have a combined weight of 10,500N what is
his maximum static friction force before he
starts to skid out and lose control of his vehicle?
 9,450N
 What is the new force of friction if Ricky starts
to skid?
 7,140N
 Why is it so hard to regain control of your car
once you lose it?

 Car Crash!!! 
Why don’t magnets fall
down?

Magnets

What do you think you would do with a
vector that is acting in both the x and y
direction?

𝐴𝑦 = Asin(𝜃)

𝐴𝑥 = A𝑐𝑜𝑠(𝜃)
Forces at an angle
Fric.
32.1N
50°
The Lawn Mower
mg
38.3N
𝐴𝑥 = 𝐴𝑐𝑜𝑠𝜃
= 50𝑁𝑐𝑜𝑠50°
𝐴𝑥 = 32.1𝑁
Normal
If Gary pushes the lawn mower at constant velocity with a
force of 50N down the handle. What is the magnitude of the
force moving the mower forward? The handle is 50 degrees
above the level ground.

What angle is the most efficient to pull
something?

As the angle of the applied force on an object
decreases from 90 degrees to 0 degrees what
happens to the X component of the force
vector?
Angle Concepts

Tony is pulling a wagon at constant velocity. The
wagon handle is 35 degrees above the level
ground. If Tony pulls the handle with his 100N
force directed 35 degrees above the horizon what
is the magnitude of the force in the direction of
the motion?

If the wagon weighs 150N what is the normal
force from the ground on the wagon?
Angle Practice

If a 40kg block is pulled across a
frictionless floor at 30 degrees above the
horizontal with a 75N force what is the
acceleration of the block in the horizontal
direction?

What is the original pull force if the Xcomponent is 25N and the angle is 60
degrees?
Checkpoint
1.
2.
3.
What have you been doing since the last
check to improve your grade?
What will you do to improve you
grade?
What do you want your grade to be at
the end of the quarter?
Grades
Bellringer – 2 mins to hand
in

What is the force of
kinetic friction between
a 100N steel plate sliding
on copper surface at
constant velocity?
DO
STOP
WORK
Be able to solve dynamic
problems at an angle.

Objective
Test next week
 https://twitter.com/Windsor_Physics

Updates


Demo
Video:

http://www.youtube.com/watch?v=kPf8FMJ5iJs
Inclined Plane
Inclined Plane
As the angle of an incline increases

The normal force decreases
 The force from gravity down the incline
increases
 The force from gravity into the incline
decreases
 The frictional force decreases

Inclined Plane
A 30 kg kid slides down a frictionless
slide that is 30 degrees above the
horizon.

What is his force down the slide?
 147.15N
 What is normal force from the slide?
 254.9N

Inclined Plane
A 2kg rubber stopper is sliding down a 35 degree ice slide.
 What is the normal force acting on the stopper?
 16.1N
 What is the friction force acting on the stopper?
 2.4N
 What is the force of the stopper down the slope?
 11.3N
 What is the net force on the stopper parallel to the slide?
 8.9N
 What is the acceleration of the stopper down the slide?
 4.5m/s^2

Inclined Plane
If a 1,500kg car is put in neutral on a 25 degree hill,
how much force will it have as it rolls down?
Ignore friction
 6,218.8N

What is the normal force of a 50kg boy that slides
down a 15 degree frictionless snow slope?
 473.8N
 What is that same boy’s force down the slope?
 127.0N

Checkpoint
Finish the rest of your
Dynamics and Statics
Packet

Homework
Online demo


https://phet.colorado.edu/en/simulatio
n/forces-1d
Force Demo

If the ramp is frictionless and m1 is 35kg
and m2 is 15kg which way will the
system move? Down the slope or up the
slope?
Incline plane

Make sure you review this homework
assignment!
Newton’s Laws HW

This is an outline of the concepts that you will be
tested on.

Make sure you understand every concept on here.

You should also know what problems I might ask
about each concept.

How do these concepts relate to the labs and
demonstrations we have done the past few weeks?
Dynamics and Statics
“Study Guide”
Bellringer – 2 mins to hand
in
What is the net force on
the block in the xdirection?
 𝐹𝑃𝑢𝑙𝑙 = 100𝑁
𝑜
 𝜃 = 45
 𝐹𝐹𝑟𝑖𝑐𝑡𝑖𝑜𝑛 = 20𝑁

DO
STOP
WORK
Be able to solve and statics or
dynamics problem I throw at you!

Objective
If you were there last week, I
expect you there again today.

Science Club

Begins Tuesday (11/4): 2:35 to 3:15
Physics Club: Our Journey
to Space
Partner Up Gang!!!

Station Activity
Bellringer – 2 mins to hand
in
What is the net force on
the block in the xdirection?
 𝐹𝑃𝑢𝑙𝑙 = 100𝑁
𝑜
 𝜃 = 45
 𝐹𝐹𝑟𝑖𝑐𝑡𝑖𝑜𝑛 = 20𝑁

DO
STOP
WORK
Bellringer – 2 mins to hand
in

What is the acceleration
of a 10kg block that is
sliding down a
frictionless slope of 30
degrees?
DO
STOP
WORK

Be a dynamics and statics master.
Objective

Begins Wednesday (11/5): 2:35 to 3:15
Physics Club: Our Journey
to Space
Part 1: 25 multiple choice
questions
Part 2: 5 long answer questions

Statics and Dynamics Test
Homework
Stations – Answers on website
Sample Part Two Questions
Study Guide
Newton's HW
Dynamics and Statics Review HW
Chapters 4 and 5

Review