# 1-dimensional forces

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```					1-dimensional forces
Forces

 Move a cart without touching it (demo)
 Stopping a cart
forces

 Cause a CHANGE in velocity
 Or acceleration

 OBJECTS IN CONTACT exert forces
Types of forces

 Weight
Types of forces

 Normal forces
Draw Force Diagrams:

 Book sitting on a table
 Book sitting on an incline
Types of forces

 Tension
Draw a Free-body diagram

 Cart being pulled across the floor by a string
Types of forces

 Friction
Free-body diagrams

 Diagram including ALL forces acting on an object
Draw FBDs

 A book sitting on a table
 A book falling to the ground

 How come the book isn’t accelerating?
One more detail

 A NET force causes acceleration
Draw 2 FBDs

 Cart rolling forward at a constant speed
 Cart sitting still on the floor
Draw 3 FBDs

 A ball is tossed upward in the air. Draw FBDs of:
 The ball, after being released, on its way up

 The ball at its peak

 On its way down, before being caught
What this means

 FBDs do NOT TELL US THE DIRECTION AN
OBJECT IS MOVING
 FORCES DO NOT INDICATE THE DIRECTION AN
OBJECT IS MOVING
Friday, October 2

 Lab – forces and acceleration
Force, Vel, and Acceleration

Scenario         Force     Velocity   Force   Accel
Diagram     sign       sign   sign
At rest

moving with
no force

Released
from rest

Initial push
backward
What this means…

 Force does not have to be in the same direction as
the motion

 Force DOES have to be in the same direction as
acceleration (change in motion)
Monday, October 5

 Finish Friday’s lab
 C-block: If your lab equipment was left out after class, you
Tuesday, October 6

 TEST next Wednesday! (motion graphs, forces)
 Practice with FBDs
 Net force/acceleration
B-block

 Homework!!!!!
Forces

 Caused by OBJECTS
 Cause ACCELERATION – or CHANGE in velocity!!!
 Measured in Newtons!
Yesterday’s lab

 What we should have learned…..
Yesterday’s lab

 What we should have learned…..

 Net force and acceleration are always in the same
direction

 Net force tells you nothing about the direction of
velocity
Draw Force Diagrams

 A weight hung from a string at rest
 A weight hung from a string being pulled upward at a
constant speed
 A weight hung from a string being lowered at a
constant speed
Remember

 NET FORCE causes acceleration
 If an object has zero acceleration, does it have to be at rest??
You are given the following force diagram

 In what direction is this
object moving?
   To the left
   To the right
   Up
   Down
   How the heck am I supposed to
know?
Net force = TOTAL force

 Determine net force in each case
 Determine direction of acceleration in each case
Determine the direction of the net force and
acceleration
Acceleration??? Net force???

 A weight hung from a string at rest
 A weight hung from a string being pulled upward at a
constant speed
 A weight hung from a string being lowered at a
constant speed
Yet again

 FBD tells us NOTHING about the direction the
object is moving

 It DOES tell us direction of net force and direction of
acceleration
Elevator Man Worksheet

 Determine the direction of the acceleration and net
force for each scenario.
 Part A: Elevator Is At Rest
 What is the direction of the man’s acceleration?
 What is the direction of the net force?
 Construct a force diagram of the man. Use the
lengths of
 your arrows to represent the relative magnitudes of
the forces.
 Part B: Elevator starts from rest and beings to
move upward.
   What is the direction of the man’s acceleration?
   What is the direction of the net force?
   Construct a force diagram of the man. Use the lengths of your
arrows to represent the relative magnitudes of the forces.
 Part C: Elevator is moving upward at a
constant speed
   What is the direction of the man’s acceleration?
   What is the direction of the net force?
   Construct a force diagram of the man. Use the lengths of your
arrows to represent the relative magnitudes of the forces.
 Part D: Elevator is moving upward and slows
to a stop.
   What is the direction of the man’s acceleration?
   What is the direction of the net force?
   Construct a force diagram of the man. Use the lengths of your
arrows to represent the relative magnitudes of the forces.
 Part E: Elevator starts from rest and begins
moving downward.
   What is the direction of the man’s acceleration?
   What is the direction of the net force?
   Construct a force diagram of the man. Use the lengths of your
arrows to represent the relative magnitudes of the forces.
 Part F: Elevator is moving downward at a
constant speed.
   What is the direction of the man’s acceleration?
   What is the direction of the net force?
   Construct a force diagram of the man. Use the lengths of your
arrows to represent the relative magnitudes of the forces.
 Part G: Elevator is moving downward and
slows to a stop
   What is the direction of the man’s acceleration?
   What is the direction of the net force?
   Construct a force diagram of the man. Use the lengths of your
arrows to represent the relative magnitudes of the forces.
Wednesday, October 7

 Test next Wednesday (October 14)
 Blair Elevator Activity
 Net forces and acceleration
Blair Elevator Activity

 Look at Normal force in the elevator on the way up
and the way down
 Draw FBDs:
   At rest
   Moving up and speeding up
   Moving up at a constant speed
   Moving up and slowing down
   Moving down and speeding up
   Moving down at constant speed
   Moving down and slowing down
PhET simulation – 1-D forces

=Forces_in_1_Dimension
 Direction of net force vs. direction of motion
(velocity)
 Net force vs. acceleration graphs
Determine the sign of acceleration and net
force
Thursday, October 8, 2009

 Normal forces demonstration – brief
 Mass, force, and acceleration
 Flea video
Practice drawing FBDs – draw a FBD of the
block on the incline
 If a block is being pulled (from rest) across a
frictionless surface with a force of 5 N to the right,
the object will:
   A) Move at a constant speed to the right
   B) Continually speed up until the force is removed
   C) Continually slow down
   D) Speed up for a while, then move at a constant speed after it
gets going fast enough
 If a block is being pulled (from rest) across a
frictionless surface with a force of 5 N to the right.
As soon as you let go of the block (stop exerting a
force) the block will:
   A) Move at a constant speed to the right
   B) slow down until it eventually stops
   C) continue to speed up
Demonstrations

 A cart is moving and you want it to stop.
 Draw a FBD of the forces

 A cart is moving and you want to speed it up.
 Draw a FBD of the forces
The motion of a car is represented with the motion diagram
below. The car is moving to the left (negative direction) and

slowing to a stop as it approaches a red light.

vfinal                      vinitial
a
o
x
Which diagram best represents the forces exerted on the car
by the Earth and the Earth’s surface?

1   y                   2    y                   3        y

x                        x                             x

4   y                    5   y

x                        x

Fsurface( ) on car   
                     Fair on car
Fsurface(ll) on car 
FEarth on car
Which of the following motions best describes the
object in the FBD above?
1 – the object is at rest
2 – the object is moving to the left at a constant
speed
3 – the object is moving to the left and slowing down
4 – the object is moving to the right and speeding up
5 – both 1 and 2 are possible
Which FBD below best
represents the forces
exerted on the ball as it
moves UP (ignore air
resistance)?
                    2
1
Fperson on ball
                      
FEarth on ball          FEarth on ball
                    
3                      4
Fperson on ball       Fperson on ball
                     
FEarth on ball        FEarth on ball
Flea Video

Normal forces activity

 Pushing and pulling objects sitting on a scale
 Draw FBDs.
What is mass?!?

 Mass = an object’s resistance to changes in motion –
 Measured in kg

 Weight – force of gravity pulling on a mass
 Measured in Newtons (N)

 WEIGHT = mass*gravity
 What is the weight of a 5-kg object?

 What is the weight of a 12-kg object?

 What is the mass of a 500-N object?
How does mass affect acceleration?
Determine acceleration, then double mass. Then
triple mass. What happens to the acceleration?
Newton’s 2nd Law

∑F = ma
Problem-solving method

 Draw a force diagram (free-body diagram)
 Write ∑F = ma equation
 Solve for unknown
Another practice problem

 A 12-kg (120 N) bucket is lowered vertically by a
rope. The tension in the rope if 163 N.
   Draw a FBD
   Calculate the acceleration of the bucket
   What is the DIRECTION of the acceleration? How do you
know?
   Describe the motion of the object
Friday, October 9
 Reminder: test Wednesday
 Turn in HW
 Draw the following FBDs:
 A 70-kg man standing in an elevator moving downward at a
constant speed
 A ball that has been tossed up in the air and is moving upward

 A cart on a track that has been given an initial push backwards

   A 2-kg cart moving at a constant speed along a frictionless
floor
 Simulation: forces in 1-D
Warm-up Problem

 A man is standing in an elevator that is accelerating
upward at a rate of 3 m/s2. If the man has a mass of
65 kg, determine the Normal force the man feels
from the floor.
Practice

 A 20-N block is being pulled across a horizontal
frictionless surface with a force of 5 N.
   Draw a force diagram
   Calculate the acceleration of the block.
Practice

 The same 2- kg block is being pulled across a surface
with a force of 5 N. There is a 2-N friction force
between the block and the surface.
   Draw a FBD
   Calculate the acceleration of the block
Tuesday, October 13

 No test tomorrow – short quiz on FBDs (review online)
 Quiz Friday as well (brief)
 Go over homework
 Superman/Incredibles activity
 Draw the following FBDs. Include numerical values:
 A 20-N stroller is rolling in the positive direction down the street.
You run after it and pull it backwards with a force of 10 N to get it to
stop.
 A 50-N bucket is being pulled upward by a rope. The tension in the
rope is 20 N.
 A 10-N block is at rest on the floor when you pull upward on it with a
force of 5 N.
 You have tossed a .5-N baseball straight up into the air. Draw a FBD
of the baseball when it comes to a stop at the peak of its motion.
Wednesday, October 14

 Quiz - FBD
 Finish Incredibles/Superman activity
 Short quiz on Friday – forces and ∑F=ma
Superman vs. Mr. Incredible

M
   Tape sheets into DAILY notebooks
   Watch both clips
   Incredibles (8 minutes)
Superman post-lab questions

 1. The masses and accelerations of the train and
airplane were the same in both cases. Why did
Superman need more force to stop the airplane than
Mr. Incredible needed to stop the train? (Hint: look
 2. Draw velocity-time graphs for the airplane and
the train as they were being saved. (2 graphs!)
 3. Draw acceleration-time graphs for the airplane
and the train as they were being saved.
 4. Sketch ∑F-time graphs for the airplane and train
as they were being saved.
Webquest

 Inertia/FBDs
Monday, October 19

 Quiz on Wednesday!
 Get out last week’s webquest
 Start drawing the following FBDs
Go over Thursday’s webquest

 What did we learn about inertia?
 What determines how much inertia an object has?
Go over Thursday’s webquest

 What did we learn about inertia?
 It is a resistance to an object’s change in motion

 What determines how much inertia an object has?
 Mass – NOTHING ELSE!!!
Clarification

 Difference between mass and weight
 Mass is a measure of inertia – never ever ever ever changes

 Which means that inertia never ever ever ever changes

 Which means that an object will still resist acceleration on the
moon and everywhere else
 Mass means inertia means resistance to acceleration

 Weight depends on gravity- changes depending on where you
are
Funny Video

 http://video.yahoo.com/network/100284668?v=38
26981&l=4418225
Simpson’s Clip

 http://www.lghs.net/teachers/science/burns/scienc
eonsimpsons/Clips_files/Newton1.mov
Use physics terms to answer the questions
Use physics terms to answer the question
Question

 If Katy, Kelly, and Kevin were on the moon, where
the acceleration of gravity is much less, would your

 If Katy, Kelly, and Kevin could somehow go to a
Tuesday, October 20
Last week’s homework

 ∑F=ma
 One of these problems on tomorrow’s quiz!!!
First law game

 http://www.stmary.ws/highschool/physics/home/a
nimations3/forces/firstLawChallenge.html
   Open the first law game
   Use the arrow keys to try to land the hot air balloon on the
target
   Is the direction of the balloon’s motion always the same as the
direction of the net force? Use the term “inertia” to explain
why or why not.
Wednesday, October 21

 Get out daily notebooks, calculator, writing utensil
Quiz video

ature=fvsr
2 more Inertia videos

Friction Forces

rces_in_1_Dimension

 When the crate is at rest, does the friction force change?
(this is static friction) (YES – it always equals the
applied force)

 After the crate is ALREADY moving, does the friction
force change? (this is kinetic friction) (NO – it stays
constant)
Thursday, October 22

 Homework questions?
 Rank the following from smallest to greatest inertia:
m=25kg
m=10kg                       m=5kg          m=10kg
v= 50 m/s
v= 5 m/s                     v= 0 m/s       v= 50 m/s

m=25kg
m=15kg                       m=30kg          v= 0 m/s
v= 0 m/s       m=5kg
v= 0 m/s
v= 5 m/s
Tuesday’s balloon game

 Is the direction of the net force always the same
direction it is moving?
 Use the term “inertia” to explain why or why not.
Inertia and FBDs

 Can we better understand why a ball moving up can
have a net force down? Use the term “inertia” to
explain what is happening to the objects.
   A ball in the air on its way up
   A train moving forward but applying the brakes
   A cart given an initial push backwards being pulled forward by
a string
Friday, September 23

 Turn in homework
 Friction lab
 Starting Monday, bring TWO DIFFERENT
COLORED PENS OR PENCILS TO CLASS!!!
Monday, October 26

 Hand back homework
 Pick up this week’s homework and get out daily
notebook
   Review Friday’s friction lab
   Two-dimensional forces
   Bring two different colored pens or pencils to class
starting tomorrow! (and for the rest of your life)
   Test next Tuesday
Friction

 Coefficient of friction (μ) – indicates the “stickiness”
between two objects
   Large μ = large friction force
   Small μ = small friction force
   Depends only on the material
Why does it stay the same???

Ff/FN = µ

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