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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 have already lost points on your lab!!! 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 http://phet.colorado.edu/simulations/sims.php?sim =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 http://www.youtube.com/watch?v=Bd4uHUMvvbc 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 http://phet.colorado.edu/simulations/sims.php?sim=Forces_ 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 http://www.youtube.com/watch?v=VvGIMHWAhn 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 at your FBDs) 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 answers change? If Katy, Kelly, and Kevin could somehow go to a place where g=0, would your answer change? 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 http://www.youtube.com/watch?v=TlrRlpyE0PI&fe ature=fvsr 2 more Inertia videos http://www.youtube.com/watch?v=0VyatNDHjqs http://www.youtube.com/watch?v=7gfWvexu4u4 Friction Forces http://phet.colorado.edu/simulations/sims.php?sim=Fo 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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