4Force A force is a push or pull Forces are vectors Represented by “F” Has SI units of NewtonsWhen all forces on an object are added, the result is called a net force.A non-zero net force causes an acceleration
5Special Types of Forces Fg: Force of gravity; weight – down towards center of EarthFN: Normal force – perpendicular to surface object is onFf: Friction – opposes motion when two surfaces slide against each otherFT: Tension – in direction of string/rope/chainFdrag: Drag – opposes motion when an object moves through a gas or liquid (i.e. air resistance)
6Vector Review Vectors have a magnitude and direction Vectors can be depicted by an arrowWhen drawing multiple vectors, their arrow lengths should be proportional to the vector magnitudes
7Free Body Diagrams Draw a dot to represent the object Draw an arrow for each force on the objectArrows should start at the dot and point awayArrow lengths should be proportional to force magnitude. For unknown forces, estimate lengthLabel each arrowChoose a direction to be positive and indicate it on the side of the diagramIf object is moving, have positive direction be in direction of motion.
8Example 1A book is sitting at rest on a table. Draw a free-body diagram for the book.
9Example 2A box is pushed along the floor at a constant speed. Draw a free body diagram for the box.
10You-Try #1A rope lifts a bucket at a constant speed. Draw a free body diagram for the bucket.
11You-Try #2A student is pushing against a table, but not hard enough to move it. Draw a free body diagram for the table.
12Warm-Up: October 24, 2011Power Tower is a thrill ride at Cedar Point, the roller coaster capital of the world. At the beginning of the ride, it accelerates the riders upwards. Draw a free body diagram of a rider at the beginning of a Power Tower ride.
13Adding Forces Same process as adding velocity vectors. The vector sum is called the net force, Fnet
14Example 3Two horizontal forces, 273 N and 131 N, are exerted on a canoe in the same direction. Draw a free body diagram and find the net horizontal force on the canoe.Now imagine the same two forces are acting in opposite directions. Draw a free body diagram and find the net horizontal force on the canoe.
15You-Try #3Al and Bob are playing tug-of-war (two teams pulling on a rope in opposite directions) against Chris and Dan. Al pulls with a force of 127 N, Bob pulls with a force of 93 N, Chris pulls with a force of 104 N, and Dan pulls with a force of 119 N. Draw a free-body diagram and calculate the net force on the rope.
16Newton’s LawsAn object at rest will stay at rest unless acted upon by a non-zero net force. An object in motion will stay in motion unless acted upon by a non-zero net force.The acceleration of an object is equal to the net force acting on the object divided by the object’s mass.For every force, there is an equal and opposite force.
17Mass The amount of matter in an object. Represented by “m” SI unit of kilograms (kg)
22Warm-Up: October 25, 2011A 2.5 kg rock is falling off the edge of a cliff. Gravity is pulling it down with a force of 24.5 N. There is a drag force of 7.0 N. Draw a free- body diagram, and solve for the acceleration of the rock.
29Balanced Forces Forces are balanced if there is no net force For two forces in opposite directions, arrows should be the same length.For multiple forces, sum of arrow lengths in opposite directions should be the same.Results in a zero acceleration (Fnet=ma) in that directionZero acceleration constant velocityThat velocity could be zero (no motion)
30Unbalanced ForcesForces are unbalanced if there is a non-zero net force.Arrows are different lengths.Results in an acceleration in the direction of the longer arrow.
31EquilibriumIf all forces are balanced (in both x and y directions), then the system is in equilibrium.There is zero accelerationZero acceleration constant velocityThat velocity could be zero (no motion)
32Terminal Velocity Fdrag = -Fg (Equal magnitudes, opposite directions) Zero net force zero acceleration constant velocityThe fastest an object can free-fall
34Warm-Up:October 28, 2011A crane attached to a chain is being lowered. If the crane has a mass of 105 kg and is accelerating downward at a rate of 2.45 m/s2, what is the tension force between the chain and crane?
37Groupwork You will work in groups of 3-4 Mr. Szwast will assign groups Work on each problem together (DO NOT split up the work)Each person should record their work and answersMr. Szwast will check each individual’s work during classThere will be some presentations of answersThe worksheet may be collected at any point in the future
38Warm-Up: October 31, 2011Every Halloween at Caltech, undergrads drop pumpkins (frozen by liquid nitrogen) from the top of Millikan Library, 42 meters above the sidewalk below. Ignoring air resistance, how long does it take until the pumpkin smashes into the sidewalk?
39Chapter 4 Groupwork Return to your groups You have 15 minutes to finishPresentations will followMr. Szwast will choose which member(s) of your group will present
42Review of ForcesFg: Force of gravity; weight – down towards center of EarthFN: Normal force – perpendicular to surface object is on (not present if object is in air)FT: Tension – in direction of string/rope/chainFf: Friction – opposite direction of motion or applied forceFdrag: Drag – air resistance; opposite direction of motion
44Example 6: Warm-UpA sphere with a weight of 49 N is dropped off the top of a building. Ignore air resistance. Draw a free-body diagram. After 1.0 second, what is the sphere’sAccelerationVelocityDistance
45Homework Corrections DO NOT WRITE ON THE HANDOUT Homework was scored out of 10 points (4 extra credit point possible)If you earned less than 7, copy all problems, work, and answers and turn them in to receive 7 pointsIf you earned more than 7 points, correct what you missed on your paper.After you finish, get groupwork from Mr. Szwast and correct what you missed with your groupTURN IN THE HANDOUT BEFORE YOU LEAVE
46Warm-Up October 14/15, 2010An object near the surface of the earth is being acted on by only one force.What must this force be?In what direction is the force?In what direction is the object accelerating?Can we conclude in what direction the object is moving?Assume the initial time, distance, and velocity are zero. After exactly 3 seconds, what is the object’sAccelerationVelocityDistance
47Warm-Up: October 18, 2010A sphere with a weight of 98 N is dropped off the top of a building. Ignore air resistance. Draw a free-body diagram. After 2.0 seconds, what is the sphere’sAccelerationVelocityDistance
48Warm-Up: October 19, 2010Draw a free body diagram showing all the forces that are acting on you right now.
49Classwork: Free Body Diagrams You will work with one partner.Your partner will be the person assigned to sit at the same table as you.If there is nobody else at your table, Mr. Szwast will assign you a partner.You will have about 25 minutes to complete the worksheet.Random students will then be called to the board to draw each free body diagram.You will turn the worksheet in before you leave.
50Warm-Up: October 21/22, 2010Define, in your own words, the following terms:BalancedUnbalancedEquilibrium
51Review of ForcesFg: Force of gravity; weight – down towards center of EarthFN: Normal force – perpendicular to surface object is on (not present if object is in air)FT: Tension – in direction of string/rope/chainFf: Friction – opposite direction of motion or applied forceFdrag: Drag – air resistance; opposite direction of motion