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Playing with FBD’s

Playing with FBD’s. We can use an FBD to find an object’s Net Force or Resultant Force The NET FORCE is the force that has resulted from all the forces acting on an object We get rid of internal forces that cancel each other out and only look at forces. Calculating Net Force.

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Playing with FBD’s

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  1. Playing with FBD’s • We can use an FBD to find an object’s Net Force or Resultant Force • The NET FORCE is the force that has resulted from all the forces acting on an object • We get rid of internal forces that cancel each other out and only look at forces

  2. Calculating Net Force Resultant Force or Net Force • Using a FBD we can sum the vectors acting on an object – this is called the resultant force or the net force (F­net). • Sum the force in the x-direction and y-direction separately

  3. Example # 1 Draw a free body diagram showing a woman lifting a bag of flour with a force of 80 N [up]. If the force of gravity on the bag is 60 N, calculate the Fnet. app=80N + = 80N + (-60N) = 80N – 60N = 20N = 20N [up]

  4. Example # 2 • A race car’s engine applies a force of 2.0 x 104N [fwd]. The force of friction is 7500 N [bwd]. If the normal force has a magnitude of 1.4 x 103N and the force of gravity has a magnitude of 1.4x103N. Draw an FBD with all forces and determine the net force in the x and y direction.

  5. Simnett Tug of War

  6. If There is an Fnet Then there is an acceleration in that same direction.

  7. Galileo’s Contribution • He noticed that object’s which had a force applied to them tended to continue in the same direction. • He called this Inertia • I.e. Dropping a cannonball from a tower • Galileo also proved that objects with the same mass accelerate towards the Earth at the same speed. http://www.pbs.org/wgbh/nova/pisa/galileo.html

  8. A Virtual Experiment • We will try Galileo’s Famous Ball Drop Experiment. Galileo’s Experiment

  9. Sir Isaac Newton • He was a 17th century scientist. • He is considered one of the most influential people to the scientific community ever. • He invented three laws of motion which help explain why objects (things with mass) move the way they do.

  10. May the Force Be With You. . . • Acceleration: Any resulting change in velocity is called an acceleration. • Velocity must either be increasing, decreasing or changing direction in order for an acceleration to be occurring.

  11. Inertia • Inertia: Tendency of objects to resist changes in their velocity (i.e. to resist acceleration) • Inertia is proportional to mass • A stationary curling stone on ice can be difficult to start moving but it is difficult to stop once it is moving • A large football player requires a lot of force to get into motion but once in motion, it takes a lot of force to stop them!

  12. Newton’s First Law of Motion • Every object in a state of uniform motion (or at rest) tends to remain in that state of motion (or at rest) unless an external, unbalanced force is applied to it. • This Law is also sometimes called “The Law of Inertia”

  13. Eureka Videos! • Eureka Episode 1: Inertia http://youtu.be/HRq-v4Gmzxg • Eureka Episode 2: Mass http://youtu.be/1i5k5mW8qdI

  14. That Explains A lot! • We can use Newton’s first law to explain and therefore predict the motion of object’s while at rest and while moving. Forces Are Balanced Objects In Motion V = 0 m/s Objects at Rest V= 0 m/s *Note that a=0 m/s2 in both Cases* Object stays moving at same speed and in the same direction Object remains at rest

  15. Comprehension Questions • Do all moving objects have a velocity? How about acceleration? • If an object has a constant velocity (i.e. a car is moving at 80km/hr with the cruise control set (assume no friction)), what is the acceleration equal to? • If an object is at rest (i.e. a car that is stopped at a red light waiting for it to turn green), what is the acceleration? Yes, all objects in motion have a velocity. Not all moving objects experience an acceleration, unless V is going up or down. Acceleration equals 0 because Velocity is constant at 80km/hr and therefore not changing. Acceleration Equals 0. Since there is no Velocity there cannot be a change in V (V=0m/s) and therefore no A (A=0 m/s2 ) .

  16. Summary • If the net force acting on an object is zero, the object will maintain its state of rest or constant velocity Important Points of Newton’s First Law: • Objects at rest tend to stay at rest • Objects in motion tend to stay in motion • If the velocity of an object is changing in either magnitude or direction or both, the change must be caused by a net external force acting on the object.

  17. Applications of Newton’s First Law Why could staying in motion be a problem???

  18. Reason #1: Safety • Restraints in a car like the seatbelt are a great application of Newton’s First Law. • Obeying local speed limits especially when weather conditions are poor is another example why Newton’s First law is very important. • If the speed limit is 60km/hr and you are doing 100km/hr and you contact black ice (frictionless surface) what will happen to the direction of car travel when you turn the wheel? Why does this happen? • What will happen when the car hits a pole? What will happen to the occupants? • Why are seatbelts important according to Newton?

  19. So Buckle Up and Drive Safe!

  20. Newton’s First Law is Fun! • Who likes amusement park rides? • Many rides create “thrilling” experiences by applying Newton’s first law. • Describe how (forces involved etc.) and why these pictures apply to Newton’s First Law:

  21. Newton’s Laws Video Quiz Practice • Think Pair Share • Write: • Law: Newton’s First • Video: • Explanation: • Create an explanation to help me understand Newton’s first law and how it applies to the video without actually stating Newton’s first law

  22. Homework • Explain the following using Newton’s 1stLaw. Make sure you create any relevant FBD’s. • getting water off your toothbrush • getting ketchup out of the bottle • Explain at least three applications where you use Newton’s First LawThe Law of Inertia on a regular basis. Include FBD’s. • Play the Inertia games at http://staweb.sta.cathedral.org/departments/science/physics/inertiagames/

  23. Demonstrations: 1.) Discuss magician’s ability to remove a tablecloth – beaker of water and paper or stack of coins and paper 2.) Pull bill between 2 bottles-1 inverted on top of the other 3.) Coin stack – flick out bottom coin with a ruler 5.) Mark a target on the floor -have a student run at it holding a tennis ball – maintain speed while dropping the ball on the target (ballistic cart if available)

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