# Playing with FBDs We can use an FBD to find an objects Net Force or Resultant Force The NET FORCE is the force that has resulted from all the forces acting.

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Playing with FBDs We can use an FBD to find an objects 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 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 (F­ net ). Sum the force in the x-direction and y-direction separately

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 F net.

Example # 2 A race cars engine applies a force of 2.0 x 10 4 N [fwd]. The force of friction is 7500 N [bwd]. If the normal force has a magnitude of 1.4 x 10 3 N and the force of gravity has a magnitude of 1.4x10 3 N. Draw an FBD with all forces and determine the net force in the x and y direction.

Simnett Tug of War

If There is an F net Then there is an acceleration in that same direction.

Galileos Contribution He noticed that objects 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/pis a/galileo.html

A Virtual Experiment We will try Galileos Famous Ball Drop Experiment. Galileos Experiment

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.

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.

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!

Newtons 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

Eureka Videos! Eureka Episode 1: Inertia http://youtu.be/HRq- v4Gmzxg http://youtu.be/HRq- v4Gmzxg Eureka Episode 2: Mass http://youtu.be/1i5k 5mW8qdI http://youtu.be/1i5k 5mW8qdI

That Explains A lot! We can use Newtons first law to explain and therefore predict the motion of objects while at rest and while moving. Forces Are Balanced Objects at Rest V= 0 m/s Objects In Motion V = 0 m/s Object stays moving at same speed and in the same direction Object remains at rest *Note that a=0 m/s 2 in both Cases*

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? 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/s 2 ). Yes, all objects in motion have a velocity. Not all moving objects experience an acceleration, unless V is going up or down.

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 Newtons 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.

Applications of Newtons First Law Why could staying in motion be a problem???

Reason #1: Safety Restraints in a car like the seatbelt are a great application of Newtons First Law. Obeying local speed limits especially when weather conditions are poor is another example why Newtons 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?

So Buckle Up and Drive Safe!

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

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

Homework 1.Explain the following using Newtons 1 st Law. Make sure you create any relevant FBDs. getting water off your toothbrush getting ketchup out of the bottle 2.Explain at least three applications where you use Newtons First Law The Law of Inertia on a regular basis. Include FBDs. 3.Play the Inertia games at http://staweb.sta.cathedral.org/departments/science/physi cs/inertiagames/ http://staweb.sta.cathedral.org/departments/science/physi cs/inertiagames/

Demonstrations: 1.) Discuss magicians 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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