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
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 Fnet.
app=80N
�⃗� 𝑔=60𝑁
+ = 80N + (-60N) = 80N – 60N = 20N = 20N [up]
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.
Simnett Tug of War
If There is an Fnet
Then there is an acceleration in that same direction.
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
A Virtual Experiment
• We will try Galileo’s Famous Ball Drop Experiment.
Galileo’s 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!
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”
Eureka Videos!• Eureka Episode 1:
Inertia http://youtu.be/HRq-v4Gmzxg
• Eureka Episode 2: Mass http://youtu.be/1i5k5mW8qdI
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 at Rest V= 0 m/sObjects 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/s2 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/s2 ) .
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 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.
Applications of Newton’s First LawWhy could staying in motion be a problem???
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?
So Buckle Up and Drive Safe!
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:
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
Homework
1. Explain the following using Newton’s 1st Law. Make sure you create any relevant FBD’s.• getting water off your toothbrush• getting ketchup out of the bottle
2. Explain at least three applications where you use Newton’s First LawThe Law of Inertia on a regular basis. Include FBD’s.
3. Play the Inertia games at http://staweb.sta.cathedral.org/departments/science/physics/inertiagames/
Demonstrations:
1.) Discuss magician’s ability to remove a tablecloth – beaker of water and paper or stack of coins and paper2.) Pull bill between 2 bottles-1 inverted on top of the other3.) Coin stack – flick out bottom coin with a ruler5.) 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)
