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Chapter 12: Forces and Motion
Transcript of Chapter 12: Forces and Motion
Are forces acting on you now?
How can forces produce motion?
What is the difference between mass and weight? A Force is....
A push or a pull acting on an object Force exerted by the blue Deer Force exerted by the orange Deer Net Force Newton's 3 Laws of Motion
They sound simple but it took Aristotle, Galileo and Newton over 2000 years to figure out. Newton's 2nd Law
(The Equation) - N2 Newton's 3rd Law
(ACTION/REACTION) -A Force can cause an object at rest to move or accelerate, can cause a moving object to accelerate or change directions. Representing Force Force has both a magnitude and a direction. The sum of all the forces acting
on an object. What is the difference between mass and inertia? The acceleration of an object is directly proportional to the net force acting on the object and inversely proportional to the mass, the net force and acceleration act in the same direction. Action-Reaction Force pairs w/o motion Do you have any examples of force?
*Tension - A pull exerted by a rope, string cable etc.
*Normal - A push exerted by a surface that an object is resting on or bounces off of; a normal force is perpendicular to the surface that exerts the force. Units: Newtons (kg*m/s ) US Customary unit : pounds
(1N = .2248 pounds)
*I Newton is the force needed to accelerate 1kg at 1m/s . 2 2 Measuring Force To measure force we use a "Newtonometer"
or scale. -Measures the force needed to support the object. (tension or normal).
-Does a spring scale measure force or mass? *Mass is the amount of matter an object contains.
*Force is the amount of Newton's needed to support the object
*Force is the product of mass and acceleration (due to gravity). Hey! This spring scale measures force but has grams on it?!? Will it be accurate on the moon where the acceleration due to gravity is different? Magnitude is... the strength of the force (number of Newtons).
Direction is... the way the force pushes or pulls the object.
Therefore forces can be represented with a vector. Free Body Diagrams (FBDs) Person Standing Apple hanging from a string Student pushing a Desk A drawing showing all the forces acting on
an object, represent the object with a dot, forces with vectors Person Apple Hand Student Desk F n F F F F F F F F F F F F n n T g g g g Apple g desk student Will any of these objects move? arm Combining Forces -When forces act in the same direction we add the vectors. 10 N
6 N = 16 N -When forces act in the opposite direction we subtract the vectors. 10 N = 4 N 6 N -When forces make an angle we use geometry to combine the vectors. 8 N = 8 N 6 N 6 N 10 N Forces Producing Motion *Will any of these objects move?
*What has to happen in order for any object to move?
*When do forces produce motion? *All the objects on the previous slide will move.
*An object moves when the sum of the forces acting on the object is not zero. -If the net force acting on the object is zero will the object move?
-So in order for an object to move or accelerate.... F = 0 N net 2 N 6 N 4 N 4 N Balanced Forces: When forces are balanced Fnet = 0N, so the object will not accelerate.
-If the object was stationary it will not move.
-If the object is moving, it will travel with a constant velocity. Unbalanced Forces: When the forces acting on an object are unbalanced Fnet = 0N....
-So the object will accelerate.
Which way does the object move?
-In the same direction as the net force acting on it. DO EX 1 & 2 Sliding book demo. Why did the book stop moving?
Is their a force acting on it while it is sliding? Friction:
A force that opposes the motion of all objects.
-Acts in the opposite direction as motion or an applied force.
-A result of the contact between two surfaces or particles of a fluid and a surface. Larger normal force=Larger friction (forces of attraction between p+ and e- play a role.) 4 Types of Friction 1. Static: Frictional force that acts on an object that is not moving; prevents motion.
-Acts in the opposite direction of the applied (orange) force and is equal in magnitude.
-Magnitude will change with applied force until static friction is over come, then the object begins to move.
-Allows us to walk. 2. Sliding (kinetic): Constant force of friction that opposes the direction of an object's motion.
-Causes sliding objects to slow down
-Smaller in magnitude than maximum static.
**Do car tires experience static or kinetic friction?
3. Rolling: Friction exerted on an object that is rolling.
-Static Friction causes wheels to roll, only a small part of friction decreases the velocity of the object
-The effect of friction on a rolling object is 100 to 1,000 times smaller than the effect of static friction on a stationary object. pull Friction 4. Fluid: Friction acting on an object as it moves through a liquid or a gas (fluid).
-Results of collisions between the moving object and the particles of the fluid.
-Directly related to object's speed.
*which piece of paper will hit the ground first a crumpled one or un-crumpled? Why?
*what if I make the un-crumpled smaller? What will happen? Book Demonstration - Can you sepparate teh two books?
-Why is this so difficult?
Watch the Mythbusters try.
Maybe this will help. Gravity & Air Resistance What is Gravity?
-An attractive force between two masses that pulls the two mass together.
- All objects fall at the same rate.
-Acceleration due to gravity is 9.8m/s regardless of mass.
Force due to gravity: Product of an object's mass and the acceleration due to gravity (AKA Weight). Universal Forces 2 Do EX 4 Then Read pg 370 *An object in Free fall is at terminal velocity when the downward force of gravity is equal to the upward force of air resistance.
*The Terminal Velocity of an object depends on the mass and cross sectional area of the object.
* For Humans skydiving Terminal Velocity is between 55m/s and 65 m/s If I launch my angry bird a the angle shown by the blue vector, why does it follow the red path? To play Angry Birds Check out: http://chrome.angrybirds.com/ While you play answer these questions?
1. After you launch the bird is the bird accelerating?
2. Which way is the birds acceleration?
3. Does this acceleration affect the bird's horizontal velocity, vertical velocity or both? (Try Launching only Horizontal, and only vertical and see what happens) Projectile Motion is... the motion of an object, after an initial forward velocity, where gravity is the only force acting on the object.
-the object traveling through the air is called the projectile. The path it follows is called its trajectory.
-Projectiles are only affected by two forces, gravity and air resistance. Red - Path Grey - Vertical Velocity
Blue - Horizontal Velocity Yellow - Acceleration (gravity) *Angry Birds ignores Air resistance, so the horizontal velocity of the bird is constant and the vertical velocity is constantly decreasing as a result of the force of gravity acting on the bird. *Demo: Shoot a ball/drop a ball - Which ball will hit the ground first? THEY HIT AT THE SAME TIME!!!!
What does this tell us about the affect of a horizontal velocity on a vertical acceleration? Maybe Mythbusters can help --> Why does this happen: The bullets hit at the same time, and two balls hit at the same time because the horizontal motion of an object is unaffected by the vertical motion, and vice versa.
*The motion of an object in one direction is unaffected by the motion in another direction. Newton's 1st Law of Motion (The Inertial Law) - N1 An object in motion will stay in motion and an object at rest will stay at rest unless acted upon by an outside force. -Inertia is... the tendency of any object to resist a change in motion.
*An object's resistance to acceleration.
-Objects maintain their current state of motion as long as... the net force on the object is zero Ex 5 & the Penny and the note card demo.
*both objects have no force acting on them so they stay in their current state of motion. Equation: a = F /m or F =ma m - mass
a - acceleration
F - net force *The acceleration is in the same
direction as the net force. net net net *When the net force acting on an object is greater than zero the object will accelerate.
*If the object is already moving and...
-the force vector points in the same direction as the motion then .... the object speeds up.
-the force vector points in the opposite dircetion as the motion then ,,, the object slows down. TRY EX 6 & 7 N2-Making car crashes safer During a collision the force exerted on the car is also exerted on the passengers, so how can we decrease the force exerted on the passengers? F=ma -Mass - directly related to force, so decreasing the mass of cars will decrease the force. (Good idea when choosing a car, but hard to change) -Acceleration - directly related to force, a = (Vf-Vi)/t
*Two Quantities that effect acceleration are time and change in speed. - Time - Inversely related to acceleration; increasing time will decrease acceleration, longer collisions = less force.
-Change in Speed - Directly related to acceleration; If the change in speed is less then less force acts on the car. N2 - Mass & Weight - What's the difference Mass is ... the amount of matter a substance is made of.
- A measure of an object's inertia
Weight is ... the force of gravity acting on an object
Equation : W=mg units - Newtons
W- Weight(N) m- mass (kg) g- acceleration due to gravity 9.8m/s 2 The difference....
*Weight depends on gravity and is measured by a scale
*Mass is constant.
DO EX 9. Every Action has an equal and opposite reaction. - When one object exerts a force on another object, the second object exerts a force equal in magnitude on the first, but in the opposite direction. Force Pairs are ... equal and opposite forces exerted on two different objects. *NORMAL & TENSION are NOT in a force pair with gravity. Draw a FBD for: the book and table; the sign and post KVILLE ROCKETS Book Table sign Post F F F F F F F F F *The force pairs in each picture are the same color g g g g N N book F T Wall Sign They are equal in magnitude but opposite in direction. Action-Reaction Force pairs W/ motion Draw a FBD for the ball & Ground, The red car & the Yellow Car F F F F F F F F F N N N g g g ball Yellow red *The force pairs in each picture are the same color They are equal in magnitude but opposite in direction. Ball Ground Red Car Yellow car Every force on your list can be classified as one of the universal forces. Make a list of every force you can think of: **They don't have to be ones you have heard of in this class, but that might be a good place to start. 1. Electromagnetic Force: A Combination of electric and magnetic forces.
-Only force that can both attract and repel.
*Electric forces: Force between charged particles
- Opposites (+ -) attract, Likes ( + +, - - ) repel
*Magnetic forces: attractive and repulsion forces between the poles of a magnet.
- Opposites (N S) attract, Likes (N N, S S ) repel
Ex. Magnet and the whiteboard, attraction between p+ and e-, Pushes, pulls, friction, Normal, tension.
2. Strong Nuclear: Powerful force of attraction between Protons and neutrons in the nucleus of an atom.
-100 x stronger than electromagnetic. (Strongest force)
-Acts over very small distances (1x10^-15m)
-Responsible for the energy released in nuclear power plants and nuclear weapons. 3. Weak Nuclear: Force responsible for ejecting particles from a decaying nucleus.
-Acts over very small distances (1x10^-18m)
-Present in radioactive process: causes particles to decay, and eject particles.
-Non contact force.
4. Gravitational Force: Attractive force between any two objects with mass
-Weakest of all universal forces.
-Result of the objects distorting the space time continuum (Einstein).
-Acts over very large distances
-Non contact force of attraction. Newton's Law of Universal Gravitation: Every object in the universe that has mass is attracted to every other object that has mass.
- Attraction is called the force of gravity.
-Each mass exerts and equal and opposite force. (The force exerted on me by the Earth is an 3rd law pair with the force exerted on the Earth by me.)
-Force of Gravitational Attraction is proportional to the mass of the object's and inversely proportional to the square of the distance between them.
-Produces a centripetal force which keeps planets and other orbiting bodies on their paths.
* Newton describes what is happening, while Einstein's theory or General relativity addresses why it happens. DO EX 13 Example 13 - Answers