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Regents Physics

Regents Physics. Agenda Introduction to Forces Intro to Newton’s three Laws of Motion HW: Read p. 117-125. What’s a Force?. We’ve learned that acceleration is the change in an object’s velocity.. And Velocity is the change in an objects position..

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Regents Physics

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  1. Regents Physics • Agenda • Introduction to Forces • Intro to Newton’s three Laws of Motion • HW: Read p. 117-125

  2. What’s a Force? • We’ve learned that acceleration is the change in an object’s velocity.. And • Velocity is the change in an objects position.. • By what causes the change in acceleration? A Force!

  3. What’s a Force? • Forces can be described as a push or a pull that is applied to an object by something else.. • Forces are vectors – magnitude and direction • The ability to understand how forces affect us is crucial to success in many fields • Ex: building of homes and bridges

  4. Kinds of Forces • Contact force – acts on an object only by touching it • Ex: book on table, friction • Long-range force – is exerted without contact • Ex: magnetic force, force of gravity

  5. Forces have agents.. • Each force has a specific, identifiable, immediate cause called the agent • The agent can be animate – such as a person or • inanimate – such as a desk, floor or magnet What’s the agent for the pull of gravity?

  6. Solving Force Problems - intro • First step is to draw a pictorial model, called a free body diagram, and identify the contact and long range forces • Draw the vectors • Example F desk on book Book on a table F book on desk

  7. Solving Force Problems - intro • Examples Ball on a rope Skydiver F rope on ball F air on diver F ball on rope F gravity on diver

  8. Practice Problems • Draw a free body diagram for each of the following: • Book held in your hand • Book pushed across the desk by your hand • Book pulled across the desk by a string • Book on a desk with you hand pushing down • Ball just after the string that was holding it broke

  9. The man…Sir Isaac Newton • 300 hundred years ago an apple fell on his head…and he wondered why? • Explained the way forces influence motion • Summed it up in three famous laws

  10. 2nd Law of Motion • The force exerted on an object is equal to the objects mass times its acceleration, or F = ma • Expressed in newtons = kg x m/s2 • Example: Mr. O • Mass = 95 kg • Acceleration = gravity = 9.80 m/s2 F = ma = (95 kg)(9.80 m/s2) = 931N

  11. 2nd law continued • F = ma can be rearranged to be • we say that the force exerted on an object is proportional to its acceleration (since the objects mass doesn’t change) • The larger the force..the greater the acceleration • We have a linear relationship! F m a =

  12. 2nd law continued • Multiple forces can combine and act on a system • They could act in the same direction or in different directions • Because forces are vectors, the total force on an object is the vector sum of all forces on the object • This vector sum is called the net force

  13. Finding a net force • Two horizontal forces act, 225 N and 165 N, are exerted in the same direction on a crate (assume no friction). Find the net horizontal force on the crate. • Step 1 – draw a free body diagram Fnet = Facting on the crate F = +225 N Fnet = 225N + 165N = +390N F = +165 N worksheet

  14. Regents Physics • Agenda • Newton’s first law of motion - Intro • Newton’s second law practice probs • HW Chap Problems: • RC #1,3,5,6,8,9 • AC # 10, 15, 16 • Probs # 20, 22, 25, 27, 30, 32, 36, 38, 40

  15. 1st Law of Motion • An object that is at rest will remain at rest or an object that is moving will continue to move in a straight line with a constant speed, if and only if the net force acting on that object is zero. • Also called the Law of Inertial Mass • What does that mean to us?

  16. 1st Law of Motion • What is inertia? Inertia is the tendency of an object to resist change Anything that has mass has inertia! We resist what tries to change us! Examples Sitting still / moving at a constant velocity Object in space Turning in a car

  17. 1st Law of Motion • What is equilibrium? If an object is at rest or it is moving at a constant velocity, the net force is zero in both cases We are happy just chillin..

  18. Some Types of Forces • See table 6.2 pg. 123

  19. Common Misconceptions • When we throw a ball, the force from our hand stays on it • A force is needed to keep an object moving • Inertia, itself, is a force • Air does not exert a force

  20. Calculations with Newton’s 2nd Law • Using F = ma • Mass and weight..what’s the difference? • Ex: a bathroom scale • Draw a freebody diagram for this and label the forces

  21. A system of solving.. • Read the problem and visualize! • Choose a coordinate system • Write your known and unknowns • Use f = ma to link acceleration and net force • Rearrange, plug in numbers and solve • Check your answer to see if it’s reasonable

  22. Practice Problem • On Earth, a scale shows that you weigh 585 N • a) What is your mass? • B) What would the scale read on the moon? (g = 1.60 m/s2)

  23. Regents Physics • Agenda • More on Newton’s Second Law • Review Intro to Forces Worksheet • Drawing Free-body Diagrams Worksheet • HW: More Advanced Newton’s Second Law Problems

  24. Practice Problem - elevator • You still weight 585 N. Find the reading on a scale in an elevator when: • a) the elevator moves up at a constant speed • b) it slows at 2.0 m/s2, while moving upward • c) It speeds up while moving 2 m/s2 downward • d) it moves downward at a constant speed • e) it slows to a stop at a constant magnitude of acceleration End

  25. Working with friction... • Friction is a force that generally opposes the direction of motion • Different surfaces have different amounts of friction that affect an objects movement • There are types of friction: • Kinetic Friction = uk • Static Friction = us Ffr = Fnu

  26. Solving Problems with Friction • Find the net force on an object that has a mass of 20.0 kg, acceleration of 2.3 m/s2 on an surface with a friction coefficient of 0.21 Known: Fn=Fg=mg=(20.0kg)(9.8 m/s2) = 196 N Fa = ma = (20.0kg)(2.3 m/s2) = 46N Ffr = Fnuk = (196N)(0.21) = 41.2 N Fn Ffr Fa Fnet = Fa - Ffr Fnet = Fa - Fnuk Fnet = 46N - 41.2N = 5N Fg Worksheet

  27. 3rd Law of Motion • Every force has an equal and opposite force, or FA on B = -FB on A Fyouon wall = Fwall onyou

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