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FORCES & NEWTON’S LAWS

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  1. FORCES & NEWTON’S LAWS

  2. Forces (Ch 6) • Why do things move? What is the cause of this motion we have studied? • Explained by Sir Isaac Newton • 3 Laws of Motion • Explain the way forces influence motion - • what is needed for objects to move • “how much” is needed • what happens when objects interact

  3. What is a Force? • Force (F) = push or a pull exerted on an object • Has magnitude & direction • Contact forces – act only on an object by touching it • Long-range forces – act on an object without touching it

  4. What causes a force? • The agent is the immediate cause of a force • There has to be an agent for a force to exist • What is the agent: • A book in your hand? • A book on a desk? • A book falling from the desk?

  5. What kind of forces are there? • Friction (Ff) – contact force opposing motion between surfaces • Normal (FN) – contact force exerted by a surface on an object (perpendicular to surface) • Weight (Fg) – long-range force due to gravitation attraction of 2 objects (usually Earth & an object)

  6. What kind of forces are there?(continued) • Tension (FT) – force exerted by a string, rope or cable when it is attached to an object & taut • Spring (Fsp) – force a spring exerts on an object

  7. Newton’s 1st Law of Motion • Law of Inertia • Inertia is an object’s tendency to resist a change in its motion • Law states… • Objects in motion will stay in motion unless a net force acts on them • Objects at rest will stay at rest unless a net force acts on them

  8. Newton’s 2nd Law of Motion • Force = mass X acceleration • Fnet = ma • Force and acceleration are directly proportional. The greater the force, the greater the acceleration. • Mass and acceleration are inversely related. The greater the mass of an object the less the acceleration if the same force is applied.

  9. Newton’s 3rd Law of Motion • For every action force there is an equal and opposite reaction force. • Two interacting objects become action-reaction pairs

  10. How do we measure force? • One “unit of force” causes a 1 kg mass to accelerate 1 m/s2 • Since F = ma, • (1 kg)(1 m/s2) = 1 kg●m/s2 • We call this “unit of force” a Newton (N)

  11. Friction • Force that opposes motion • Static Friction – opposes the start of motion • Kinetic Friction – opposes motion between 2 surfaces • Coefficient of Friction ( μ)– a ratio of the magnitude of the 2 forces • It’s a constant depending on the 2 surfaces • No unit! • Calculating Friction • Ff = μFN

  12. What is a Net Force? • The net force is the sum of all forces acting on a body • The net force causes the acceleration of the object • Acceleration is always in the direction of the Fnet • So, 2nd law: Fnet = ma • When the Fnet = 0 – system is in equilibrium

  13. How do you find Fg? • Recall that F = ma (2nd law) • So, the force of weight Fg = mg • g = 9.8 m/s2 on earth • Mass in KILOGRAMS!

  14. Free-Body Diagrams • Use a dot (or box) to represent the object • Represent each force acting on the object with an arrow (vector) • Remember to point it in the right direction • Let’s Practice!

  15. Book held in your hand

  16. Book pushed across a desk by your hand

  17. Book pulled across a desk by a string

  18. A ball hanging from a string

  19. Same ball just after the string breaks

  20. Calculating Lift Problems • Is the object being lifted at a constant speed? Or is it accelerating? • If constant speed, the forces are balanced. So, Fappied = Fg • Why? • If accelerating, Fnet = ma

  21. Calculating Friction Problems • Is the object moving at a constant speed? Or is it accelerating horizontally? • If constant speed, the forces are balanced. So, Fapplied = Ff • Why? • If accelerating horizontally, Fnet = ma

  22. Let’s Practice # 1 • A horizontal force of 45 N is needed to keep a 125 N block sliding at a constant speed over a horizontal surface. Calculate the coefficient of friction.

  23. Let’s Practice # 2 • A force of 40 N accelerates an 8 kg block 2 m/s2 along a horizontal surface. • What is the net force? • What is the frictional force? • What is the normal force? • What is the coefficient of friction?

  24. What is the net force?

  25. What is the frictional force?

  26. What is the normal force?

  27. What is the coefficient of friction?

  28. Let’s Practice # 3 • A 6000 kg helicopter accelerates upward at 3 m/s2. • What is the net force? • What is the weight force? • What lift force does the air exert on the propellers ?

  29. What is the net force?

  30. What is the weight force?

  31. What lift force does the air exert on the propellers ?

  32. What if??? • What if the object is moving horizontally, but the applies force is at an angle? Now, we are working with a 2-D force!

  33. A force includes magnitude & direction, making it a VECTOR. So, we can break it down! Resolution of Forces – Breaking down a single vector into 2 or more vectors (SOH CAH TOA)

  34. Push vs. Pull

  35. A car is pulled with a force of 60N at angle of 37° from the horizontal. Find the vertical and horizontal components of the applied force .                                                                                                                                                               If the cart is rolling with constant velocity, what is the frictional force? Why?

  36. Reminders – • Don’t forget your kinematic equations (motion) • Remember to ID all forces acting on the object. Draw the diagram! • If a force is applied at an angle, break it into its x- and y-components • Mass needs to be in kg! • Formulas: • Fnet = ma = sum of all forces acting on the object • Fg = mg • Ff = μFN