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Forces

Forces. Chapter 2. Section 1: The Nature of Force. Force A push or pull on an object Has both Size & Direction Size: Measured in SI units called newtons (N) Spring Scale. Section 1: The Nature of Force. Direction: Same direction: Add (+) Diff. direction: Subtract (-)

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Forces

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  1. Forces Chapter 2

  2. Section 1: The Nature of Force • Force • A push or pull on an object • Has both Size & Direction • Size: • Measured in SI units called newtons (N) • Spring Scale

  3. Section 1: The Nature of Force • Direction: • Same direction: Add (+) • Diff. direction: Subtract (-) • Net force = combination of all forces acting on object • Unbalanced Forces • Result in motion

  4. Section 1: The Nature of Force • Balanced Forces • Net force = 0 • No motion • Practice Calculating Net Force

  5. Lab: The Nail Challenge! • Objective: • Balance nails on single nail head • 1st & 2nd highest # for each period gets bonus coupon • Work in pairs

  6. Section 1: Newton’s 1st Law • Inertia • Tendency of object to resist a change in it’s motion

  7. Section 1: Newton’s 1st Law • Inertia depends on Mass • “Amount” of inertia depends on objects’ mass

  8. Section 1: Newton’s 1st Law • Newton’s 1st Law of Motion • Object at rest will remain at rest • Object in motion will remain in motion unless acted on by an unbalanced force.

  9. Practice Problem 1 Imagine a place in the cosmos far from all gravitational and frictional influences. Suppose an astronaut in that place throws a rock. The rock will: • a) gradually stop. • b) continue in motion in the same direction at constant speed.

  10. Practice Problem 2 An 2-kg object is moving horizontally with a speed of 4 m/s. How much net force is required to keep the object moving with the same speed and in the same direction? • 0 N (no force)

  11. Practice Problem 3 Ben Tooclose is being chased through the woods by a bull moose which he was attempting to photograph. The enormous mass of the bull moose is extremely intimidating. Yet, if Ben makes a zigzag pattern through the woods, he will be able to use the large mass of the moose to his own advantage. Explain this in terms of inertia and Newton's first law of motion.

  12. 2-1: Newton’s 1st Law Review • Unbalanced force from another car changes your CAR’s motion • You continue as before until your seatbelt changes YOUR motion

  13. 2-2: Force, Mass & Acceleration • Newton’s 2nd law of Motion • Force, Mass & Acceleration are related • Force = Mass X Acceleration OR Acceleration = Force Mass • FYI, 1 N = 1kg X 1 m/s2

  14. 2-3: Force, Mass & Acceleration

  15. 2-3: Force, Mass & Acceleration • A 52 kg water skier is being pulled by a speedboat. The force causes her to accelerate @ 2 m/s2. Calculate the FORCE that causes this acceleration. • F = 52 kg x 2 m/s2 • = 104 kg x m/s2 = 104 kg*m/s2 • = 104 N

  16. 2-3: Force, Mass & Acceleration • What is the force on a 1000 kg elevator accelerating at 2 m/s2? • 1000 kg X 2 m/s2 = 2000 N • How much force is needed to accelerate a 55 kg cart at 15 m/s2? • 55 kg X 15m/s2 = 825 N

  17. 2-4: Newton’s 3rd Law • Newton’s 3rd law of Motion • For every action, there is an equal and opposite reaction • “Action” & “Reaction” are names of forces

  18. 2-4: Action & Reaction Forces • Forces ALWAYS occur in pairs. • Single forces NEVER happen • 2 objects are involved in every force • Action force: “A pushes B” • Reaction force: “B pushes A”

  19. 2-4: “Equal” Forces • In Newton’s Third Law, “equal” means: • Equal in size • Equal in time • “Opposite” Means: • Opposite in direction

  20. 2-4: Possible Problems • Don’t Action & Reaction forces cancel each other? • Action & Reaction forces act on DIFFERENT objects • In “Net force” problems, we are talking about opposing forces acting on the SAME object

  21. 2-4: Try These!! • If forces are equal and opposite why don't they cancel each other out? • They occur on two different objects.  Forces can only cancel out when the forces are acting on the same object.

  22. If the forces are equal and opposite how do two different objects obtain different accelerations in the same interaction? • Different accelerations are obtained when the objects have different masses.

  23. When a small bug is splattered across a fast moving windshield what experiences more force- the bug or the windshield? • Some what of a tricky question, they both experience the SAME force. 

  24. Why does the force have a greater effect on the bug? • Because the bug's mass is much much smaller than the car's, it will experience a much greater change in acceleration than the car.  This change in acceleration over a very small fraction of time is why the bug experiences a greater effect.

  25. 2-3: Friction • Friction • Resistance to motion • Opposite direction of travel • Caused when 2 surfaces rub together • resistive force (slows down objects)

  26. 2-3: Friction • Friction depends on… • Types of surfaces • How hard surfaces push together

  27. 2-3: Types of Friction • Types of Friction • Sliding Friction: solid surfaces slide over each other • Rolling Friction: object rolls over surface

  28. 2-3: Types of Friction • Types of Friction • Fluid Friction: object moves through fluid (or air) • Static Friction: objects not moving

  29. 2-3: Uses of Friction • Is Friction harmful or helpful? • Ways to reduce friction • Ways to increase friction

  30. 2.3: Gravitational Force • Gravitational Force • Force of attraction between 2 objects • Pulls things toward each other • Depends on: • Mass • Distance

  31. Mass vs. Weight • Mass • Amount of matter • Same no matter where you are • SI units = kilograms (kg) • 1 kg = 1000 grams (g) • Weight • Force of gravity • SI units = newtons (N) • Depends on where you are

  32. 2.3: Gravity & Freefall Acceleration • Free Fall • Only force acting on an object is gravity • Objects in free fall accelerate as they fall • All objects free fall at the same rate (9.8 m/s2)

  33. 2.3: Gravity & Freefall

  34. 2.3: Free Fall Acceleration • For every second an object falls its downward velocity increases by 9.8 m/s

  35. 2.3: Air Resistance • Air resistance • Type of fluid friction • Opposes motion of objects through air • Depends on: • Size, Shape, Speed

  36. 2.3: Terminal Velocity • Terminal Velocity • As an object falls it picks up speed • Increased speed  increased air resistance • Eventually force of air resistance = force of gravity  TERMINAL VELOCITY • Object stops accelerating!

  37. 2.3: Terminal Velocity

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