Forces Ch. 6

1. ForcesCh. 6 Milbank High School

2. Sec 6.1Force and Motion • Objectives • Define a force and differentiate between contact forces and long-range forces • Recognize the significance of Newton’s second law of motion and use it to solve motion problems • Explain the meaning of Newton’s first law and describe an object in equilibrium

3. I. Gravitational: • attraction bet. masses • tides, gravity, weight • III. Weak Nuclear: • helps to explain atomic collisions • II. Electromagnetic: • friction •tension • adhesion •lift • electrostatic •drag • buoyant •magnetic • IV. Strong Nuclear: • binds atomic nuclei The Forces of Nature

4. Force • How do forces influence motion? • Force– a push or pull exerted on an object having magnitude and direction • System—object that experiences the force • Environment—world around the system that exerts the force

5. Two Categories of Forces… • Contact Force • Acts on an object only by touching it • Long-Range Force • Exerted without contact • Magnets • Gravity Agent: a specific, identifiable, immediate cause of a force

6. Types of Forces • Ff - - Friction (opposes sliding) • FN - - Normal (surface) • Fsp - - Spring (push or pull of a spring) • FT - - Tension (spring, rope, cable) • Fthrust - - Thrust (rockets, planes, cars) • Fg - - Weight (force due to gravity)

7. Representing Forces... • Forces are vectors • Forces are drawn as arrows (vectors) • forces add like vectors. • the sum of all the forces is called the net force. • A picture of a body with arrows drawn representing all the forces acting upon it is called a FREE BODY DIAGRAM.

8. Free Body Diagrams

9. Try it... Draw a picture of your book sitting on the desk. Identify all the forces acting on it.

10. Free Body Diagrams... T (table) Book W (weight)

11. Free Body Diagrams... What forces are acting on a skier as she races down a hill?

12. The Answer... FN d & f W

13. The Answer... FN f and d W

14. Draw free body diagrams for the following • An egg is free-falling from a nest in a tree. Neglect air resistance. • A skydiver is descending with a constant velocity. Consider air resistance. • A car is coasting to the right and slowing down.

15. Newton’s Second Law • F = ma • a = Fnet / m • Expressed in Newtons (N) • Force required to give 1kg mass a 1m/s2 acceleration

16. Example • A race car has a mass of 710 kg. It starts from rest and travels 40.0 m in 3.0 s. The car is uniformly accelerated during the entire time. What net force is exerted on it?

17. Newton’s First 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 constant speed, if and only if the net force acting on that object is zero.”

18. Newton’s First Con’t • Inertia—the tendency of an object to resist change. • Equilibrium—object at rest or moving at a constant velocity

19. Finally…Misconceptions about forces • When a ball has been throw, the force of the hand that threw it remains on it. • A force is needed to keep an object moving • Inertia is a force • Air does not exert a force • The quantity ma is a force

20. Sec. 6.2Using Newton’s Laws • Objectives • Describe how the weight and the mass of an object are related • Differentiate between the gravitational force weight and what is experienced as apparent weight • Define the friction force and distinguish between static and kinetic friction • Describe simple harmonic motion and explain how the acceleration due to gravity influences such motion.

21. Mass and Weight • The weight force, Fg , is used to find the downward force of an object. • Both the net force and acceleration are downward. Fg = mg

22. Example Problems • Pg. 128 • Practice Problem 12. • Pg. 129

23. Friction • Static friction force • The force that opposes the start of relative motion between the two surfaces in contact • Friction force with object isn’t in motion • Kinetic Friction Force • The force that opposes relative motion between surfaces in contact • Friction force when object is in motion

24. Calculating Friction Kinetic Friction Force Ff ,kinetic = µkFn Static Friction Force 0< Ff, static < µsFN

25. Typical Coefficients of Friction Surface µs µk Rubber on concrete 0.80 0.65 Rubber on wet concrete 0.60 0.40 Wood on wood 0.50 0.20 Steel on steel (dry) 0.78 0.58 Steet on steel (with oil) 0.15 0.06 Teflon on steel 0.04 0.04

26. Example Problems • Pg. 131-133 • Balanced Friction Forces • Unbalanced Friction Forces

27. Terminal Velocity • The constant velocity that is reached when the drag force equals the force of gravity • Objects can only fall so fast due to their size and shape and density of the air/fluid • Ping-pong ball – 9 m/s • Basketball – 20 m/s • Baseball – 42 m/s • Skydiver: >62 m/s w/o chute 5 m/s w/ chute

28. Periodic Motion • Pendulums, springs, strings • Simple Harmonic Motion • Motion that returns an object to its equilibrium position as a result of a restoring force that is directly proportional to the object’s displacement • Period (T) • Time needed to repeat one complete cycle of motion • Amplitude • Maximum distance the object moves from equilibrium

29. Amplitude, Frequency, Period The Amplitude is the displacement. The Frequencyis the number of cycles/sec. The Period is the time for one cycle T = 1/f

30. Period of a Pendulum

31. Problems • Pg. 136 • 17-19

32. Sec. 6.3Interaction Forces • Objectives • Explain the meaning of interaction pairs of forces and how they are related by Newton’s third law • List the four fundamental forces and illustrate the environment in which each can be observed. • Explain the tension in ropes and strings in terms of Newton’s third law

33. Interaction forces • Two forces that are in opposite directions and have equal magnitude • Newton’s Third Law—all forces come in pairs • FA on B = -FB on A