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Forces (Part 3)

Friction, Terminal Velocity, Weight and Density. Forces (Part 3). Topics. Effects of Friction Air Resistance & Terminal Velocity Mass vs Weight Density & Floating. Effects of Friction. Effects of Friction.

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Forces (Part 3)

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  1. Friction, Terminal Velocity, Weight and Density Forces (Part 3)

  2. Topics • Effects of Friction • Air Resistance & Terminal Velocity • Mass vs Weight • Density & Floating

  3. Effects of Friction

  4. Effects of Friction • Typically, we think of friction as resisting motion. (e.g. when we drag an object on the floor) • However, can you imagine what it would be like to walk on a totally frictionless surface (e.g. on ice)? • Similarly, what would happen if a car attempts to travel over a layer of ice?

  5. Effects of Friction • In these cases, friction is actually helps motion, not resist it. • Consider the action of walking: • Foot is pushing backwards on floor • By N3L, floor is pushing back on foot • However, if there is no friction, the foot is unable to “grip” the floor, and the action-reaction pair does not form. • This is why sprinting shoes have spikes and soccer boots have studs – greater friction helps you to run faster.

  6. Exercise 1 • (a) Using a FBD, describe how friction between the tires and the road is necessary for a car to move forward. • (b) Suggest 1 reason why drivers in colder countries change tires when it starts to snow.

  7. Half-Time • The Olympic Sport of Curling • http://www.youtube.com/watch?v=ZqwJ_DpEY94 • Draw Shot (backspin) in Pool / Billiards • http://www.youtube.com/watch?v=qKwulo_ag0k

  8. Terminal Velocity

  9. Terminal Velocity • Recall in Kinematics, we learned that an object in free fall, falls at a constant acceleration of 10 ms-2? • Why is this so? (hint: Consider the resultant force of a free-falling object.) • In real life, as long as the object is falling in Earth’s atmosphere, there will be air resistance. Hence the acceleration will not be constant.

  10. Terminal Velocity • Air Resistance is actually part of a category of forces called Drag Force. • Drag Force is the force opposing a motion of an object in a fluid. • (Note: In Physics, fluid = Gas or liquid) • Two Properties of Drag Forces: • Only a moving object experience drag force • The greater the speed of the object, the greater the drag force

  11. Different Stages of Free fall • Stage 1 – Just before object starts to fall, it’s velocity is zero. Since velocity is zero, drag force is zero. • Resultant Force = mg • ma = mg • a = g (10 ms-2) mg

  12. Different Stages of Free fall • Stage 2: object falls and has a downward velocity. There is air resistance. • Resultant Force = mg – Fair • ma = mg – Fair • a < g FAir mg

  13. Different Stages of Free fall • Stage 3: as object’s velocity increases, air resistance increases. Eventually, Fair = mg. • Resultant force = mg - Fair = 0 • Balanced Forces = no acceleration • Object falls at constant velocity • This is called terminal velocity FAir mg

  14. Class Activity • On 14 Oct 2012, Felix Baumgartner attempted the highest sky dive ever attempt by a human. • Watch the video of Felix jumping • We will record his speed every 5 seconds • Starting from t = 21 s to t = 45 s (0:52 mark of video) • Plot the velocity-time graph of Felix and note the shape of the graph • http://www.youtube.com/watch?v=Ncc_ZZ9iCyk • Note: In this skydive, Felix became the only human to break the sound barrier (MACH 1) outside a vehicle.

  15. Velocity Time Graph of Felix BaumGartner

  16. Terminal Velocity • Qn: what is the purpose of a parachute? How does it work? • The parachute increases the air resistance of the falling object • Stage 3 occurs sooner • Terminal velocity is lower, and safe enough for the person to land without injury

  17. Mass VS Weight

  18. Mass VS Weight • Mass is the amount of matter in a body • Weight is the gravitational force acting on a body. • Weight changes when a human travels further away from the Earth (or on the Moon) but mass does not change • A mass can only be measured by a calibrated beam balance. • Electronic balances / weighing scales can only measure weight

  19. Gravitational Field • A gravitational field is a region in which a mass experiences a force due to gravitational attraction • Gravitational field strength g is the gravitational force per unit mass • g = Fgravity/m

  20. Inertia • In Physics, we are often more interested in mass as a measure of inertia, than as a measure of amount of substance. • The more mass an object has, the greater the inertia of an object. • Inertia is the reluctance of the object to change its state of rest or motion, due to its mass. • Concept of inertia is closely linked to Newton’s First and Second Laws.

  21. Density and Floatation

  22. Density • Recall equation for density: • Density = Mass/ Volume • Density is represented using the greek symbol ρ (roe)

  23. Floating • (floating applet) • If an object’s density is greater than the fluid, it will always sink to the bottom. It does not matter how much greater • If an object’s density is less than the fluid, it will always float to the surface. It does not matter how much less. • There is no “suspended” in the middle of the liquid, unless the density is exactly the same as the liquid’s density.

  24. Floating • Qn: A ship is made of iron, which is denser than water. Why then does a ship still float? • Ans: There are large air spaces within a ship. The total mass divided by total volume is still less than the density of water. • For self research: how is a submarine able to float and sink when it wants to? How about fish?

  25. Explain these using density… Lava Lamps Galileo Thermometer

  26. Recap Forces (Part 3) • Effects of Friction • Friction is useful in walking. wheels, etc. • Air Resistance • Only when object moving, increases with velocity • 3 stages of Free Fall (with air resistance) • Mass vsWeight • Gravitational Field, Gravitational Field Strength • Inertia • Density & Floating • Equation for Density • Floatation and Sinking

  27. Recap (Forces Part 1-2) • Component vs Resultant Forces • 5 types of component forces • Free Body Diagrams and Vector Diagrams • N1L: object at rest and in uniform motion • N2L: F=ma • N3L: Action-Reaction Pairs • Useful effects of Friction • Air Resistance and Terminal Velocity • Mass vs Weight • Density and Floating

  28. I don’t need to teach 3E next lesson YAYYYYYYYYY =P End of Forces Lesson

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