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Work

Work. Work has three specific conditions which must be met in order of “work” to be done A force must be exerted on an object The object must be displaced by the force At least part of the force must be in the same direction as the displacement This means

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Work

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  1. Work • Work has three specific conditions which must be met in order of “work” to be done • A force must be exerted on an object • The object must be displaced by the force • At least part of the force must be in the same direction as the displacement • This means • Work can only be done if the force and direction of movement are collinear • All, or part of the force must act in the direction of the motion

  2. Work • Work is calculated using the following formulaW = FΔd • This means that the units of work will be the Newton meter or Nm

  3. Joules • 1 Nm was named the “joule” in honor of the physicist James P. Joule. • The joule is represented by “J” • Thus, 1 Nm = 1 J • One joule is the work done when a one newton force moves an object one meter in the direction of the force

  4. A Scalar? • Despite being calculated from two vectors, work is considered a scalar quantity and as such requires no direction to be associated with it or its calculation

  5. More graphing…HURRAY! • To find work from a graph, you must create a force vs displacement graph • Once the graph is created you must calculate the area under the graph to find work • Area under a force vs dispalcement graph = work • Total area = total work • Partial area = partial work

  6. Force (F in N [q]) W = ½ base x height Stretch displacement (Δd in m [q])

  7. Try it • How much work is done if you must use a 300N force to lift a box onto a 70cm high table?

  8. Try it • You lift up a box 1.5 meters using a force of 210 N. How much work have you done?

  9. Try it • When writing notes you likely write approximately 15.6 cm left to right across the page for each line. If you write at a constant velocity with a force of friction of 27 N.How much work do you do for each line you write?

  10. Try it • A 61 kg person is free falling towards the earth. The person falls 16.5 m in 2.4 seconds. How much work is done… • On the person by the earth • On the earth by the person

  11. Try it • A person runs at constant velocity of 12m/s [N]. If the force of friction acting on the person is 128N.How much work with the person have done after running 1 km?

  12. Try it • A tennis racket, in contact with a 55g ball for 0.0050s changes the balls velocity from 30m/s [E] to 40m/s [W] over a distance of 0.0001m.How much work is done on the tennis ball by the racket?

  13. Textbook Practice Questions • Page 199 #1 – 3 • Page 203 #4 – 10 • Page 207 #11 – 13 • Page 213 # 14 – 18 • Read pages 214 – 249(kinetic and gravitational potential energy) Questions related to work

  14. Energy – A summary • Energy is transferred from one object to another whenever work is done • Energy comes in many forms that are interchangeable • Energy can be stored and used at a later time to do work • Energy is ALWAYS conserved

  15. Energy – A summary • Rest mass energy • Comes from the famous equation E = mc2 • It predicts that energy can be converted to mass and mass to energy • Nuclear energy is an example (pg 718)

  16. Energy – A summary • Potential energy • Energy stored in an object • This energy can be released when specific conditions are met • Four main types • Chemical • Elastic • Gravitational • Nuclear • ALL objects have gravitational potential energy!

  17. Energy – A summary • Kinetic Energy • Is the energy associated with objects in motion • An object can only have kinetic energy while it is in motion. (non-moving objects have zero kinetic energy)

  18. Energy – A summary • Energy is the result of work. • Both can have the same unit, the joule (J) • More energy is always consumed then useful work done (because of things like friction, air resistance and heat loss) • However, if we ignore these wasteful forces, then work = energy*** THIS IS ONLY TRUE IF WE IGNORE THINGS LIKE FRICTION!!!!!***

  19. Energy – A summary • Units for energy are often given in factors of J such as • kJ • MJ • GJ • Welcome back to unit conversions! (Think back to sig digs, and conversions, you'll need this info)

  20. Gravitational energy ΔEG = mgΔh ΔEG Change in gravitational energy (J) m  mass of the object (kg) g  gravitational field intensity (N/kg) Δh  vertical displacement of the object (m)

  21. Energy – A summary • Gravitational energy • No fixed reference point (pick your own) • Energy will be vertical displacement relative to your chosen "zero" value • The true value for gravitational potential energy would actually be the vertical displacement away from the absolute center of the planet

  22. Energy – A summary • Solving for gravitational potential energy • Set your lowest point as zero (unless given) • Calculate height differences between points • Use equation to solve between points(use absolute values for this) • If the object moves down then energy = negative • If object moves up then energy = positive • Excess movement is not accounted for. The change in height is merely the difference between the reference point and the new point (not total) • Without friction/air resistance, WORK = ENERGY

  23. Energy – A summary • Kinetic energy Ek = ½mv22 Ek Kinetic energy (J) m  mass of the object (kg) v2 final velocity of the object (m/s)

  24. Energy – A summary • Kinetic energy • Energy associated with moving objects • Often refers to bulk kinetic energy • Again, if friction is ignored, then work done is equal to kinetic energy because all work turns to kinetic energy

  25. Energy – A summary • Solving for (bulk) kinetic energy • Determine variables you have, you'll need some or all of the following • Force, acceleration, velocity, mass, distance and/or time • Kinetic energy can often be solved using our previous kinematics equations (for either constant velocity, or constant acceleration) Remember: constant force = constant acceleration • Decide which equation will solve for what you need • Determine which variables in this equation are still unknown • Use kinematics to determine unknown variables • Use equation to solve question

  26. Energy – A summary • Energy can never be created or destroyed. Therefore whatever energy your object has at the beginning must also be present at the end, although it may have changed forms/type. • This means • The sum of the energies at the beginning of the questions must equal the sum of the energies at the end of the question. • Energy cannot be created or destroyed. It can be changed from one form to another, but the total amount of energy in the universe stays constant.

  27. Energy – A summary • Conservation of energy application • Whenever an object has energy, all or part of that energy can be transferred into another form. Potential can become kinetic, kinetic can become elastic etc. • In general though, most energy transfers between potential and kinetic or kinetic and kinetic. (It's difficult to transfer potential energy to potential energy)

  28. Energy – A summary • Efficiency is a representation to show how much of our work is wasted % efficiency = (useful work done / actual work (or energy) input) x 100 • Efficiency is a means of determining how much work or energy is lost in an interaction • You need two energy/work amounts to calculate efficiency 1) The actual amount of work/energy put into the system 2) The resultant energy in the system at the end of the interaction

  29. Energy – A summary • Power is the RATE at which work is done • Thus we get the equation P = W / Δt = E / Δt • Power is measured in Watts (W) • One watt is equal to the power available when one joule of work is done in one second 1W = 1J / 1s • The average sustainable power for a human is about 75 W

  30. Energy – A summary • Power and speed can also be related using the equation P = F(vav) • Where P  is the power in (W) F  is the applied force (N) Vav  is the average speed of the object (m/s) • Appliances are often measured using kilowatt hours 1kWh = 3.6 MJ (see page 156)

  31. The Equations Chart

  32. Today: Power Lab • Read the lab carefully • A marking rubric/submission guideline is included on the back • You may work with a partner/group for measurements, but EVERY PERSON needs to do their own mass and power • This means EVERY PERSON must submit their own laboratory which will not be the same as anybody else's.

  33. A quick review of things • The three conditions of work • A force must be exerted on an object • The object must be displaced by the force • At least part of the force must be in the same direction as the displacement • This means • Work can only be done if the force and direction of movement are collinear • All, or part of the force must act in the direction of the motion

  34. 1 Nm = 1 J • One joule is the work done when a one newton force moves an object one meter in the direction of the force • Work is a scalar quantity(no direction required) • On a force vs. displacement graph, work can be found by calculating the area under the graph

  35. Energy is transferred from one object to another whenever work is done • Energy comes in many forms that are interchangeable • Energy can be stored and used at a later time to do work • Energy is ALWAYS conserved • Potential energy • Energy stored in an object • This energy can be released when specific conditions are met • Four main types • Chemical • Elastic • Gravitational • Nuclear • ALL objects have gravitational potential energy!

  36. Kinetic Energy • Is the energy associated with objects in motion • An object can only have kinetic energy while it is in motion. (non-moving objects have zero kinetic energy) • Units for energy are often given in factors of J such as • kJ • MJ • GJ

  37. Kinetic Energy • Is the energy associated with objects in motion • An object can only have kinetic energy while it is in motion. (non-moving objects have zero kinetic energy) • Units for energy are often given in factors of J such as • kJ • MJ • GJ

  38. Energy is the result of work. • Both have the same unit, the joule (J) • More energy is always consumed then useful work done (because of things like friction, air resistance and heat loss) • However, if we ignore these wasteful forces, then work = energy*** THIS IS ONLY TRUE IF WE IGNORE THINGS LIKE FRICTION!!!!!***

  39. Gravitational energy • ΔEG = mgΔh • ΔEG Change in gravitational energy (J) • m  mass of the object (kg) • g  gravitational field intensity (N/kg) • Δh  vertical displacement of the object (m) • Gravitational energy • No fixed reference point (pick your own) • Energy will be vertical displacement relative to your chosen "zero" value • The true value for gravitational potential energy would actually be the vertical displacement away from the absolute center of the planet

  40. Kinetic energy Ek = ½mv22 Ek Kinetic energy (J) m  mass of the object (kg) v2 final velocity of the object (m/s) • Kinetic energy • Energy associated with moving objects • Often refers to bulk kinetic energy • Again, if friction is ignored, then work done is equal to kinetic energy because all work turns to kinetic energy

  41. Energy – A summary • Solving for gravitational potential energy • Set your lowest point as zero (unless given) • Calculate height differences between points • Use equation to solve between points(use absolute values for this) • If the object moves down then energy = negative • If object moves up then energy = positive • Excess movement is not accounted for. The change in height is merely the difference between the reference point and the new point (not total) • Without friction/air resistance, WORK = ENERGY

  42. Energy – A summary • Solving for (bulk) kinetic energy • Determine variables you have, you'll need some or all of the following • Force, acceleration, velocity, mass, distance and/or time • Kinetic energy can often be solved using our previous kinematics equations (for either constant velocity, or constant acceleration) Remember: constant force = constant acceleration • Decide which equation will solve for what you need • Determine which variables in this equation are still unknown • Use kinematics to determine unknown variables • Use equation to solve question

  43. Energy can never be created or destroyed. Therefore whatever energy your object has at the beginning must also be present at the end, although it may have changed forms/type. • This means • The sum of the energies at the beginning of the questions must equal the sum of the energies at the end of the question. • Energy cannot be created or destroyed. It can be changed from one form to another, but the total amount of energy in the universe stays constant.

  44. Efficiency is a representation to show how much of our work is wasted % efficiency = (useful work done / actual work (or energy) input) x 100 • Efficiency is a means of determining how much work or energy is lost in an interaction • You need two energy/work amounts to calculate efficiency 1) The actual amount of work/energy put into the system 2) The resultant energy in the system at the end of the interaction

  45. Power is the RATE at which work is done • Thus we get the equation P = W / Δt = E / Δt • Power is measured in Watts (W) • One watt is equal to the power available when one joule of work is done in one second 1W = 1J / 1s • The average sustainable power for a human is about 75 W

  46. Power and speed can also be related using the equation P = F(vav) • Where P  is the power in (W) F  is the applied force (N) Vav  is the average speed of the object (m/s) • Appliances are often measured using kilowatt hours 1kWh = 3.6 MJ (see page 156)

  47. Practice Time • Calculate the force needed to do 40 kJ of work to push a load of bricks 25m. • You exert a force 50N [S] while pushing your friend. If you do 300 J of work, how far did you push your friend?

  48. Practice Time • You do 140 J of work to lift your backpack onto your back. If you lift the bag 180cm how much does it weigh? • Mars is one third the mass of earth and one half the diameter. You lift a 4kg object a height of 2m. • On which planet does the object have the greatest gravitational potential energy? • On which planet did you expend the most energy? • Calculate the change in gravitational potential energy for both planets.

  49. Try it • A 61 kg person is free falling towards the earth. The person falls 16.5 m in 2.4 seconds. How much work is done… • On the person by the earth • On the earth by the person

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