1 / 23

Work, Energy & Power

Work, Energy & Power. Work. What does WORK mean to you? Are you doing WORK when… Lifting weights? Walking with a big bag of grocery in your hand? Completing your homework assignment? Writing an essay?. Physics concept of WORK.

aimee
Télécharger la présentation

Work, Energy & Power

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Work, Energy & Power

  2. Work • What does WORK mean to you? • Are you doing WORK when… • Lifting weights? • Walking with a big bag of grocery in your hand? • Completing your homework assignment? • Writing an essay?

  3. Physics concept of WORK • WORK is done only when a constant force applied on an object, causes the object to move in the same direction as the force applied.

  4. Physics concept of WORK • What IS considered as work done in Physics: • You push a heavy shopping trolley for 10 m • You lift your school bags upwards by 1 m

  5. Physics concept of WORK • What is NOT considered as work done: • You push against a wall • Jumping continuously on the same spot • Holding a chair and walking around the classroom

  6. Physics concept of WORK WORK can be calculated by: Work done = Constant x Distance moved force (N) in the direction of force (m) W = F x d = F x d x cosΘ Units: [J] = [N] x [m] SI Unit for Work is JOULE (J)

  7. More Examples of WORK • You are helping to push your mother’s heavy shopping cart with a force of 50 N for 200 m. What is amount of work done? Work done, W = F x d x cos Θ = 50 x 200 x cos 0 = 10,000 J or 10 kJ (kilo-Joules)

  8. More Examples of WORK: • Jack put on his bag-pack of weight 120 N. He then starts running on level ground for 100 m before he started to climb up a ladder up a height of 10 m. How much work was done? Think about the angle between the Force and the displacement -> If the angle is 90 degrees, then cos Θ = 0, so in physics terms, no work is done.

  9. Energy – Quick Re-cap • Energy is the capacity to do work • SI Unit: Joule (J) • Many forms • Common ones: • Kinetic • Potential • Electric • Chemical • Solar • Nuclear

  10. Kinetic Energy (KE) • A form of energy that a body in motion possess. • A body a rest, will it possess any KE? • Examples: • Bullet shot out from pistol • Helicopter flying at 120km/h

  11. Kinetic Energy (KE) • The amount of KE of a moving body depends on: • Mass of body (kg) • Velocity (ms-1) • When either mass or velocity of moving body is increased, KE will also increase.

  12. Kinetic Energy = x Mass x (Velocity)2 KE = x m x v2 Units: [ J ] = [kg] x [ms-1]2 Kinetic Energy (KE) • Formula: • SI Unit: Joule [ J ] … same as Work Done

  13. Velocity, V Mass = m kg Kinetic Energy (KE) KE = ½  m  v2

  14. Examples of KE • Find the KE of an empty van of mass 1000kg moving at 2m/s. • Find the KE of van when it is loaded with goods to give a total mass of 2000kg, and moving at 2m/s. • Find KE of unloaded van when it speeds up to 4m/s. KE of van at 2m/s = ½ x 1000 x (2)2 = 2000 J = 2 kJ KE of van at 2m/s = ½ x 2000 x (2)2 = 4000 J = 4 kJ KE of van at 2m/s = ½ x 1000 x (4)2 = 8000 J = 8 kJ

  15. Kinetic Energy (KE) • Formula: KE = ½ mv2 • From the formula, what can you infer about the change in KE when… • Mass doubles • Velocity doubles KE doubles KE increases by FOUR times

  16. Examples of KE • A motorcycle accelerates at 2m/s2 from rest for 5s. Find the KE of motorcycle after 5s. Mass of motorcycle is 200 kg. Velocity of motorcycle after 5s, a = (vf- vi) t v = 2(5) + 0 = 10m/s KE of motorcycle at 10m/s = ½ x 200 x (10)2 = 10,000 J = 10 kJ

  17. Potential Energy • Potential energy is the energy possessed by an object as a result of its POSITION or CONDITION. • Two common kinds: • Gravitational PE • Elastic PE (not in syllabus)

  18. Elastic PE • Energy that can be possessed by an object due to its CONDITION. Examples: • “Slinky” … when stretched or compressed • Spring … when stretched or compressed • Rubber band … when stretched • Balloon with air … when compressed

  19. Gravitational PE • Energy that can be possessed by an object due to its POSITION. • In Physics, ground level is normally assumed to be at ZERO GPE. • Any object that is at ground level has ZERO GPE. • If object is lifted a certain height above ground, its GPE has increased.

  20. Gravitational PE • Examples: • When a chair lifted from ground a distance of 1m • You sitting on the 3rd storey of this building

  21. Object on top of building, of mass, m g earth Distance from ground, h Ground, 0 GPE Gravitational PE • Can be calculated with: GPE = mass  gravitational  height above acceleration ground level = m  g  h Units: [J] = [kg] x [m/s2] x [m] SI Units of GPE : Joule [J]

  22. Example of GPE • You lifted your bags to the top of your table. What can you say about the GPE of your bag? • Zero, increase, decrease • Lift the same bag on the Moon. What happens to GPE? • Zero, increase, decrease • Will the GPE be the same on Earth and Moon? • Same, less on Moon, more on Moon?

  23. Examples of GPE • You lifted a set of books of mass 3kg, for 2m. What is the GPE gained by the books? Take g=10m/s2. • Find the work done by you to lift the books. GPE = mgh = 3  10  2 = 60 J • Work done, W = F  d x cos Θ (F = weight of books) • = (m g)  d • = 3 x 10 x 2 • = 60 J (Note: same as GPE)

More Related