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Work, Energy & Power

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  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)