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## Work, Energy & Power

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**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**• 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.**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**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**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)**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)**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.**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**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**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.**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**Velocity, V**Mass = m kg Kinetic Energy (KE) KE = ½ m v2**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**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**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**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)**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**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.**Gravitational PE**• Examples: • When a chair lifted from ground a distance of 1m • You sitting on the 3rd storey of this building**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]**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?**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)