Work, Energy & Power Physics 11

Work  In physics, work is defined as the dot product of force and displacement  The result is measured in Joules (J) and a Joule is a Newton metre  Work is a scalar

Zero Work Conditions  Work will equal zero if: Force applied is equal to zero Displacement is equal to zero The angle between the force and the displacement is equal to ninety degrees

Comprehension Check 1.How much work is done if you push on a wall with 3500N but the wall does not move? 2.How much work is done carrying a book down the hall at constant velocity? 3.If you pull a crate with a force of 550N at an angle of 35° to the horizontal and it moves 25m horizontally, how much work was done?

Work Done By Changing Forces  How could you determine the work done in the graph?

Work Done By Changing Forces  How could you determine the work done in the graph? AREA UNDER CURVE!

Positive and Negative Work  Positive work occurs when the angle between the force and displacement is 0°- 90°  Negative work occurs when the angle between the force and displacement is 90°-180° F d F d

Kinetic Energy Physics 11

Work on an moving object A 2kg object is moving at 10 m/s when a force is applied to it accelerating it to 20m/s over a distance of 5m. What is the work done on the object?

Work on a Moving Object 10m/s20m/s 5m

If you Insist on Numbers…

Work-Energy Theorem  These terms have a special name: Kinetic Energy  Work is the change in Kinetic Energy  Energy is defined as the potential to do work

Work done by Friction  A car is travelling at 100kph when the driver sees a moose on the road ahead. The driver slams on the brakes, bringing the car to a stop just before hitting the moose. If the car's mass is 1200kg, how long does it take to stop?

Work done by Friction

Numbers again…

Potential Energy Physics 11

Potential Energy and Work What is the work done on a 12kg object to raise it from the ground to a height of 1.5m?

Potential Energy and Work What is the work done on a 12kg object to raise it from the ground to a height of 1.5m?

Potential Energy  The work done on the system can be recovered, so it must be stored. This stored energy is Potential Energy, Ep or PE.  Energy is defined as the potential to do work. KE: a moving object can apply a force through a distance PE: An object in the air, when dropped, can apply a force through a distance (dropping textbook)

Forms of Energy: Brainstorm Kinetic  Moving things Potential  Gravitational  Elastic (springs)  Chemical (stored in molecular bonds)  Thermal (heat)  Electrical (separated charges)  Nuclear (E=mc 2 ) Physics 11 Physics 12 Chemistry

Spring Energy Physics 11

Hooke’s Law Δ F s : Applied force Δx : displacement of the spring from the equilibrium position k: the spring constant

Where is the Energy?  If a vertical spring is stretched 12cm by the weight of a 0.10kg. How much energy is in the spring?  Derive an expression for the energy stored in a spring.

Analogy with Gravity

What we know about Energy so Far Kinetic Spring Gravitational Potential Work

Work Energy Theorem, Power, and Efficiency Physics 11

Conservation of Energy  The total energy of a closed system is constant.  The total energy in the system is the sum of the kinetic and potential energy of it’s various parts.

Examples  Book Drop  Collision into a spring  Car coasting up a hill h v v x hv v

Energy is a “State Function”  Each type/form of energy we have seen so far is a ‘state function’  A state function only cares about the current state of the closed system  How the system got into that state does not matter  path-independent h v m All Paths result in the same final velocity

Consider the following situation a)What is the velocity at the top of the loop? b)What is the maximum compression of the spring? h r k m

Consider the following situation a)What is the velocity at the top of the loop? r k m h

Consider the following situation a)b b)What is the maximum compression of the spring? r k m

Rate of Change in Energy  So what is different about each of these paths? The time it takes to go down the longer ramp will be greater due to a lower acceleration  Power: h v m All Paths result in the same final velocity

Efficiency  Not all systems are closed. Energy lost through friction  Efficiency of energy conversion in open systems.

Efficiency  Not all systems are closed. Energy lost through friction  Consider the following: A rocket has 3.50x10 3 J of chemical potential energy. The stored chemical energy is transformed into gravitational potential energy when it is launched. What is the efficiency of the rocket’s transformation of energy if the 0.5kg rocket travels 1.00x10 2 m.