- Work/Energy Theorem Notes Teacher Key -

Transcription

1 NAME: DATE: PERIOD: HON. PHYSICS - Work/Energy Theorem Notes Teacher Key - - Mechanical Energy - The MECHANICAL ENERGY that an object has is simply the SUM of potential and kinetic energy forms. Gravitational Potential (PE g ) Spring Potential (Pe s ) Kinetic (KE) Expressed as an equation: - Mechanical Energy Example #1 - During a physics-stunt-gone-wrong, a 10kg bowling ball rolls along the top of a 2m high book case with a velocity of 2 m/s. Determine the mechanical energy of the ball. - Mechanical Energy Example #2- During a physics-lab-gone-wrong, two naughty students load a 1 kg water balloon into a water balloon launcher that is strategically located on the top of the bleachers. If a 100N force is used to draw the launcher back a distance of 2 m, and the top of the bleachers are 10m above the ground, determine the mechanical energy that the balloon has in the loaded position.

2 - Total Energy - The TOTAL ENERGY that an object has can be found by taking the SUM of all energy forms. Potential (PE) Kinetic (KE) Heat (Q) or INTERNAL / THERMAL energy Expressed as an equation: - Heat Energy Facts - HEAT energy, sometimes called INTERNAL energy or THERMAL energy, is a special type of kinetic energy. HEAT energy is a measure of how much the MOLECULES INSIDE a material are VIBRATING. In physics, HEAT energy is the result of WORK DONE BY FRICTION IF an object moves and experiences friction, then it will experience an INCREASE in HEAT ENERGY

3 - Changing Energy - How could we change each of the following energies? To change KE you must change VELOCITY To change PE g you must change HEIGHT To change PE s you must change STRETCHED/COMPRESSED POSITION - Changing Energy - What must we do in order to change the velocity of an object? What must we do in order to change the height at which an object is located? What must we do in order to change the amount that a spring is stretched or compressed? In all of these cases, we must apply a FORCE through some DISTANCE we must DO WORK! This is known as the WORK-ENERGY THEOREM

4 The WORK-ENERGY THEOREM states: - Work/Energy Theorem - The WORK done on a system (or object) will result in a CHANGE IN THE TOTAL ENERGY Expressed as an equation: of the system (or object). This CHANGE can be a GAIN in energy if POSITIVE work is done, or a LOSS in energy if NEGATIVE work is done. - What is a system? - A system is simply an object, or group of objects, that are isolated from outside forces yet allowed to interact with one another. - Energy in a system - The total energy of the system can be found by taking the sum of the energies of each object. Expressed as an equation: E T = PE + KE + Q - Changing System Energy - When an OUTSIDE FORCE acts on the system, WORK will be done to the system, which will cause a change in energy. W = Fd = ΔE T = ΔPE + ΔKE + ΔQ The WORK done can cause: PE to increase or decrease KE to increase or decrease Q to increase or decrease

5 - Changing System Energy - Work/Energy Theorem Example #1 How much work must be done to stop a 1,000 kg car traveling at 30 m/s along a flat road? If the brakes apply an average force of 4,500N in stopping the car, determine the distance required for the car to stop.

6 Work/Energy Theorem Example #2 A 0.1 kg arrow is fired from a bow whose string exerts an average force of 95 N on the arrow over a distance of 0.80 m. What is the speed of the arrow as it leaves the bow? Work/Energy Theorem Example #3 As a toy dart gun is loaded, the spring inside is compressed a distance of m from its uncompressed position. If the spring has a spring constant of 200 N/m, determine the amount of work done in compressing spring? What average force must be used to compress the spring? Work/Energy Theorem Example #4 A 0.145kg baseball traveling at 35 m/s moves a fielder s glove backward 0.25m when the ball is caught. How much work must the fielder do in bringing the ball to rest? What was the average force exerted by the ball on the glove?