GIG Read the passage and mark your answers on your whiteboard. NOT ON THE PAPER. Question 10-11 or 25-26.

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GIG Read the passage and mark your answers on your whiteboard. NOT ON THE PAPER. Question 10-11 or 25-26.

Potential and Kinetic Energy

Work at an Angle Sometimes the direction in which a force is applied is not the same as the object’s movement. In this case, only the component of the force in the direction of movement does work.

Work at an Angle

Work at an Angle Formula for work with displacement at an angle W = Fdcosθ Work = force ✕ distance ✕ cosineθ

Force Perpendicular to Motion Does No Work You push a 10kg box 1.0m across the floor, using 100N of force. Then you push a 0.1kg box 1.0m across the floor, again using 100N of force. How much work did you do each time?

Force Perpendicular to Motion Does No Work You push a 10kg box 1.0m across the floor, using 100N of force. Then you push a 0.1kg box 1.0m across the floor, again using 100N of force. How much work did you do each time? W = Fd = 100N ✖ 1.0m = 100J Gravity and normal force do not change this calculation.

Negative Work If the object moves in the opposite direction of the force, negative work has been done, equal to: W= -Fd

Types of Energy Energy exists in several different states, such as the chemical energy in covalent bonds and nuclear energy holding atoms together. In physics, we are especially concerned with kinetic energy and potential energy.

Kinetic Energy Kinetic energy is the energy an object has because of its motion. The formula for kinetic energy is: KE = .5m(v2) Kinetic energy = ½ mass times velocity2

Sample Problem A cat weighing 5.3kg runs across the floor at 8.7m/s. What is its kinetic energy?

Sample Problem A cat weighing 5.3kg runs across the floor at 8.7m/s. What is its kinetic energy? KE = .5mv2 = .5(5.3kg)(8.7m/s)2 = 200J

The Work-Energy Theorem Work and kinetic energy are related. The total work done on an object is equal to its change in kinetic energy. Wtotal = ΔKE = .5mvf2 - .5mvi2 Total work = ½ mass times final velocity2 - ½ mass times initial velocity2

Sample Problem A 78kg parachutist jumps from an airplane. How much work has gravity done when the parachutist’s velocity is 55m/s?

Sample Problem A 78kg parachutist jumps from an airplane. How much work has gravity done when the parachutist’s velocity is 55m/s? Wtotal = .5(78kg)(55m/s)2 - .5(78kg)(0m/s)2 = 120,000J or 120kJ

Potential Energy Potential energy is energy that is stored for later use. One of the most common types of potential energy is gravitational potential energy, that is, energy available if an object above the ground falls down.

Potential Energy Gravitational potential energy formula PEgravity = mgh Gravitational potential energy = mass ✕ gravity ✕ height

Sample Problem A 3.5kg bowling ball rests precariously on the edge of a shelf 1.4m above the ground. How much gravitational potential energy does it have?

Sample Problem A 3.5kg bowling ball rests precariously on the edge of a shelf 1.4m above the ground. How much gravitational potential energy does it have? PEgravity = mgh = 3.5kg ✕ 9.81m/s2 ✕ 1.4m = 48J

Potential and Kinetic Energy Lab Follow the instructions on your lab report sheet.

Spring Potential Energy A spring and similar objects which have the ability to deform and return to their original state have spring potential energy, also known as elastic potential energy.

Spring Potential Energy Formula PEspring = .5kx2 Spring Potential Energy = ½k ✕ distance deformed2

Practice Problem A spring with a k value of 120N/m is stretched 2.3cm. What is its potential energy?

Practice Problem A spring with a k value of 120N is stretched 2.3cm. What is its potential energy? PEspring = .5kx2 = .5(120N/m) ✕ (.023m)2 = .032J

Homework None.

Closure A 50.0kg diver stands on a high dive 3.00m above the water, then dives. How much has his potential energy changed when he is 2.00m above the water? At 1.00m? At 0.00m? Does his kinetic energy change in the same linear fashion?

Closure A 50.0kg diver stands on a high dive 3.00m above the water, then dives. How much has his potential energy changed when he is 2.00m above the water? At 1.00m? At 0.00m? When he is ⅓ of the way down, he has lost ⅓ of his PE. ⅔, he has lost ⅔. When he reaches the water, he has 0 PE. Does his kinetic energy change in the same linear fashion? No, because it depends on velocity2.