1. Unit 5: Work, Power and Energy

2. Work Work is done when a force causes a change in motion of an object, or work is a force that is applied to an object and then multiplied by the movement of an object. – This force must be applied in the direction of the motion of the object.

3. Formula: Work = Force x displacement or w = f x d If we apply a force on an object that does not move we have done NO work.

4. – Sitting and looking at a computer screen is not work. Tapping on the keyboard and making the keys move is work. Your fingers are moving and applying a force.

5. If a father lifts his child 2.0m into the air and exerts a force of 190N, what is the work done by the father? w = f x d, so; 190N x 2.0m = 380J Both work and energy are expressed in units of joules (J).

6. Power Power is a quantity that measures the rate at which you do work. Formula: Power = work/time or p = w/t – A watt (W) is the unit for power. A watt is the amount of power required to do 1 joule (J) of work in 1 second, or watt = 1 joule/1 second

7. It takes 100J of work to lift an elevator 18m. If this is done in 20s, what is the average power of the elevator? p=w/t, so; p = 100J/20s = 5W

8. Energy Energy is the ability to do work (measure of how much work is done. Energy is a word to describe the power and effort of a force). Energy is also measured in Joules Two types of energy: – Potential Energy (Stored) – Kinetic Energy (Active)

9. Active Energy vs. Stored Energy Potential Energy (PE) is the energy of an object due to its position. – Coaster has PE when it is at the top of the hill. Even if it isn’t moving yet, it may if a force is applied and it begins to move

10. Gravitational potential energy (GPE) is relative to the ground. – Potential Energy = mass x acceleration due to gravity x height OR – PE = mgh m = mass g = acceleration due to gravity… gravity = 9.8 m/s 2 h = height

11. Find the PE of a rock climber as she climbs 35m above the base of a cliff. Her mass is 65kg. PE = mgh PE = 65kg x 9.8 m/s 2 x 35m = 22, 295J

12. Kinetic Energy Kinetic Energy is the energy due to an objects motion. Formula: Kinetic Energy = ½ x mass in kg x velocity in (m/s) or KE = ½ x m x v 2 m = mass v = velocity

13. What is the KE of a 44kg cheetah running at 31m/s? KE = ½ x m x v 2 KE = ½ x 44kg x (31m/s) 2

14. Law of Conservation of Energy The Law of conservation of mas/energy states that energy cannot be created or destroyed but it can be transformed from one form to another. – Other forms of energy: * Mechanical* Electrical * Light* Chemical * Sound* Nuclear * Heat (friction)

15. These forms of energy can be transformed from one form to another. – Ex) Rubbing our hands together Mechanical energy → Heat energy

16. – Ex) On a roller coaster ride all of the energy for the entire ride comes from the conveyer belt a the beginning of the ride. This energy is then stored as GPE at the top of the first hill. As the coaster travels down the first hill, the GPE is converted to KE In the real world, a small amount of this energy is lost as heat to the wheels and vibrations that produce the roaring sound!

17. Energy Efficiency We all use devices every day that use energy – or more accurately, transfer energy from one form to another. Everything we use wastes energy – some of the energy transfers into forms that are not useful to use.

18. For example, when driving a car, energy from burning fuel is transferred into kinetic energy of the car. However, more than half the energy is lost as heat and sound. How effectively devices transfer energy – i.e. how much energy used is useful – is called its efficiency.

19. Efficiency is normally calculated as a percentage - something 90% efficient is considered good at its job. Devices that transfer only 5% of the energy they use into something useful are inefficient (very wasteful).

20. We can calculate efficiency if we know the total energy used, and how much is transferred into useful forms.

21. To solve for efficiency think of it this way, the resulting work that a machine does for you (work output) divided by the net work you do (work input). Output/input Example: If you do 80 Joules of work (work input) on a something and get 40 Joules of work out of it (work output), you have 50 % efficiency. 40 J / 80 J =.50 or 50% IT WILL ALWAYS BE PERCENTAGE!!!!