Work and Energy Mr. I

Students need to be able : Explain what displacement is Calculate force Calculate weight Use formulas

Work Work is when a force acts on an object and it causes displacement in the direction of the applied force. Work is a scalar quantity, but it can be negative. ( We will talk about this later) The key is it needs to have 3 pieces: 1- Force 2- Displacement 3- Cause WORK IS MEASURED IN JOULES.

Δx=displacement (m)=xf-xi W=work in Joules (J) Calculating Work W=f.Δx f= Force in Newtons (N) Δx=displacement (m)=xf-xi W=work in Joules (J)

Joule One Joule is equivalent to one Newton of force causing a displacement of one meter. Work is measured in Joules 1 J = 1 N * m

Negative Work When a force is applied and it goes against the displacement then we call it negative work. Example : a car skidding to a stop on a roadway surface or a baseball runner sliding to a stop on the infield dirt. In such instances, the force acts in the direction opposite the objects motion in order to slow it down. The force doesn't cause the displacement but rather hinders it. These situations involve what is commonly called negative work

4 Types of Work Problems : 1. The force is given and displacement is given What is the work done if a football player is running with a force of 150N a distance of 20m? w=f.∆x W=150N x20m = 3000J 2. The mass is given, the acceleration is given and displacement is given. What is the work done by a football player with mass of 100kg and an acceleration of 9m/s2, and he runs for 20m. Since F=ma, and w=f.∆x then . w= (100kg x 9m/s2) 20m =18000J

4 Types of Work Problems : 3. What is the work done when a 100kg football player is lifted 0.5m into the air. When someone is lifted w=mgh. w=100kg x 9.8 m/s2x.5m =490J 4. When energy is changing and we are being asked for the amount of work done to cause that energy change. Calculate the work done by a car that is changing its velocity from 20m/s to 30m/s. Mass is 500kg. w=∆E=1/2mvf2-1/2mvi2 = (.5x500kgx(30m/s)2-(.5x500kgx(20m/s)2 = 125000J

Energy Energy is the ability to cause change. There are a variety of forms of energy: Electrical - energy from moving electrons Gravitational energy – Energy due to change in position and gravity Kinetic Energy – Energy of a moving object Chemical Potential Energy – Energy stored within bonds Elastic Potential Energy – Energy stored in a stretched objected Nuclear Energy – Energy stored in the nucleus of the atom Thermal Energy – Heat Energy (Energy due to the vibration of molecules)

Law of Conservation of Energy Energy is never created or destroyed, it is simply converted from one form to another.

Kinetic Energy (KE) Kinetic Energy (in Joules) =1/2 mass (in kg) x [speed (in m/s)]2 KE=1/2mv2 Kinetic Energy is the energy an object has because of its motion. If Mr. I throws a football that has a mass of .5kg at 2m/s, what is its kinetic energy? KE = ½ .5(2)2 = 1 J

Gravitational Potential Energy (GPE) Anything that can fall has GPE. GPE =mgh What is the gravitational potential energy of a dancer who has a mass of 50kg and is 2m above the ground? GPE=(50kg)(9.8)(2m) = 980J

Relationship between Energy and Work Work and energy are very closely related Work is the change in energy W=∆E =Ef-Ei

Transforming Energy Due to the law of conservation of energy we can transform or convert energy from one form to another. For instance in your car engine gasoline, which is chemical potential energy is converted into thermal (heat), sound, and kinetic energy. A more efficient engine is going to convert more of the chemical energy into kinetic energy and less into sound and thermal energy. That is why modern engines are more efficient than the old engines.

Conversions between Kinetic and Potential Energy We encounter many situations where we have convert potential energy to kinetic, can you demonstrate some?.... If an object is being dropped, GPE=KE + GPE until the object hits the ground. Therefore At the top or bottom of a drop/ramp use : GPE TOP= KEBottom In the middle of a drop/ ramp use : GPETOP=ke +gpe OR KEBOTTOM= ke + gpe

Mechanical Energy Mechanical energy is the total amount of potential energy and kinetic energy in a system and can be expressed by this equation. Mechanical energy = potential energy + kinetic energy

Solving Conservation of Energy Problems Steps : 1- Ask yourself, where are you in the drop or ramp. Are you at the bottom or top, or are you somewhere else! 2- If you are at the top or bottom then GPE TOP= KEBottom 3. If you are somewhere else then use GPETOP=ke +gpe OR KEBOTTOM= ke + gpe 4. Then substitute in the values that you are given in the problem. You may use gpe=mgh and ke=1/2mv2 here 5.Solve for what you are asked to solve for.