Work and Kinetic Energy

1- Outline what is meant by Kinetic Energy. 2 - List different forms of energy and describe examples of the transformation of energy from one form to another. 3 - State and apply the Work - Kinetic Energy Theorem. 4 - Solve problems involving Work, Energy and Power. Assessment Standards

Mechanical  Kinetic, Potential (gravitational, elastic) Thermal  Microscopic mechanical Electromagnetic Nuclear Energy is always conserved! Forms of Energy

Energy and Work Energy: the capacity to make things happen Work: is the transfer of energy Kinetic Energy – Energy of motion Depends on two quantities.  mass  velocity Work & Energy

Energy – Is one of the conserved quantities in nature (momentum, mass, charge) – The capacity to do work Work – The mechanical means of transferring energy. – work = force · distance Unit of energy & work: 1 N·m = 1 J (Joule) Work & Energy

Kinetic Energy is one form of mechanical energy. Work is a transformation from one type of energy into Kinetic Energy or Kinetic Energy into another form of energy (potential energy, dissipation into heat, chemical, etc.) Work’s Relation to Energy

The kinetic energy of an object is: E K – kinetic energy (J) m – mass of the object (kg) v – the velocity of the object (m/s) Units: mass x velocity 2 = kg (m/s) 2 = Nm = J (Joule) Kinetic Energy

How does the kinetic energy of the car change in each case?

Suppose a constant net force Fnet acts on an object of mass m over some distance ∆s. Newton's Second Law tells us that the object will have an acceleration a = Fnet/m. If the object's initial velocity was u and its final velocity is v, kinematics tells us that: Multiplying both sides of this equation by the object's mass, m, gives: Rearranging: Work - Kinetic Energy Theorem

Since F net = ma (Newton's Second Law), we can substitute: then divide both sides of the equation by 2: This is the Work/Energy Equation. Alternatively, here Work - Kinetic Energy Theorem

The final kinetic energy minus the initial kinetic energy is the change in kinetic energy. Remember that work is the applied force times the displacement so we have: Work - Kinetic Energy Theorem

This is the work energy theorem. Work - Kinetic Energy Theorem

We have already seen that there is a connection between the work done by a force and the change in the velocity of the object: If W > 0 J: velocity increases If W = 0 J: velocity remains constant If W < 0 J: velocity decreases

A dog is running across the yard at a speed of 15 m/s. He weighs 42 kg. What is his kinetic energy? He turns and runs in the opposite direction with 250 Joules of energy. How fast is he going? His friend is running at this same speed and has 175 Joules of energy. How much does he weigh? Examples

A 1500 kg vehicle is traveling at a speed of 27 m/s. What amount of work must the brakes do in order to bring the vehicle to a stop over a distance of 30 m? examples

A 250 gram lacrosse ball is shot with a velocity of 15 m/s from the attack man’s stick. It hits the goal net and causes it to move 150 cm in stopping the ball. What is the average force exerted on the ball? Examples

William Tell uses a 50 gram arrow to split an apple. He pulls back with a force of 550 N a distance of 50 cm on his bow. It takes the arrow 1.2 seconds to hit the apple and split it. How far away is the volunteer apple holder from William Tell assuming once it leave the bow it moves at a constant speed? examples