Work and Kinetic Energy. What is kinetic energy?  If an object is moving, it has energy. You can think of kinetic energy as the energy of motion, and.

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Presentation transcript:

Work and Kinetic Energy

What is kinetic energy?  If an object is moving, it has energy. You can think of kinetic energy as the energy of motion, and it is one type of energy. Energy is the ability to do work.  Another type of energy is potential, which is energy based on position.

The math behind kinetic energy  An object’s kinetic energy depends on:  mass  velocity or

How do you measure kinetic energy?  Kinetic energy is a scalar quantity.  Unit of kinetic energy: Joule  An object with mass of 1 kg, moving at 1 m/s, has a kinetic energy of 0.5 Joule.  SI base unit: Kg m 2 s -2  Why Joule? Because this guy (James Prescott Joule)

Example problems Solve the following problems involving kinetic energy: a) What is the kinetic energy of a car with a mass of 1400kg moving at 22ms -1 ? b) What is the kinetic energy of a 150g ball moving at 2ms -1 c) A 200kg go-cart starts at rest and then accelerates to 18ms -1 after 50m. What is the KE at that point?

Example problems (solutions) Solve the following problems involving kinetic energy: a) What is the kinetic energy of a car with a mass of 1400kg moving at 22ms -1 ? J b) What is the kinetic energy of a 150g ball moving at 2ms -1 ? 300J a) A 200kg go-cart starts at rest and then accelerates to 18ms -1 after 50m. What is the KE at that point? 32400J

Thinking about kinetic energy Let’s say you are going to shoot a person out of cannon. At any point as they are flying through the air they have kinetic energy. When they land they no longer have kinetic energy. What are two ways you can increase the amount of kinetic energy they have?

What is work?  Work = force x distance moved in direction of the force or W = Fs or W = Fd This is for work done by a constant force.  Unit of work: Joule  Work is a scalar quantity.

Understanding the idea of work  Work is always done by a force  Work is only done when there is movement  A force only does work when it has a component in the same direction the object is moving.

Has work been done?  A man running from here to Guangzhou.  A woman stepping up onto a chair.  A woman stepping up onto a chair and then jumping back down.  A student holding a bowling ball.  A student holding a bowling ball and walking up stairs.  A student swinging a bowling ball back and forth.  A man running to Guangzhou and then back here.

Work and friction When there is friction then energy is dissipated as heat or sound.  Your shoes on the ground  A gun being shot Friction is a dissipative force Gravity on the other hand is a conservative force, and the energy is not lost. It is stored as potential energy. For example a book placed on an upper shelf has potential energy equal to the work done against gravity to place it there.

Positive and negative work  The man does positive work lifting the box and negative work lowering the box.  Gravity does positive work when the man lowers the box and negative when he lifts it. Gravity is a conservative force. We will discuss more about gravitational potential energy in the next lesson.

Example problems Solve the following problems involving Work: a) Find the work done by pushing a box 45m using a force of 50N. b) What is the work done by pushing a 100kg box 70m across a rough floor with 150N resistance? The box starts and stops at rest. c) How does the answer to (b) change if the box is still moving at 2ms -1 when it reaches 70m? d) A car with a mass of 1200kg is driving at 10ms -1 and over a period of 20 seconds increases the speed to 20ms -1. Assuming constant acceleration, what is the work done?

Example problems (solutions) Solve the following problems involving Work: a) Find the work done by pushing a box 45m using a force of 50N. 2250J b) What is the work done by pushing a 100kg box 70m across a rough floor with 150N resistance? The box starts and stops at rest J c) You will add the KE to the total J d) A car with a mass of 1200kg is driving at 10ms -1 and over a period of 20 seconds increases the speed to 20ms -1. Assuming constant acceleration, what is the work done? J

Work and Kinetic Energy Kinetic energy is a type or energy and its units are Joules just like the equation for Work. How are they connected? Let’s start with the formula for Work: W = Fs

Work-energy principle The total work done by the forces acting on a body is equal to the increase in the kinetic energy of the body.

When to use each formula. Type of problem where you use W = Fs If you have a problem where an object is moved from one location to the other against a resistive force and starts and ends at rest. Problems where you use W = Ke f – Ke i If you have a problem with a change in velocity. Problems where you use might use both If you have a problem where an object is moving against a resistive force and has a change in velocity.