Chapter 6 Work and Energy

*1. TRUE or FALSE The work done on a body is simply calculated by multiplying the net force acting on a body by the distance that the object travels.

2. TRUE or FALSE The work done on a body is simply calculated by multiplying the net force acting on a body by the distance that the object travels.

*4. TRUE or FALSE Energy can be defined as the ability of a body or system of bodies to perform work.

*5. TRUE or FALSE Objects A and B have the same mass. Object A is traveling at a velocity of 4 m/s North. Object B is traveling at a velocity of 12 m/s East. Object B has three times as much translational kinetic energy (kinetic energy) as object A.

6. TRUE or FALSE The stopping distance of a car depends on the square of the car’s speed.

A satellite is moving in uniform circular motion around the earth. Is the force of gravity between the earth and the satellite equal to zero? What is the angle between the direction of the force of gravity on the satellite and the direction of the instantaneous velocity of the satellite?

Is the work done by the force of gravity of the earth on the satellite equal to zero? Explain.

*Suppose that in a certain situation the force acting on an object is not constant. Can we use the four equations of motion V = Vo + at etc., to calculate the final velocity and final kinetic energy? Explain.

*If we are told the amount of work done on the object can we calculate the change in kinetic energy for the object? (You may wish to review the Work-Energy Principle.)

A certain object has 20 Joules of kinetic energy. It is moving on a horizontal, frictionless surface. If its speed is doubled what is the new value of its kinetic energy?

Work = F d cos(theta) spring F = -kX KE = ½ m V²

the metric unit for work and energy is the Joule

Chapter 6 PHY 151/161

*1. TRUE or FALSE Potential energy is the energy associated with a body due to its position or configuration.

2. TRUE or FALSE If there is no friction and no change in potential energy for an object, the amount of work done on an object is equal to the change in the kinetic energy which an object possesses. (This is a review question.)

3. TRUE or FALSE The location where the potential energy is zero (the reference level) is always at ground level.

*Which situation has the greatest change in gravitational potential energy?

a) A 5 kg ball is lifted from the ground to a point 2 meters above the ground.

b) A 5 kg ball is moved from a table that is 3 meters above the ground to a shelf that is 5 meters above the ground.

How is the work value related to the value of potential energy?

4. TRUE or FALSE A certain spring is extended by 6 cm by an applied force of 5 Newtons. The spring is now at rest. If the force applied to the spring remains at 5 Newtons the spring will not stretch any farther.

5. TRUE or FALSE For the spring discussed above, the work done in extending the spring 6 cm is 0.3 Joules. (This is 0.06 meters * 5 Newtons.)

Know how they differ in regard to total work done on a closed path.

*What force is not "conservative" ?

W_NC = ΔKE + ΔPE

*Describe the value of the sum of the potential energy and kinetic energy for an object which is moving in a frictionless environment.

KE₁ + PE₁ = KE₂ + PE₂

In class we will watch a video: Mechanical Universe #13 Conservation of Energy

gravitational PE = mgh elastic PE = ½ k X²

6.7 Problem Solving

Problems that involve variable forces and accelerations can not be solved with our 4 kinematic equations. They can be solved using energy principles. The KE and PE of the system at the start and end of the problem are the important quantities.

*Be able to explain the conceptual example 6-10.

What questions do you have on the examples worked in the text? You should be able to work all of these problems by the end of today’s class.

OK, I have a question for you. In example 6-13 the ball falls a distance h. Why does the text include a calculation for the potential energy using the distance Y?

Is it important that the bungee jump operator understands conservation of energy?

You will see Conservation of Energy principles in various chapters throughout the remainder of the text.

Memorize the Law of Conservation of Energy

The concept here is that the amount of energy available at the start of the problem is not all available for KE and PE at the end of the problem because work is done by dissipative forces.

KE₁ + PE₁ = KE₂ + PE₂ + F_frd or KE₁ + PE₁ - F_frd = KE₂ + PE₂

*You are told that two people do the same amount of work. If you are not given any other information can you be sure that the power ratings of the two people are equal?

Power_AVG = work/time Power = F V

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