• One form of energy can be converted into another form of energy.

• Conservative and non-conservative forces

• CONSERVATION OF ENERGY

Chapter 8: Potential Energy and Conservation of Energy part 2

Reading assignment: Chapter 9.1-9.4

Homework : (due Wednesday, Oct. 5, 2005):

Problems: Q2, 3, 6, 15, 20, 22

Work due to friction

If friction is involved in moving objects, work has to be done against the kinetic frictional force.

This work is:dfW kf

Work done by non-conservative forces

1. Work done by an applied force.(System is not _____________)

An applied force can _________ energy into or out of the system.

Example. Applying a ____________ to an object and lifting increases the energy of the object.

cos dFdFW

Black board example 7.4

Angus is pulling a 10,000 kg truck with all his might (2000N) on a frictionless surface for 10.0 m.

(a) How much work is the man doing?

(b) What is the speed of the truck after 10 m.

(c) What is the speed of the truck after 10 m if there is friction?

(friction coefficient: 0.0153)

Power

Power is the at which work is done:

dt

dWP

Average power:

(work done per time interval t) t

WP

vFdt

sdF

dt

dWP

The power can also be expressed as:

The units of power are __________ (__/__) = watt (W)

(Dot product)

Black board example 7.7

An elevator having a total mass of 3000 kg moves upward against the gravitational force at a constant speed of 9.13 m/s.

(a) What is the power delivered by the motor?

Review Important energy formulas:Work:

2

2

1 :energy potential Elastic kxUe

ygmU g :energy potential nalGravitatio

zzyyxx dFdFdF

dF

dFW

cos

2

2

1 :energy Kinetic vmK

f

i

x

x

dxxFW

Forms of energy:

A mass m is bobbing up and down on a spring.

Describe the various forms of energy of this system.

(a) At the highest point

(b) At the point where the kinetic energy is highest

(c) At the lowest point

Black board example 8.2

Black board example 8.3

Three balls are thrown from the top of a building, all with the same initial speed.

The first is thrown horizontally, the second with some angle above the horizontal and the third with some angle below the horizontal.

(a) Describe the motion of the balls.

(b) Rank the speed of the balls as they hit the ground.

Black board example 8.5

Nose crusher?

A bowling ball of mass m is suspended from the ceiling by a cord of length L. The ball is released from rest when the cord makes an angle A with the vertical.

(a) Find the speed of the ball at the lowest point B.

(b) Assume a cord length L = 5m and an angle A = 20°.

(c) The ball swings back. Will it crush the operator’s nose?

Reading potential energy curves

_______Edx

dU____

Remember:

Black board example 8.6

During a rock slide, a 520 kg rock slides from rest down a hillside that is 500 m long and 300 m high. The coefficient of friction between the rock and the hillside is 0.25.

(a) What is the gravitational potential energy of the rock before the slide?

(b) How much energy is transferred into thermal energy during the slide?

(c) What is the kinetic energy of the rock as it reaches the bottom of the hill?