PHY 111 - CH 6

As a satellite gets closer to Earth...

It SPEEDS UP

What are the combined units for a W (watt)?

Joule/s

Does normal force (FN) do any work?

NO (neither does weight)

Is energy a scalar or vector quantity?

SCALAR

Is work a scalar or vector quantity?

SCALAR

Average power

The rate at which work is done

What are the two different definitions of a conservative force?

The work a force does on a moving object is independent of the path between the object's initial and final positions (does NOT depend on the path); (2) a force is conservative when it does no work on an object moving around a closed path, starting and finishing at the same point

What are the units for power?

W (watt)

Does friction depend on the path?

YES

What is the formula for work using the formula for F (force)?

mas (mass times acceleration times displacement)

What is the formula for KE? (kinetic energy)

1/2 times m (mass) times (velocity)^2

What does conserved mean in physics?

CONSTANT

Formula for work?

F times s (force times displacement)

If the work done by the net force is positive, then the kinetic energy of the object?

INCREASES

Examples of nonconservative forces?

Static and kinetic frictional forces; air resistance; tension; normal force; propulsion force of a rocket

Friction is?

Wasteful energy

Work-Energy Theorem

When a net external forces does work on an object, the kinetic energy of the object changes by the amount of work done on it

Formula for PE? (potential energy)

mgh (mass times force of gravity times height)

Engine A has a greater power rating than engine B. Which one of the following statements correctly describes the abilities of these engines to do work? (a) Engines A and B can do the same amount of work, but engine A can do it more quickly. (b) Engines A and B can do the same amount of work in the same amount of time. (c) In the same amount of time, engine B can do more work than engine A.

A

If the force doesn't line up with s (displacement), what do you have to do?

Component method (make F line up with s)

A box is being moved with a velocity (v) by a force (P) (in the same direction as v) along a level horizontal floor. The normal force is Fn, the kinetic frictional force is fk, and the weight is mg. Which one of the following statements is correct? (a) P does positive work, fk and Fn do zero work, and mg does negative work. (b) Fn does positive work, P and fk do zero work, and mg does negative work. (c) fk does positive work, Fn and mg do zero work, and P does negative work. (d) P does positive work, mg and Fn do zero work, and fk does negative work

D

If the work done by the net force is negative, then the kinetic energy of the object?

DECREASES

Does the force of gravity (Fg or g) move upwards or downwards?

DOWNwards

How is average power obtained?

Dividing the work by the time required to perform the work

Symbol for total mechanical energy?

E

What is GPE?

Energy that an object of mass (m) has by virtue of its position relative to the surface of the earth

Position (in terms of GPE) measured by height h of the object is relative to?

An arbitrary zero level

For a perfect circular orbit, Fc (centripetal force) does NO work. Why?

B/c the angle is 90 degrees (angle between s and centripetal force) so there is NO change in KE (so constant speed)

A ball has a speed of 15 m/s. Only one external force acts on the ball. After this force acts, the speed of the ball is 7 m/s. Has the force done (a) positive, (b) zero, or (c) negative work on the ball?

C

What is the work-energy theorem? (in terms of KE)

Final KE minus initial KE

Examples of conservative forces?

Gravitational force; elastic spring force; electric force

What does GPE stand for?

Gravitational potential energy

KE and Potential Energy are?

MECHANICAL energy

In terms of conservative and nonconservative forces, what happens in normal situations?

The two types of forces act simultaneously on an object

On a curve showing Force (y-axis) plotted against displacement (s on the x-axis), what is the area under the curve?

WORK

What are the two equations for P (power)?

Work divided by time (to do the work) OR force times average velocity

Does work involve energy?

YES

At the highest point of an object, what is the value for 1/2m(v^2)?

ZERO (b/c the velocity at the maximum height is ZERO)

Equation for the net work done by an external nonconservative force?

change in KE (kinetic energy) plus change in PE (potential energy)

W subscript nc equals? (nonconservative work) - two different formulas

delta KE (change in kinetic energy) plus delta PE (change in potential energy) OR final energy (Ef) minus initial energy (Eo)

Formula for GPE? (W subscript gravity)

mass times g (small g) times change in height

W subscript c equals? (conservative work)

negative delta PE (negative change in potential energy)

When does the total mechanical energy (E = KE + PE) of an object remain constant as the object moves?

Provided that the et work done by external nonconservative forces is ZERO

IF you have friction, what do you do to the value of kinetic energy?

Subtract the amount of friction from KE (kinetic energy - b/c friction opposes motion)

For a conservative force, the Work (W) done by gravity equals?

0 J

What is the formula for Work-Energy Theorem?

1/2 times m (mass) times (final velocity)^2 MINUS 1/2 times m (mass) times (initial velocity)^2

What is the value for the net W (net amount of work)?

1/2 times m (mass) times (final velocity)^2 MINUS 1/2 times m (mass) times (initial velocity)^2

In a simulation on earth, an astronaut in his space suit climbs up a vertical ladder. On the moon, the same astronaut makes the exact same climb. Which one of the following statements correctly describes how the gravitational potential energy of the astronaut changes during the climb? (a) It changes by a greater amount on the earth. (b) It changes by a greater amount on the moon. (c) The change is the same in both cases

A

A net external force acts on a particle that is moving along a straight line. This net force is not zero. Which one of the following statements is correct? (a) The velocity, but not the kinetic energy, of the particle is changing. (b) The kinetic energy, but not the velocity, of the particle is changing. (c) Both the velocity and the kinetic energy of the particle are changing.

C

In one case, a sports car, its engine running, is driven up a hill at a constant speed. In another case, a truck approaches a hill, and its driver turns off the engine at the bottom of the hill. The truck then coasts up the hill. Which vehicle is obeying the principle of conservation of mechanical energy? Ignore friction and air resistance. (a) Both the sports car and the truck (b) Only the sports car (c) Only the truck

C

In terms of work, s stands for?

Displacement

A net external nonconservative force does positive work on a particle. Based solely on this information, you are justified in reaching only one of the following conclusions. Which one is it? (a) The kinetic and potential energies of the particle both decrease. (b) The kinetic and potential energies of the particle both increase. (c) Neither the kinetic nor the potential energy of the particle changes. (d) The total mechanical energy of the particle decreases. (e) The total mechanical energy of the particle increases.

E

Principle fo Conservation fo Energy

Energy can neither be created nor destroyed but can only be converted from one form to another

What are the units for work?

J (joules)

What are the combination units for a J? (joule)

N times m (newton times meter)

Work done by kinetic frictional force is always? SO?

NEGATIVE; so it is impossible for the work it does on an object that moves around a closed path to be zero

A sailboat is moving at a constant velocity. Is work being done by a net external force acting on the boat?

NO

Can energy be created or destroyed?

NO

Is the concept of potential energy defined for a conservative force?

NO

If a force is perpendicular to the displacement, does it do any work?

NO (because cos(90) is 0)