Interactions between the system and the environment will cause a change in the system.

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Interactions between the system and the environment will cause a change in the system.

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Interactions between the system and the environment will cause a change in the system.

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- Interactions between the system and the environment will cause a change in the system.
- Change in the speed of a system due to an external force.
- Change in the direction of motion of a system due to an external force.
- Change in the temperature of a system due to thermal conduction.

In order to change the energy of a system we must do work.

What is work?

Work is the change in the mechanical energy of a system due to an external force.

Work can be done to store energy in a system.

Store energy in a spring by compressing the spring.

Work can be done to change the state of motion of a system.

Change the velocity of a system.

When the external force is constant we can determine the work done using the following expression:

J - Joules

W – Work [J = Nm = kgm2/s2]

F – Force [N]

Dr – Displacement [m]

q – Angle between F and Dr

Work is a scalar quantity – no direction!

The maximum amount of work is done when F is parallel (or anti-parallel) to Dr.

Work is a means of transferring energy to or from the environment.

Positive work – Energy transferred to the system.

Negative Work – Energy is transferred from the system.

- Example: Discuss whether work is done for the situations below and explain your reasoning.
- You lift a package off of the ground.
- Yes, work is done. The package is moved a distance against the gravitational force.
- You carry the package across the room.
- No, work is not done. You are not moving the package with or against a force.
- You push a package across a rough surface.
- Yes, work is done. You do work against friction to move the package across the floor.

Dot Product (Scalar Product)

This product of two vectors results in a scalar quantity. You multiply one vector by the component of the second vector that is parallel to the first vector.

If A = B: We use the same rules when multiplying a vector by itself.

The square of a vector only gives magnitude.

A person pulls a box along the ground at a constant speed. If we consider Earth and the box as our system, what can we say about the net external force on the system?

1. It is zero because the system is isolated.

2. It is nonzero because the system is not isolated.

3. It is zero even though the system is not isolated.

4. It is nonzero even though the system is isolated.

5. none of the above

The force the person exerts on the box is equal and opposite to the force the person exerts on the earth in order to have the box move at a constant velocity. The net force the person exerts on the box Earth system is zero.

A person pulls a box along the ground at a constant speed. If we consider Earth and the box as our system, the net force exerted by the person on the system is

1. zero

2. nonzero

The force the person exerts on the box is equal and opposite to the force the person exerts on the earth in order to have the box move at a constant velocity. The net force the person exerts on the box Earth system is zero.

Explanation

A person pulls a box along the ground at a constant speed. If we consider Earth and the box as our system, the work done by the person on the system is:

1. zero

2. nonzero

The displacement of Earth is zero, and so the work done by the person on Earth is zero. The displacement of the box is nonzero, however, and therefore the person does work on the box. The sum of the work done on the box and that done on Earth is then nonzero.