1. Newton’s First Law (Balanced Forces) Examples
Modified Atwood’s Machine A
Constant Velocity
Method 1 B
Treat as two separate objects tied together by a string.
Floor
Box A
Since Box A has four interactions it has four forces acting upon it.
Earth
String
Since Box B has two interactions it has two forces acting upon it.
Box B

2. Newton’s First Law (Balanced Forces) Examples
Modified Atwood’s Machine A
Constant Velocity
Method 1 B
Treat as two separate objects tied together by a string. A B

3. Newton’s First Law (Balanced Forces) Examples
Example 3 continued
Modified Atwood’s Machine A
Constant Velocity
Method 1 B
Treat as two separate objects tied together by a string.
Method 1
Begin by writing an equation for the horizontal forces acting on Box A. A B

4. Newton’s First Law (Balanced Forces) Examples
Example 3 continued
Modified Atwood’s Machine A
Constant Velocity
Method 1 B
Treat as two separate objects tied together by a string.
Method 1
Write an equation for the vertical forces acting on Box B. A B

5. Newton’s First Law (Balanced Forces) Examples
Example 3 continued
Modified Atwood’s Machine A
Constant Velocity
Method 2 B
Treat as a single system since all of the parts move together.
Box A
Earth
Floor
Box B
String
Since the forces of tension are internal to the system, we can ignore those interactions.

6. Newton’s First Law (Balanced Forces) Examples
Example 3 continued
Modified Atwood’s Machine A
Constant Velocity
Method 2 B
Treat as a single system since all of the parts move together.
Box A
Earth
Floor
Box B
String
Since the system has four external interactions, it has four forces acting upon it.

7. Newton’s First Law (Balanced Forces) Examples
Example 3 continued
Modified Atwood’s Machine A
Constant Velocity
Method 2 B
Treat as a single system since all of the parts move together.
Write an equation for the horizontal forces acting on the system. A B