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

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

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

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

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.

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.

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