Question

physics - 3.1 section review worksheet
name:
date:
class:

directions:
read the section carefully, then answer the following questions in complete sentences. be sure to show your thinking when necessary.

1. list three action and reaction pairs shown in the picture on page 64 in your textbook.

a.
b.
c.

1. why don’t action and reaction forces cancel?

1. use impulse to explain how force is related to changes in momentum.

1. explain the law of conservation of momentum and how it relates to newton’s third law.

Explanation:

1. Without seeing the picture on page 64, common action - reaction pairs could be: a person pushing on a wall (action) and the wall pushing back on the person (reaction); a book resting on a table (book exerts a downward force on table - action, table exerts an upward normal force on book - reaction); a swimmer pushing water backward (action) and water pushing swimmer forward (reaction).
2. Action and reaction forces act on different objects. For equilibrium, forces must act on the same object. So they don't cancel each other out.
3. Impulse ($J$) is defined as $J = F\Delta t$, and also $J=\Delta p$ (change in momentum). So force is the rate of change of momentum over time, $F=\frac{\Delta p}{\Delta t}$.
4. The law of conservation of momentum states that in an isolated system, the total momentum before an interaction is equal to the total momentum after the interaction. Newton's third law implies that the forces between two objects in an interaction are equal and opposite. These equal - and - opposite forces act for the same time interval, resulting in equal and opposite changes in momentum, which is consistent with the conservation of momentum.

Answer:

1. a. A person pushing on a wall and the wall pushing back on the person.

b. A book resting on a table (book's downward force on table and table's upward normal force on book).
c. A swimmer pushing water backward and water pushing swimmer forward.

1. Action and reaction forces act on different objects, not the same object, so they don't cancel.
2. Impulse $J = F\Delta t=\Delta p$, so $F=\frac{\Delta p}{\Delta t}$, meaning force is the rate of change of momentum over time.
3. The law of conservation of momentum: In an isolated system, total momentum before an interaction equals total momentum after. It relates to Newton's third law as the equal - and - opposite forces from Newton's third law cause equal and opposite changes in momentum, maintaining total momentum in an isolated system.