Forces and Interactions - Action / Reaction:

Objectives:

1.    Define force as part of an interaction..

2.    State Newton's third law of motion..

3.    Given an action force, identify the reaction force.

Key Terms:

interaction     action force     Newton's third law    reaction force

Web Resources:

Notes:

Newton's Third Law:

"Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on  the first object."

(http://www.glenbrook.k12.il.us/gbssci/phys/Class/newtlaws/u2l4a.html)

Forces come in pairs.  For every action there is an equal and opposite reaction.  All the forces of everyday life can either be labeled as an action force or a reaction force.  Follow these simple rules:

• All forces (action & reaction) come in pairs (Newton's 3rd)
• It does not matter which one is labeled action and which is labeled reaction
• The action & reaction forces are always opposite and equal in size
• If the net forces (action & reaction) are balance there is no acceleration (Newton's 1st)
• If the net forces are unbalanced there is acceleration (Newton's 2nd)

A common misconception when dealing with the action / reaction pairs is that all the forces will cancel each other out because all the forces appear to be balanced.  The trick is to look ONLY at the forces that apply to a single object as a system to determine if movement is taking place.

To solve for the orange, you must only look at the system of forces affecting the orange (blue).

Label the action /reaction forces below.

Each molecule of exhaust exerts a tiny amount of force on the rocket as it is ejected from the burner.  When the combined weight (mass * acceleration) of the exhaust becomes greater than the weight of the rocket, acceleration of the rocket begins.  We can see this in the formula:

F = m * a = m * a = F

If the weight of the cannon is 10,000N and the weight of the cannon ball is 10N, how much does the cannon accelerate if the ball leaves the 5m barrel at 5m/s. (hint: you must first find the acceleration of the ball a = v/t)

step 1:    find the time it took for the ball to leave the barrel.(t = d / v)    5m / 5m/s = 1s

step 2:    find the acceleration (a = v/t)                                                  5m/s / 1s = 5m/s2

step 3:    find the masses of the cannon and the ball (estimate using 10m/s2 for gravity)

10,000N/10m/s2 = 1,000kg & 10N/10m/s2 = 1kg

step 4:    setup the final equation (ma = ma)                                          1,000kg * a = 1kg * 5m/s2

a = (1kg * 5m/s2) / 1,000kg = 0.005m/s2