0.2 Force, momentum and impulse  (Page 13/35)

Tumi presses a book against a vertical wall as shown in the sketch.

1. Draw a labelled force diagram indicating all the forces acting on the book.
2. State, in words, Newton's Third Law of Motion.
3. Name the action-reaction pairs of forces acting in the horizontal plane.

1. Draw a force diagram

   A force diagram will look like this:

   

   Note that we had to draw all the forces acting on the book and not the action-reaction pairs. None of the forces drawn are action-reaction pairs, because they all act on the same object (the book). When you label forces, be as specific as possible, including the direction of the force and both objects involved, for example, do not say gravity (which is an incomplete answer) but rather say 'Downward (direction) gravitational force of the Earth (object) on the book (object)'.

2. State Newton's Third Law

   If body A exerts a force onto body B, then body B will exert a force equal in magnitude, but opposite in direction, onto body A.

3. Name the action-reaction pairs

   The question only asks for action-reaction forces in the horizontal plane. Therefore:

   Pair 1: Action: Applied force of the girl on the book; Reaction: The force of the book on the girl.

   Pair 2: Action: Force of the book on the wall; Reaction: Force of the wall on the book.

   Note that a Newton III pair will always involve the same combination of words, like 'book on wall' and 'wall on book'. The objects are 'swapped around' in naming the pairs.

Experiment : balloon rocket

Aim:

In this experiment for the entire class, you will use a balloon rocket to investigate Newton's Third Law. A fishing line will be used as a track and a plastic straw taped to the balloon will help attach the balloon to the track.

Apparatus:

You will need the following items for this experiment:

1. balloons (one for each team)
2. plastic straws (one for each team)
3. tape (cellophane or masking)
4. fishing line, 10 meters in length
5. a stopwatch - optional (a cell phone can also be used)
6. a measuring tape - optional

Method:

1. Divide into groups of at least five.
2. Attach one end of the fishing line to the blackboard with tape. Have one teammate hold the other end of the fishing line so that it is taut and roughly horizontal. The line must be held steady and must not be moved up or down during the experiment.
3. Have one teammate blow up a balloon and hold it shut with his or her fingers. Have another teammate tape the straw along the side of the balloon. Thread the fishing line through the straw and hold the balloon at the far end of the line.
4. Let go of the rocket and observe how the rocket moves forward.
5. Optionally, the rockets of each group can be timed to determine a winner of the fastest rocket.
   1. Assign one teammate to time the event. The balloon should be let go when the time keeper yells “Go!” Observe how your rocket moves toward the blackboard.
   2. Have another teammate stand right next to the blackboard and yell “Stop!” when the rocket hits its target. If the balloon does not make it all the way to the blackboard, “Stop!” should be called when the balloon stops moving. The timekeeper should record the flight time.
   3. Measure the exact distance the rocket traveled. Calculate the average speed at which the balloon traveled. To do this, divide the distance traveled by the time the balloon was “in flight.” Fill in your results for Trial 1 in the Table below.
   4. Each team should conduct two more races and complete the sections in the Table for Trials 2 and 3. Then calculate the average speed for the three trials to determine your team's race entry time.
   Results:

   	Distance (m)	Time (s)	Speed (m $·$ s ${}^{-1}$ )
   Trial 1
   Trial 2
   Trial 3
   		Average:

   Conclusions: The winner of this race is the team with the fastest average balloon speed.