Uniform-Motion Problems: Just Playing With Cars

David Drymiller Morgan Park High School
1744 W. Pryor
Chicago IL 60643
(312)535-2550

Objectives:

To solve uniform motion word problems using rate X time = distance.
To practice measuring in the metric and English systems.

Materials Needed:

At least two battery operated cars with constant velocity,
Measuring tape with both English and metric systems at least 50 feet or
15 meters, meter and yard sticks,
Stop Watches at least two or more if possible,

Strategy:

The students perform the following demonstration for their classmates under the
direction of the classroom teacher. No prior knowledge of the activity is
needed by the students.

First, with masking tape mark the starting and finishing lines for calculating
velocity over a fixed distance. (I used 800 cm.) Find the time needed for each
car to transverse the fixed distance. Record the time and distance on the
blackboard and then calculate the velocity of each car.
(25 cm/sec and 36 cm/sec)

Now introduce the first type of Uniform-Motion Problem, "If the cars are place
back to back and travel at the same time in opposite directions, when will they
be 1000 cm apart?". Explain the problem first as the cars are placed on the
floor back to back and run off in opposite directions. Next use the standard
associated pictures and charts for this type of problem, write the equation and
solve it in the usual manner.

25t + 36t = 1000
61t = 1000
t = 16.39

Place the cars back to back on the floor and let them run for 16.4 secs. and
measure the distance between the two cars. (It should work. I got 1020 cm.)

Now introduce the second type of Uniform-Motion Problem, "If the two cars are
750 cms apart and they each travel towards each other at the same time, when and
where will the two cars meet?". Set the cars on opposite sides of the room and
let them run into each other as you begin your explanation. Again use the
standard associated pictures and charts for this type of problem, write the

equation and solve it in the usual manner.

25t + 36t = 750 (12.29)(25) = 307.25 cm
61t = 750
t = 12.29

They should collide in approximately 12 secs, 307 cm from where the slower car
starts. Mark that spot on the floor. Place the cars 750 cms apart and run them
at each other. (I missed by 6 cm.)

Performance Assessment:

With two faster cars mark the starting and finishing lines for calculating
the velocity over a fixed distance. (I used 20 yards.) Also mark two starting
lines on opposite ends of the hallway. (65 yards) In teams of 3 or 4, the
students find the velocities, make the associated picture and chart, write the
equation and solve it in the usual manner, and mark the spot in the hall where
the two cars will collide. (This is competition at its best.)