Comparing motion diagrams

Same speed first

The car matches the bike's speed while it is still behind. Then it keeps speeding up and passes later. Same position is the pass. Same spacing between dots is the same speed. Those are two different times.

The setup

A car is stopped at a light. A bike is already moving at constant speed. The instant the bike is next to the car, the car starts. The car speeds up steadily until it passes the bike.

At t = 0 they share a position. The bike pulls ahead right away. The car is still starting from rest.

How the diagrams look

Number every dot with the same clock. Bike dots stay evenly spaced. Car dots start almost stacked, then the gaps grow.

The numbers on a real key might start at a different second. The pattern is what matters. Bike gaps stay one size. Car gaps grow through that size, then keep growing.

Circle the pass, not t = 0

They are next to each other twice. First is the start, when the bike arrives and the car is still stopped. That is not a pass. The pass is the later time both dots sit on the same mark again.

On the sketch that is t = 4. After that the car's next gap is bigger than the bike's, so the car is already ahead.

Yes, they match speed

Look at neighboring gaps, not at who is in front. When one car gap equals the bike's constant gap, those two seconds are the same speed. On the sketch that is about t = 2 to t = 3. The car is still behind.

If you said they never match because one is accelerating, that is the trap. Instantaneous speed can match for a stretch of the diagram even while the car is still speeding up across the whole run.

What the MA will poke

Seeley's list for Friday is this worksheet plus the warm-up, ping pong, and twin tracks.