Study guide: Motion, Momentum and Collisions
Extend force learning with motion graphs and momentum accounting.
Ideas to explain in your own words
- Average speed equals distance in meters divided by time in seconds. Average speed need not equal every moment’s speed.
- Momentum equals mass in kilograms multiplied by signed velocity in meters per second. Mass does not become negative when motion reverses.
- Total momentum equals the two signed supplied momenta added. Opposite signs can partly cancel.
- Slope on a position-time graph gives velocity. A horizontal line represents constant position. Curving graphs indicate changing slope; their direction must be interpreted from the chosen coordinate system.
Useful vocabulary
- m/s
- Meters per second expresses speed or a signed velocity component.
- Average speed
- Average speed equals distance in meters divided by time in seconds.
- kg·m/s
- Kilogram-meter per second is a momentum unit; direction is included in signed model values.
- Momentum
- Momentum equals mass in kilograms multiplied by signed velocity in meters per second.
- Total momentum
- Total momentum equals the two signed supplied momenta added.
- positive slope
- position increases with time
Watch your reasoning
- Using the distractor operation a*b instead of a/b; Use total distance for speed.
- Using the distractor operation a+b instead of a*b; Include velocity direction through its sign.
- Using the distractor operation a-b instead of a+b; Add signed momenta.
A worked review example
Worked distinct example: A distinct route is 18 m in 3 s: 18/3=6 m/s average speed. Method: identify the quantities in the stated rule, substitute using compatible units, and perform the operations in the correct order. Use total distance for speed. Time belongs in the denominator. Interpret the final unit and keep the model assumption visible; this calculation is not a measurement of a real learner or site. Worked distinct example: A distinct object has 3 kg mass and -4 m/s velocity: 3×(-4)=-12 kg·m/s. Method: identify the quantities in the stated rule, substitute using compatible units, and perform the operations in the correct order. Include velocity direction through its sign. Momentum is not kinetic energy. Interpret the final unit and keep the model assumption visible; this calculation is not a measurement of a real learner or site. Worked distinct example: A distinct model has +10 and -6 kg·m/s: total +4 kg·m/s. Method: identify the quantities in the stated rule, substitute using compatible units, and perform the operations in the correct order. Add signed momenta. State the isolated-system assumption for conservation. Interpret the final unit and keep the model assumption visible; this calculation is not a measurement of a real learner or site. Worked supplied problem: Position rises by 6 m over 3 s, then stays fixed for 2 s. Explain both graph segments. Solution: The first segment has average slope 6/3=2 m/s. The horizontal segment has zero velocity. Five seconds of elapsed time do not mean five seconds of motion. This conclusion uses the stated observations and the mechanism explained above; it does not establish every unmeasured feature of a real organism, device or place.
Make a practice plan
- Explain the mechanism in Calculate average speed using a labeled case.
- Complete Calculate signed momentum and correct the specific misconception in its feedback.
- Resolve a changed case from Account for two-object momentum, then combine the four ideas in the transfer task.