Eleventh Grade · Science · week 18

Momentum and Impulse

Compute linear momentum and impulse, and apply the impulse-momentum theorem to relate a force acting over time to a change in momentum.

Momentum is a measure of "how hard it is to stop something moving," combining both mass and velocity: p = m*v, a vector measured in kilogram-meters-per-second (kg*m/s), pointing in the same direction as the velocity. A slow-moving truck and a fast-moving bicycle can have comparable momentum, since one makes up in mass what the other makes up in speed. Momentum will turn out to behave very differently from energy in one crucial way, explored next week: it is always conserved in a collision, no exceptions.

Impulse connects momentum to force and time: J = F*t = the change in momentum, delta-p = m*v_final - m*v_initial. This impulse-momentum theorem explains a huge range of safety engineering: airbags, crumple zones, and padded gym floors all work by extending the TIME over which a momentum change happens, which -- since the change in momentum itself is fixed by the crash -- reduces the average FORCE needed to cause it. Same change in momentum, stretched over more time, means less force felt.

📋 For the teacher
Teach it (10–15 min):
  • This week's lab: catch a gently tossed egg (over a soft landing surface just in case) two ways -- with a rigid, unmoving hand, and by "giving" with your hand as you catch it, moving it backward with the egg over a longer distance and time. The egg survives the second way for exactly the impulse-momentum reason above.
  • Have them restate the impulse-momentum theorem in plain English before using the formula: "a given change in momentum can come from a big force over a short time, or a small force over a long time" -- that sentence is the entire safety-engineering unit in one line.
  • Brainstorm a list of household or sports safety features together (bike helmets, gym mats, boxing gloves, phone cases) and identify how each one works by extending impact TIME.
  • Keep momentum's direction (its sign) explicit in every problem -- a bounce that reverses direction is the detail students most often drop when computing impulse.
Talk about it:
  • Why do momentum and kinetic energy, both built from mass and velocity, behave so differently (one squares velocity, one does not)?
  • How do airbags and crumple zones use the impulse-momentum theorem to reduce injury?
  • Why does the SIGN of velocity matter so much when a collision involves a bounce?

Mastery looks like: They compute momentum and impulse correctly, including sign changes from a bounce, and apply the impulse-momentum theorem to explain real safety scenarios.

Common stumbles: Dropping the sign when an object bounces back (treating the speed as if direction never reversed); confusing impulse (a change in momentum) with momentum itself.

Homework for this lessonType it, write it with a stylus, or print it for pencil & paper.open →
Records attendance + 15 min of Science for this learner.
← Back to todayAll coursesYear planner