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9th grade · Science · Unit 9 of 28

Forces, Motion & Momentum

Newton's second law, momentum and safer collisions.

What your child practices

Practices Newton's second law (F = ma), momentum and its conservation, and designing devices that reduce the force of a collision.

New York State Learning Standards learning standard: HS-PS2-1; HS-PS2-2; HS-PS2-3 · Newton's second law, conservation of momentum, and designs that reduce collision forces

How we explain it

Children can open this short lesson before they practice.

  1. Newton’s second law: force = mass × acceleration (F = ma).
  2. Momentum = mass × velocity. In a collision, total momentum is conserved.
  3. Making a collision last longer, as airbags do, lowers the force.

Example: A 4 kg cart accelerates at 2 m/s². What net force acts on it? F = m × a. 4 × 2 = 8. Answer: 8 N

Forces, Motion & Momentum in New York

In the New York State Learning Standards, this is unit 9 of 28 in 9th grade Science, after Chemical Change and before Energy & Magnetism.

Gradelings describes a child's practice on this unit as Beginning, Approaching, Meeting or Exceeding (Four practice steps). That reflects practice, not a report-card mark: the teacher decides the mark.

Source: New York State Education Department (NYSED)

Sample questions

A taste of this unit. In the app, questions change every time and get easier or harder to match your child.

  1. Question 1

    Doubling the net force on an object (same mass) makes the acceleration…

    • halve
    • stay the same
    • quadruple
    • double

  2. Question 2

    Newton's second law is…

    • F = ma
    • p = F/t²
    • E = mc²
    • V = IR

  3. Question 3

    A rocket moves forward because…

    • air pulls it
    • gases pushed backward push the rocket forward
    • it has no mass
    • gravity pushes it up

  4. Question 4

    Momentum equals…

    • mass ÷ volume
    • mass × velocity
    • mass × acceleration
    • force × distance

  5. Question 5

    The SI unit of force is the…

    • watt
    • newton
    • joule
    • pascal

  6. Question 6

    Why is landing with bent knees safer than with stiff legs?

    • It adds momentum
    • It increases stopping time, lowering force
    • It stops gravity
    • It increases force

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