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9th grade · Science · Unit 10 of 10

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.

Arizona Academic Standards learning standard: HS.P3U1.6; HS.P3U2.7 · Newton's second law, 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 Arizona

In the Arizona Academic Standards, this is unit 10 of 10 in 9th grade Science, after 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: Arizona Department of Education

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

    Velocity differs from speed because velocity includes…

    • color
    • direction
    • mass
    • temperature

  2. Question 2

    A 2 kg cart accelerates at 3 m/s². What net force acts on it?

    • 5 N
    • 6 N
    • 9 N
    • 1.5 N

  3. Question 3

    Momentum equals…

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

  4. Question 4

    Doubling the mass (same force) makes the acceleration…

    • double
    • quadruple
    • stay the same
    • halve

  5. Question 5

    If the net force on an object is zero, it…

    • must speed up
    • must slow down
    • must turn
    • keeps a constant velocity

  6. Question 6

    A graph of velocity against time has a constant upward slope. The object has…

    • constant acceleration
    • constant velocity
    • zero velocity
    • decreasing mass

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