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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.

Alaska content 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 Alaska

In the Alaska content 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: Alaska Department of Education and Early Development

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

    A car's crumple zone is designed to…

    • make the car stiffer
    • add weight only
    • increase impact force
    • absorb energy and lengthen collision time

  2. Question 2

    Momentum equals…

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

  3. Question 3

    A 10 N net force acts on a 5 kg box. Its acceleration is…

    • 15 m/s²
    • 0.5 m/s²
    • 50 m/s²
    • 2 m/s²

  4. Question 4

    Airbags reduce injury by…

    • increasing the force
    • increasing the time of the collision
    • making the car heavier
    • adding momentum

  5. Question 5

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

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

  6. Question 6

    When designing a helmet, which criterion best reduces injury?

    • Make it heavier
    • Make it as hard as possible with no padding
    • Make it smaller than the head
    • Lengthen the time to stop the head

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