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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.

Maine Learning Results 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 Maine

In the Maine Learning Results, 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: Maine 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

    A rocket moves forward because…

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

  2. Question 2

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

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

  3. Question 3

    A 4 kg ball moves at 5 m/s. Its momentum is…

    • 1.25 kg·m/s
    • 20 kg·m/s
    • 45 kg·m/s
    • 9 kg·m/s

  4. Question 4

    Velocity differs from speed because velocity includes…

    • color
    • temperature
    • mass
    • direction

  5. Question 5

    A car's crumple zone is designed to…

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

  6. Question 6

    When designing a helmet, which criterion best reduces injury?

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

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