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8th grade · Science · Unit 3 of 13

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.

Texas Essential Knowledge and Skills (TEKS) teks student expectation: 8.7(A); 8.7(B) · Newton's second law and how all three laws act in collisions and vehicle safety

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 Texas

In the Texas Essential Knowledge and Skills (TEKS), this is unit 3 of 13 in 8th grade Science, after Chemical Change and before Light & Radiation.

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: Texas Education Agency (TEA)

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

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

  2. Question 2

    Impulse equals…

    • force ÷ area
    • mass × volume
    • force × time
    • energy × time

  3. Question 3

    In a closed system, total momentum before a collision…

    • is always larger after
    • becomes zero
    • doubles
    • equals total momentum after

  4. Question 4

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

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

  5. Question 5

    A rocket moves forward because…

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

  6. Question 6

    A 1000 kg car going 20 m/s stops in 4 s. The average force on it is…

    • 250 N
    • 20,000 N
    • 80,000 N
    • 5000 N

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