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Physics · Grades 8–11

Force, mass and acceleration

Apply the resultant force rule, including its sign and units.

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Learn the idea

Equation

For constant mass in this model, net force equals mass times acceleration.

Resultant first

Find the net force before dividing by mass. A pulling force alone may not be the resultant.

Proportions

At fixed mass, doubling net force doubles acceleration. At fixed force, doubling mass halves acceleration.

Units

One newton is one kilogram metre per second squared. Mass is not measured in newtons.

Worked examples

A 4 kg cart experiences a 20 N net force. Find acceleration magnitude.

a = Fnet/m = 20/4 = 5 m/s².

5 m/s²

A 3 kg object accelerates at −2 m/s². Find its signed net force.

Fnet = 3 × (−2) = −6 N.

-6 N

Check your understanding

  1. At fixed mass, tripling the net force does what to acceleration?

    1. Leaves it unchanged
    2. Triples it
    3. Divides it by three

  2. A 5 kg cart is pulled right by 28 N with 8 N friction left. Find acceleration, right-positive.

  3. Why is using just the pulling force incorrect when friction also acts?

Answers and marking notes

Answers and marking notes
  1. Triples it

    Acceleration is directly proportional to net force at fixed mass.

  2. 4 m/s²

    Net force is 28 − 8 = 20 N; divide by 5 kg.

  3. The equation requires the vector sum of forces, not a single selected force.

    Credit calculating the resultant first.

Further reading and review references

These are reference links, not copied exercises, institutional endorsement or proof of independent review.

Independent educational review required. Grade bands are approximate; these are not officially aligned curricula.