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Grade 8 · lesson 1 of 4

Find the net force

Combine opposite forces and distinguish zero net force from zero motion.

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Supplied materials

  • Paper-only fictional cart model. Choose right as positive. Cart A has a 6 N rightward push and a 6 N leftward force. Ignore other horizontal forces.
  • New case B has an 8 N rightward push and a 3 N leftward force. N means newtons, a unit of force.

Learn the idea

Forces have direction. With right positive, a rightward force is positive and a leftward force is negative. The net force is their sum.

In this simple model, zero net force means no acceleration: an object at rest stays at rest, and one already moving continues with constant velocity. It does not mean every moving object must stop. A nonzero net force changes velocity in its direction.

  1. Write Cart A’s forces as +6 N and −6 N.
  2. Add them and state the net force.
  3. Describe both a resting and an already-moving Cart A.
  4. Repeat the addition for Cart B.
Worked example

See the method in action

Cart A: +6 + (−6) = 0 N. If it is already travelling right at constant speed, these balanced horizontal forces do not change that velocity in the model.

Your turn

Check your reasoning

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What is Cart B’s net horizontal force?

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See the answer and explanation

5 N right

With right positive, +8 + (−3) = +5 N, so the net force is 5 N right. Adding the magnitudes without directions would incorrectly give 11 N.

Take it further

If the leftward force becomes 10 N while the rightward force stays 8 N, the net is 2 N left. Explain why a cart could still be moving right while slowing down.

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Longer courses for learners ages 14+

These related courses have their own preparation and projects. They are not early-grade assignments.

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