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Newton’s second law practice
Newton’s second law relates net force to acceleration, not necessarily to the direction an object is already moving. Add all forces along an axis before using F = ma.
By ExamCat · Original study lesson · Published September 24, 2026
Understand the method
Choose a positive direction and keep signs consistent. Mass is measured in kilograms, acceleration in m/s² and force in newtons. A force diagram should contain forces acting on the chosen object, not the forces that object applies to something else.
Worked example, step by step
A 4 kg cart experiences 18 N to the right and 6 N to the left on a horizontal track. Find its acceleration.
- Choose right as positive.
- Calculate net force: 18 − 6 = 12 N.
- Divide by mass: a = 12/4 = 3 m/s² to the right.
- If vertical forces balance, there is no vertical acceleration; this does not remove the horizontal net force.
A mistake to watch for
Using 18 N alone ignores the opposing force. Also, an object with zero net force can move at constant velocity; zero acceleration is not the same as zero velocity.
Try it yourself: three questions
Write an answer before opening the explanation. For calculations, keep your units and intermediate steps; for concepts, state the reason for your choice.
1. A 3 kg object has net force −9 N. Find acceleration.
Show answer and explanation
−3 m/s² along the chosen axis. The negative sign states direction.
2. What net force keeps a cart moving at a constant velocity?
Show answer and explanation
Zero. An applied force may still be needed to balance friction, but the sum is zero.
3. If mass doubles while net force stays fixed, what happens to acceleration?
Show answer and explanation
It halves because a = Fnet/m. This comparison assumes the net force really remains unchanged.
Build your next practice session
Mark the first step where your reasoning differed from the explanation. Turn that step into a question, answer it without looking, then change one condition in the example and explain what would change. A correct guess is a reason to revisit the method.
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Further reading
OpenStax Physics provides textbook background. The worked scenario and practice questions on this page were written independently for ExamCat.
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