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A resultant force changes velocity. If resultant force is zero, an object stays at rest or moves with constant velocity; it does not necessarily stop. Newton’s second law is F = ma with the resultant force. Weight is a gravitational force, W = mg, whereas mass measures the amount of matter and inertia. In free-body diagrams draw only forces acting on the chosen object, using arrows that show direction.
Video by Cognito. The video’s own board label is retained; follow the specification reference and scope notes on this lesson for assessed content.
Viewing focus: Apply Newton’s laws
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: What does zero resultant force imply?
Constant velocity, possibly zero. Balanced forces do not require rest.
After watching 2: Where do a third-law force pair act?
On different objects. They are the two sides of one interaction.
What you'll be able to do
A resultant force changes velocity. If resultant force is zero, an object stays at rest or moves with constant velocity; it does not necessarily stop. Newton’s second law is F = ma with the resultant force. Weight is a gravitational force, W = mg, whereas mass measures the amount of matter and inertia. In free-body diagrams draw only forces acting on the chosen object, using arrows that show direction.
Newton’s third-law forces are equal and opposite forces of the same interaction acting on different objects. They do not cancel on one object’s free-body diagram. A falling object initially accelerates; drag grows with speed until it balances weight at terminal velocity. At that point speed can remain non-zero while acceleration is zero. Changing shape can change drag and lead to a new terminal speed.
Momentum is p = mv and includes direction. Total momentum is conserved in a closed system with no resultant external force. During a collision, forces change the momentum of individual objects. Increasing the stopping time for the same momentum change reduces average force, explaining cushions, airbags and crumple zones. Conservation of momentum does not imply conservation of kinetic energy in every collision; some energy transfers to thermal stores, sound or deformation.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: A 2 kg trolley at 3 m s⁻¹ joins a stationary 1 kg trolley. Find their common speed if external impulse is negligible.
2 m s⁻¹ in the initial direction.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.