A question can ask how an object moves without yet asking why it moves that way. Straight-line motion at constant speed describes a pattern. A question about a change in speed or direction asks about the cause of a change. Start by deciding which kind of account the wording requires.
Keep the object in view throughout the explanation. Record the path, the elapsed time, and any stated change separately. When forces enter the question, ask about their combined effect on that object rather than treating any single push as the complete description. This guide uses the elementary motion conditions of the three examples.
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Read distance and time as a pair
Uniform linear motion has constant speed and a fixed direction along a straight line. Both conditions belong to the description. A statement about speed alone does not establish that the path is straight. Look for the directional wording before selecting the name of the motion.
Average speed is total distance travelled divided by elapsed time. In the example, 100 m over 20 s gives 5 m/s. The calculation describes the whole interval. It does not establish the speed at every individual moment. Retain the time interval when interpreting the result.
Ask what changes and what acts
Acceleration is the rate of change of velocity, which can involve a change in speed, direction, or both. In the force example, the net force is the combined force acting on the object. Newton’s second law connects that combined force to acceleration for the model under consideration.
After reviewing the answer, write one sentence describing motion and a second sentence identifying the relevant cause. If you cannot complete the second sentence from the supplied conditions, leave it as a question rather than inventing an unseen force. The two sentences have different jobs.
TRY & READ
Check your understanding with examples
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Example 1 · Physics: motion and forces
What is the motion of an object moving in a straight line at constant speed called?
Definitions, laws, and equations in this chapter. The Japanese questions and explanations were newly authored rather than copied from the source or existing exercises.
Source checked: 2026-10-03
Example 2 · Physics: motion and forces
An object travels 100 m in 20 s. What is its average speed?
2000 m/s
5 m/s
0.2 m/s
80 m/s
Read the answer and explanation
Answer: 5 m/s
Average speed is distance divided by time: 100/20 = 5 m/s.
Definitions, laws, and equations in this chapter. The Japanese questions and explanations were newly authored rather than copied from the source or existing exercises.
Source checked: 2026-10-03
Example 3 · Physics: motion and forces
What causes an object's speed or direction of motion to change?
Its volume alone
Its color alone
Its temperature alone
The net force
Read the answer and explanation
Answer: The net force
The combined force acting on an object produces acceleration.
Definitions, laws, and equations in this chapter. The Japanese questions and explanations were newly authored rather than copied from the source or existing exercises.
Topics: Physics: motion and forces. Range: Everyday knowledge, Broader knowledge. Difficulty: Basic, Standard. These are selected initially. You can change these on the setup screen.
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