Describing Motion Around Us · Lesson 1 of 10
Motion in a Straight Line
“The object moved, but first we need to agree on where zero lives.”
• Recognise motion in a straight line in familiar situations. • Describe position using a reference point, distance and direction. • Distinguish an instant of time from a time interval. • Use an origin and a sign convention consistently. • Explain why rest and motion depend on the chosen reference point.
At the start of a race, every runner appears motionless beside a starting line. A moment later, their positions change continuously. To describe what happened, saying that a runner “went forward” is not enough. We need a fixed point, a direction and a time. These simple choices turn an everyday observation into a precise description of motion.
Motion occurs everywhere: a butterfly changes direction in air, a stone falls vertically, a train travels along a track and the tip of a clock hand follows a circle. A useful first step is to study idealised forms. Motion along a straight path is the simplest because only two opposite directions are possible.
Linear motion is the motion of an object along a straight line.
| Situation | Why it can be treated as straight-line motion |
|---|---|
| Swimmer in a straight lane | The swimmer’s main position changes along the length of the lane |
| Ball falling vertically | Its path is approximately a vertical straight line |
| Train on a straight track | Its position changes along one track direction |
| Car on a straight highway segment | Its motion can be represented along a single line |
Describing position
Position tells where an object is at a particular instant. A position must be stated relative to a fixed reference point. In a race, the starting line can be the reference point. On a road, a milestone or a building may be used. Without the reference point, a statement such as “the car is 200 metres away” remains incomplete.
A reference point is a fixed point relative to which the position or motion of an object is described.
The origin is the point selected as the zero position on a line or coordinate system.
In straight-line motion, positions on one side of the origin are usually assigned positive values and positions on the other side negative values. The choice itself is a convention. Rightward or upward is often selected as positive, but another choice is allowed. Once a choice has been made, it must not change midway through an analysis.
Problem
The starting point is the origin. An athlete is first at 100 m and later at 40 m. Describe both positions.
- 1.State the reference point: the starting point is (O=0 m).
- 2.State the positive direction: take the direction from the start toward the far end as positive.
- 3.The first position is (+100 m).
- 4.The later position is (+40 m).
- 5.The smaller later position shows that the athlete moved back toward the origin, although both positions remain on its positive side.
An object is in motion relative to a reference point when its position with respect to that point changes with time.
An object is at rest relative to a reference point when its position with respect to that point does not change with time.
Problem
Is a seated passenger at rest or in motion?
- 1.Relative to the seat, the passenger’s position does not change, so the passenger is at rest.
- 2.Relative to a station platform, the passenger’s position changes as the train moves, so the passenger is in motion.
- 3.Both statements are correct because they use different reference points.
Think It Over
Safe motion also depends on position relative to other moving objects. A driver must know not only the distance to a truck ahead but how that distance is changing. Greater velocity generally requires a larger safe separation because the vehicle travels farther during the driver’s reaction and while the brakes reduce its velocity.
Note
An instant of time is one particular clock reading, such as (t=4 s).
A time interval is the duration between two instants, calculated as final time minus initial time.
Problem
A ball is released at 2 s and caught at 7 s. Identify the instants and the interval.
- 1.The release occurs at the instant (t_1=2 s).
- 2.The catch occurs at the instant (t_2=7 s).
- 3.The time interval is (t_2-t_1).
- 4.Therefore, the interval is (7-2=5 s).
A position, displacement or direction is ambiguous if the origin and positive direction have not been stated.
Quiz
Which description best matches Linear Motion?
Which description best matches Reference Point?
Which term matches this description: Linear motion is the motion of an object along a straight line.
Which term matches this description: A reference point is a fixed point relative to which the position or motion of an object is described.
Which statement is a key takeaway from this lesson?
Practice Problems
- Give three examples of motion that can be approximated as straight-line motion.
- A point lies 30 m left of an origin. Write its position if rightward is positive.
- Explain how a book on a train can be simultaneously at rest and in motion.
- Distinguish the instant 12 s from the interval between 12 s and 20 s.
Key Takeaways
• Position requires a reference point, distance and direction. • Motion and rest are relative to a chosen reference point. • Signs distinguish opposite directions along a straight line. • An instant is a clock reading, while an interval is a duration.
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Next · Lesson 2
Distance travelled and displacement