Simple Movements and Speeds
Problematic — How do we describe and measure an object's movement? How can we characterize its movement by a speed?
- Understand what a movement is and how to describe it.
- Identify different types of trajectories (straight, circular, etc.).
- Define and calculate the average speed of a moving object.
- Apply these concepts to concrete daily examples.
- Acquire the basics to interpret a distance-time graph.
Part 1: Movement and Its Description
Movement corresponds to a change in the position of an object relative to a reference point or another object. Movement is described by the path the object follows and the speed at which it moves.
To study movement, it is necessary to choose a frame of reference, that is, an object or point considered to be stationary. Then, we describe how the object moves in relation to this frame of reference.
The Trajectory
- The trajectory is the set of positions occupied by an object over time.
- It can be straight (a straight line), circular, or curvilinear (a more general curve).
For example, a car driving straight on a road follows a straight trajectory. A satellite orbiting the Earth describes a circular trajectory.
Movement is first described by the trajectory, which provides the shape of the path traveled. Understanding the nature of this trajectory is essential to then analyze the speed of the moving object. This first step allows situating the object in space over time.
Part 2: The Notion of Speed
Speed is a quantity that measures how fast an object in motion is. It indicates the distance traveled per unit of time.
To calculate an average speed, you need to know the distance traveled and the time taken for that movement. The formula is:
average speed = distance traveled ÷ duration
Speed is usually expressed in meters per second (m/s) or kilometers per hour (km/h).
Concrete Example
A runner covers 400 meters in 80 seconds.
- Distance traveled = 400 m
- Time = 80 s
- Average speed = 400 ÷ 80 = 5 m/s
We can also convert this speed to km/h: 5 m/s × 3.6 = 18 km/h.
Speed is a fundamental quantity that allows us to quantify an object's movement over time. Knowing how to calculate average speed is essential for understanding moving phenomena in our daily environment.
Part 3: Study of Simple Movements
The simplest movements are often straight-line movements, meaning the trajectory is a straight line. We distinguish:
- Uniform rectilinear motion, where speed is constant.
- Varied rectilinear motion, where speed changes over time.
In uniform rectilinear motion, the distance traveled is proportional to time. This is represented by a distance-time graph that is a straight line.
Example
A bicycle travels at a constant speed of 12 km/h.
- In 1 hour, it travels 12 km.
- In 30 minutes (0.5 h), it travels 6 km.
Uniform rectilinear motion means the speed is constant in both magnitude and direction, so the object covers equal distances in equal time intervals.
The distinction between uniform and varied rectilinear motion allows us to analyze the regularity of movement. These simple concepts prepare for studying instantaneous speed and later acceleration.
Part 4: Interpretation of Distance-Time Graphs
The distance-time graph is a powerful tool to visualize movement. The horizontal axis (x-axis) represents time, and the vertical axis (y-axis) represents the distance traveled from the start.
- An inclined straight line means movement at constant speed.
- A horizontal line means the object is not moving (fixed position).
- A curve can represent variable speed.
For example, a steeper slope of the line indicates a higher speed.
Simplified Example
An object remains still for 10 s (flat graph), then starts moving at a constant speed for 20 s (inclined line), and finally stops again.
Reading a distance-time graph is a key skill to understand and precisely describe movement. This type of representation helps intuitively identify phases of motion and rest.
This course covered fundamental notions of movement, starting with its description through trajectory, then defining average speed to quantify movement. We distinguished different types of simple movements, including uniform rectilinear motion, and learned how to interpret distance-time graphs. These concepts are essential to understand the world around us and provide a solid foundation for more advanced physics studies. By mastering these notions, you will be able to describe and analyze movements in many everyday situations.