1.0 — Motion: Background Content and Teacher Information

Content Information

Motion

The laws of motion apply to everything that moves. In physics, motion is a change in position of an object with respect to time. Motion is typically described in terms of displacement, distance, velocity, acceleration, time and speed.

To a physicist, the state of motion and the state of rest are not fundamentally different. For a physicist, “rest” is a special case of motion where an object has a constant speed of zero. It can be thought of as the natural state where there are no forces acting on an object. Although motion and rest are not fundamentally different to a physicist, students usually think of them as two separate ideas.

There are two kinds of motion: uniform motion and accelerated motion. Uniform motion describes objects that stay still or move in a straight line at a constant speed. For example, a book on a shelf has uniform motion, as does a car on cruise control or a spaceship traveling in deep space. Accelerated motion describes a state where the speed is changing over time. Some examples of acceleration include a book falling to the ground after a cat knocked it off of a shelf, a car when the driver presses on the gas pedal, or a rocket at blastoff.

Distance

Distance is the total amount of ground covered by an object in motion. The lessons in this unit focus on distance, but knowing the larger idea of displacement can be useful when describing motion in physics.

Displacement refers to how far an object has moved with respect to the point where the object started. Just like the difference between speed and velocity, displacement differs from distance by including a direction. For example, if a physics teacher walks forward 4 meters,
then turns around and walks back for 2 meters, the total distance the teacher traveled is 6 meters, but the teacher’s displacement is 2 meters forward because they only ended up 2 meters away from their starting point.

Speed

Speed is the rate at which an object is moving regardless of its direction. The measurement of speed does not include how fast/slow that rate changes over time, or change in the direction of the object. The measurement of speed itself does not include the direction the object
is traveling, so the observer has no idea if the object is going to the left, right, backwards, forwards, etc.

There are two common ways of talking about speed: as an average and as instantaneous. Average speed is calculated by dividing the total distance traveled by the total time it took for that object to travel the distance (s=d/t). Instantaneous speed, however, refers to the exact speed that an object is travelling at a specific instant.

Velocity

Velocity builds on speed by indicating not only the rate the object is traveling at, but also the direction in which it is traveling. For example, say you throw a ball to your friend. The ball leaves your hand at 30 mph and travels away from you. The speed of the ball is 30 mph. The velocity of the ball is 30 mph forward.

Acceleration

Acceleration is the change in motion that occurs when an object changes direction, speeds up or slows down. Acceleration is calculated with the following equation:

Acceleration = change in velocity/change in time

The scientific definition for acceleration is not intuitive because in everyday terms, we think about acceleration as when we go faster. Think of a rollercoaster reaching the top of a hill, we are used to describing its increasing speed as it heads down the hill as acceleration. It is less common however to hear someone refer to the decreasing speed of the rollercoaster as we come to the end of the ride as an acceleration of speed. Well, less common unless you spend a lot of time in amusement parks with physicists! In physics acceleration can mean both speeding up and slowing down. Either way, you can feel acceleration whether you are braking sharply at a red light or picking up speed riding down a big hill on a bike. Physicists use the word acceleration instead of speeding because the phrase “speeding up” can mean both an increase in the number value of the speed and the speed increasing with time.

Patterns of Motion: Playing Catch

Many of the lessons in this unit revolve around the physical activity of playing catch. These lessons focus on noticing the path of the ball, describing its motion, and looking for the patterns in the motion of the ball while playing catch. Below are some of the patterns students may notice about the motion of the ball in these activities.

• The path of the ball always forms an arc (parabola) between the person throwing the ball and the person receiving the ball.
• The vertical speed of the ball will be the slowest at the top of the arc and faster on each side (when thrown and caught).
• The horizontal speed of the ball will be fastest when it is first released from the hand of the person throwing it.

Common Misconceptions

• Have students describe the motion they observe and the data that they read from the graphs in everyday language first (slowing down, speeding up, changing direction) before introducing the academic concepts of speed, velocity and acceleration. When the vocabulary is presented, make sure students understand when the scientific definitions or uses for the word vary from the everyday meanings of the same words.
• Because the words speed, velocity and acceleration are used differently in everyday language than in physics, determining the students’ understandings of those terms can help clarify their preconceptions of what the term means. Students often confuse the concepts of velocity and acceleration.
• Some students may have trouble interpreting the graphs of speed and distance traveled since many students interpret graphs as pictures rather than representations of data. Listen to the ways in which the students describe what the graph shows so as to catch misconceptions. Have students construct their own graphs so that they can have the experience of creating a representation of the motion of the object.
• Students may use words such as force, momentum and energy before they have been introduced in the unit. Listen to how the students use the words. Sometimes students use these terms but are applying their own definitions to them. Instead of focusing on the vocabulary students are using while first exploring concepts in physics, focus on the ideas they are describing and what they think is happening. Proper use of academic terms will develop as students talk through their ideas and learn more about physics.