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Home Concept Builders 1D Kinematics Motion Diagrams Help Help 17

Motion Diagrams - Questions 17 Help

When the forces acting upon an object are balanced, the object will maintain its state of motion; that is, its velocity remains constant. If the forces are unbalanced, the object will either speed up or slow down. The reverse logic is also true: if an object has a constant velocity, then the forces are balanced. And if an object is speeding up or slowing down, the forces are unbalanced.

In Activity 3, you will have to construct a motion diagram for six different verbal descriptions. The fifth question is randomly selected from a set of three elgible questions, each of which describes the same type of motion. Here is one of the three versions.
 

Question 5:

A leftward-moving skateboarder speeds up, increasing its speed by 0.50 m/s each second.

Dot Spacing

This verbal description is of a speeding up motion. An object that speeds up over the course of its motion covers an increasing distance each successive second. So it is important that the dot separation distance increase as the object moves from the right towards the left.
 

Velocity Vectors

There are two important questions to answer when selecting the set of velocity vectors. First, which direction do they point? And second, is the length staying constant, increasing, or decreasing? The first question is easiest to answer. Velocity can be thought of as speed with a direction. The direction of the velocity is simply the direction that the object is moving; in this case, that is to the left. The second question pertains to the length of the vector arrow and that is where the speed comes in. A speeding up object is increasing its velocity; the magnitude or size is becoming greater over time. Thus, the length of the velocity vector will be increasing as the object moves from the left towards the right. This should narrow down the choices to one.
 

Acceleration Vectors

The first question to ask is: Is the object accelerating? Accelerating objects are either speeding up, slowing down or changing directions. This object is speeding up so it is accelerating. That rules out the choice of a zero acceleration.

The second question to ask is: Which direction is the acceleration directed? You must consider two things: (1) Which way is the object moving? and (2) Is the object speeding up or slowing down? An object that is slowing down will have an acceleration direction that is opposite the direction it moves. And if it is not slowing down (as in the case of this object), then the acceleration is in the direction that the object moves.

The third question to ask is: Is the length staying constant, increasing, or decreasing? The correct answer requires a thorough understanding of acceleration. First, acceleration is not velocity. An object can have a changing velocity but a constant acceleration. Acceleration has to do with the rate at which the velocity (or speed) is changing. If the change rate is the same each second, then the acceleration value is the same value from one second to the next. That is, the acceleration is a constant value if the amount by which the speed changes each second is a constant amount. Such instances would be represented by a set of acceleration vectors that are the same length. There's only two such choices.

The following pages from The Physics Classroom Tutorial provide additional information related to this topic:

Velocity versus Speed

Acceleration

Dot Diagrams (or Ticker Tape Diagrams)

Vector Diagrams

Concept Builder Teacher Notes Directions