Video Transcript
The graph shows the velocities of three objects during a time interval. Which of the objects has no net force acting on it during the interval?
Okay, so in this question, we have shown a graph of velocity against time. This graph has three lines on it. There’s line one, the red line; line two, the blue line; and line three, the orange line. Each of these three lines represents the velocity of an object during the time interval shown on the graph. We are asked to work out which of the objects has no net force acting on it during this time interval.
Let’s begin by looking at each of the lines on the graph in turn and seeing what kind of motion they represent. We’ll begin with the line labeled one, so that’s the red line. We can see that this line is horizontal, which means the value of velocity is the same for all values of time. In other words, line one represents an object moving with a constant velocity during this time interval.
If we now look at line two, the blue line, we see that the velocity increases as time goes on. For example, at this time here, the blue line has this height on the velocity axis. Meanwhile, at this later or larger value of time, the blue line has a greater height, which means a greater velocity. So line two, the blue line, represents an increasing velocity.
Lastly, we’ll look at line three, the orange line. Since this line has a negative slope, we can see that the velocity decreases as time increases. This means that line three represents a decreasing velocity.
So we now know what the three lines on the graph show. But we still need to work out which of these types of motion means that the object has no net force acting on it. To help us do this, we can recall Newton’s first law of motion. This law says that an object at rest will remain at rest and an object moving with a constant velocity will continue to travel at that velocity unless acted on by an unbalanced force.
What this law means is that if there is no net force acting on an object, then that object experiences no change in velocity. We have seen that lines two and three both represent objects whose velocity is changing. Line two shows a velocity that is increasing, while line three shows a velocity that is decreasing. Using Newton’s first law of motion, we can see that both of these two lines must represent objects that do have a net force acting on them because these objects are not moving with a constant velocity.
However, we saw that line one does show an object with a constant velocity. And Newton’s first law tells us that this constant velocity means that that object has no net force acting on it. So our answer to the question is that the object which has no net force acting on it during the time interval shown is the object represented by line one.
