View Resource. Trowbridge, D. American Journal of Physics, 49 3 , Saltiel, E. European Journal of Physics, 1 2 , Forces and Motion Many students think that, if an object has a speed of zero even instantaneously , it has no acceleration Number of Resources 1 Number of References 1 Number of Diagnostic Resources 1.
Add a comment. Active Oldest Votes. Improve this answer. JackI JackI 1, 7 7 silver badges 24 24 bronze badges. No, it does not. Sign up or log in Sign up using Google. Sign up using Facebook. Sign up using Email and Password. Post as a guest Name. Email Required, but never shown. Featured on Meta. The reason for the units becomes obvious upon examination of the acceleration equation. Since acceleration is a vector quantity , it has a direction associated with it.
The direction of the acceleration vector depends on two things:. The general principle for determining the acceleation is:.
This general principle can be applied to determine whether the sign of the acceleration of an object is positive or negative, right or left, up or down, etc. Consider the two data tables below. In each case, the acceleration of the object is in the positive direction. In Example A, the object is moving in the positive direction i.
When an object is speeding up, the acceleration is in the same direction as the velocity. Thus, this object has a positive acceleration. In Example B, the object is moving in the negative direction i. According to our general principle , when an object is slowing down, the acceleration is in the opposite direction as the velocity.
Thus, this object also has a positive acceleration. This same general principle can be applied to the motion of the objects represented in the two data tables below. In each case, the acceleration of the object is in the negative direction. In Example C, the object is moving in the positive direction i. According to our principle , when an object is slowing down, the acceleration is in the opposite direction as the velocity.
Thus, this object has a negative acceleration. This is reasonable because the train starts from rest and ends up with a velocity to the right also positive. So acceleration is in the same direction as the change in velocity, as is always the case.
Now suppose that at the end of its trip, the train in Figure 7 a slows to a stop from a speed of What is its average acceleration while stopping? Figure 9. In this case, the train is decelerating and its acceleration is negative because it is toward the left. As in the previous example, we must find the change in velocity and the change in time and then solve for acceleration. The minus sign indicates that acceleration is to the left.
This sign is reasonable because the train initially has a positive velocity in this problem, and a negative acceleration would oppose the motion. Again, acceleration is in the same direction as the change in velocity, which is negative here. This acceleration can be called a deceleration because it has a direction opposite to the velocity.
The graphs of position, velocity, and acceleration vs. We have taken the velocity to remain constant from 20 to 40 s, after which the train decelerates. Figure Its position then changes more slowly as it slows down at the end of the journey.
In the middle of the journey, while the velocity remains constant, the position changes at a constant rate. It remains the same in the middle of the journey where there is no acceleration. It decreases as the train decelerates at the end of the journey. The train has positive acceleration as it speeds up at the beginning of the journey. It has no acceleration as it travels at constant velocity in the middle of the journey. Its acceleration is negative as it slows down at the end of the journey.
What is the average velocity of the train in part b of Example 2, and shown again below, if it takes 5. Average velocity is displacement divided by time. It will be negative here, since the train moves to the left and has a negative displacement.
Finally, suppose the train in Figure 2 slows to a stop from a velocity of As before, we must find the change in velocity and the change in time to calculate average acceleration. The change in velocity here is actually positive, since. This is reasonable because the train initially has a negative velocity to the left in this problem and a positive acceleration opposes the motion and so it is to the right.
Again, acceleration is in the same direction as the change in velocity, which is positive here. As in Example 5, this acceleration can be called a deceleration since it is in the direction opposite to the velocity.
Perhaps the most important thing to note about these examples is the signs of the answers. In our chosen coordinate system, plus means the quantity is to the right and minus means it is to the left. This is easy to imagine for displacement and velocity. But it is a little less obvious for acceleration. Most people interpret negative acceleration as the slowing of an object. This was not the case in Example 2, where a positive acceleration slowed a negative velocity.
The crucial distinction was that the acceleration was in the opposite direction from the velocity. In fact, a negative acceleration will increase a negative velocity.
For example, the train moving to the left in Figure 11 is sped up by an acceleration to the left. In that case, both v and a are negative. The plus and minus signs give the directions of the accelerations. If acceleration has the same sign as the change in velocity, the object is speeding up. If acceleration has the opposite sign of the change in velocity, the object is slowing down.
If we take east to be positive, then the airplane has negative acceleration, as it is accelerating toward the west.
It is also decelerating: its acceleration is opposite in direction to its velocity. Learn about position, velocity, and acceleration graphs. Move the little man back and forth with the mouse and plot his motion.
0コメント