| Problem statement Solution video |
DISCUSSION THREAD
Ask and answer questions here. You learn both ways.
Discussion
Consider the animation below of the motion for particle P:
- For all time, P moves with a positive x-component and a negative y-component of both velocity and acceleration. Why? Can you see this in the animation?
- As we will see in the next class period when we talk about the "path description" of kinematics, the velocity of a point is always tangent to the path of the point, and the acceleration always points "inward" on the path of the point. Do you see this in the animation?
The components of velocity and acceleration of P are found directly from the time derivatives of the position components:
vP = x_dot i + y_dot j
aP = x_ddot i + y_ddot j
In the “given” section, if we’re doing our work digitally, can we use a screenshot of the diagram from the problem or do we have to redraw it? I know the information has to be written in our own words.
Yes, it is OK to use the screen shot of the diagram.
Should the velocity and acceleration be written as vectors or as scalar magnitudes?
Yes, velocity and acceleration are always vectors. If we want the magnitude of the velocity vector, we will ask for speed, and if we want the magnitude of the acceleration vector, we will as for the magnitude.
where can I find the template for the homework?
On the top banner, click Course material > Course Information > Homework format. It should take you to a PDF template of sorts.
It is also in the syllabus that was emailed to us. Hope that helps!
Does this problem have a traditional or flipped coordinate system? That is, does pin P have a negative horizontal position since it lies to the left of the y-axis? I'm wondering this because the expression for the x position is always positive and it's stated that "P moves with a positive x-component", even though the picture and animation show the pin staying left of the y-axis. Usually, the right side of the y-axis has the positive x values.
I took the x axis shown to be the positive axis even though it extends to the left because of the statement that you quoted, and the fact that I have used that "flipped coordinate system" in the past.
I believe the position of the pin is not important because we only need to find the velocity and acceleration of the x and y-components.
Should we have our answers as vectors or magnitudes?
I believe it is vector notation.
The answers should be in vector notation but make sure to use the i-hat, j-hat, and k-hat notation instead of the notation.
In the animation, are the y-components negative due to the force of gravity?
The y-components of both velocity and acceleration are negative because both the first and second derivative of y(t) with respect to time are negative for t>0. Gravity is not an issue on problems involving strictly kinematics.