Velocity & Speed Homework: Average Speed & Velocity

[-EquinoX-]

Homework Statement

A person walks first at a constant speed of 5 m/s along a straight line from point A to
point B and then back along the line from B to A at a constant speed of 3 m/s. a. What is
her average speed over the entire trip? b. What is her average velocity over the entire
trip?

Homework Equations

The Attempt at a Solution

I am confused with the total distance between A and B. As far as I know for the average
speed I need to find the total distance from A to B and B to A. However how do I find
that? I also need the time as well.

 

[LowlyPion]

Homework Statement

A person walks first at a constant speed of 5 m/s along a straight line from point A to
point B and then back along the line from B to A at a constant speed of 3 m/s. a. What is
her average speed over the entire trip? b. What is her average velocity over the entire
trip?

Homework Equations

The Attempt at a Solution

I am confused with the total distance between A and B. As far as I know for the average
speed I need to find the total distance from A to B and B to A. However how do I find
that? I also need the time as well.

You can always solve it by defining some distance variable and calling it D.

Note: They are asking you for Avg Speed and Avg Velocity. Do you know what the difference is between them?

 

[-EquinoX-]

average speed is the total distance traveled divided by delta t and average velocity is delta x divided by delta t

 

[LowlyPion]

average speed is the total distance traveled divided by delta t and average velocity is delta x divided by delta t

What are the equations then for figuring the amount of time for the first leg and the second.

I know you don't know the distance D between A and B, but put it in the equation anyway.

 

[-EquinoX-]

What are the equations then for figuring the amount of time for the first leg and the second.

I know you don't know the distance D between A and B, but put it in the equation anyway.

that's my confusion, of finding out the time. I think the equation is s = v/t, so t is equal to v/s?

 

[ValenceE]

Homework Statement

A person walks first at a constant speed of 5 m/s along a straight line from point A to
point B and then back along the line from B to A at a constant speed of 3 m/s. a. What is
her average speed over the entire trip? b. What is her average velocity over the entire
trip?

Homework Equations

The Attempt at a Solution

I am confused with the total distance between A and B. As far as I know for the average
speed I need to find the total distance from A to B and B to A. However how do I find
that? I also need the time as well.

average speed ... i came up with 4m/s... velocity... ?


VE

Edit: I'm sure this is wrong... certainly missing some subtelties...

 

[LowlyPion]

that's my confusion, of finding out the time. I think the equation is s = v/t, so t is equal to v/s?

That's the wrong equation.

V = s/t which means t = s/V

With S set to D and substituting the values of the velocities given then what is the total time? (time of the first leg plus time of the second)

 

[ValenceE]

ok... 3.75 m/s... velocity ?...


VE

 

[-EquinoX-]

ok... 3.75 m/s... velocity ?...VE

where did you get 3.75 from? and I know the average speed is 0 here as the total distance traveled is 0, as it went from A to B and B to A again, therefore they went back to the same place again where it started.

 

[LowlyPion]

average speed ... i came up with 4m/s... velocity... ?


VE

Edit: I'm sure this is wrong... certainly missing some subtelties...

4 m/s is wrong.

Actually not so subtle. The math lays it all bare. Follow where it leads.

 

[LowlyPion]

where did you get 3.75 from? and I know the average speed is 0 here as the total distance traveled is 0, as it went from A to B and B to A again, therefore they went back to the same place again where it started.

No you have confused speed with velocity.

Velocity is a vector. Magnitude and displacement.

Speed is a scalar magnitude.

 

[LowlyPion]

where did you get 3.75 from?

Do the math and you will see how to find the average speed.

 

[-EquinoX-]

so you mean that average speed is not 0?

 

[-EquinoX-]

Do the math and you will see how to find the average speed.

first I need to find delta x and delta t, how do I find delta t? even I don't know the time

 

[LowlyPion]

so you mean that average speed is not 0?

Average speed is not 0.

Average velocity is 0. Velocity implies both direction of displacement as well as magnitude for the time taken.

The first paragraph of this article might be instructive to you:
http://en.wikipedia.org/wiki/Velocity

In physics, velocity is defined as the rate of change of position. It is a vector physical quantity; both speed and direction are required to define it.

 

[-EquinoX-]

Average speed is not 0.

Average velocity is 0. Velocity implies both direction of displacement as well as magnitude for the time taken.

The first paragraph of this article might be instructive to you:
http://en.wikipedia.org/wiki/Velocity

Oh yea, sorry, I got that reversed.. now for the average speed. I need the total distance travelled, which is 2D and the delta t, which is??

 

[LowlyPion]

first I need to find delta x and delta t, how do I find delta t? even I don't know the time

V = x/t

This means

t = x/V

If x=D how long to do the first leg?

 

[-EquinoX-]

never mind, I got 3.75. Thanks all