# Homework Help: Velocity and speed

1. Aug 28, 2008

### -EquinoX-

1. The problem statement, all variables and given/known data
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?

2. Relevant equations

3. 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.

2. Aug 28, 2008

### LowlyPion

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?

3. Aug 28, 2008

### -EquinoX-

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

4. Aug 28, 2008

### LowlyPion

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.

5. Aug 28, 2008

### -EquinoX-

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

6. Aug 28, 2008

### ValenceE

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

VE

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

7. Aug 28, 2008

### LowlyPion

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)

8. Aug 28, 2008

### ValenceE

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

VE

9. Aug 28, 2008

### -EquinoX-

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.

10. Aug 28, 2008

### LowlyPion

4 m/s is wrong.

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

11. Aug 28, 2008

### LowlyPion

No you have confused speed with velocity.

Velocity is a vector. Magnitude and displacement.

Speed is a scalar magnitude.

12. Aug 29, 2008

### LowlyPion

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

13. Aug 29, 2008

### -EquinoX-

so you mean that average speed is not 0?

14. Aug 29, 2008

### -EquinoX-

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

15. Aug 29, 2008

### LowlyPion

Average speed is not 0.

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

http://en.wikipedia.org/wiki/Velocity

16. Aug 29, 2008

### -EquinoX-

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??

17. Aug 29, 2008

### LowlyPion

V = x/t

This means

t = x/V

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

18. Aug 29, 2008

### -EquinoX-

never mind, I got 3.75. Thanks all