# Physics question, solved but need clarification

1. Mar 3, 2009

### RS7

Physics question, solved but need clarification!!

1. The problem statement, all variables and given/known data
A bird take 8.5 s to fly from position A to position B. Determine the bird's avg acceleration
Vb = 7.8 m/s [25 degrees N of E]
Va = 4.4 m/s [ 31 degrees S of E]

2. Relevant equations
Vax/Va = cos31
Vax/4.4 m/s = cos31
3.8 m/s

Vay/Va = sin31
Vay/4.4 m/s = sin31
2.3 m/s

Vbx/Vb = cos25
Vbx = 7.8cos25
7.1m/s

Vby/Vb = sin25
Vby= 7.8sin25
3.3m/s

Ay = Vby- Vay / t
3.3-2.3/ 8.5 s
0.12 m/s^2

Ax = Vbx- Vax / t
7.1 m/s - 3.8 m/s / 8.5 s
0.4 m/s^2

a^2 = Ax^2 + Ay^2
0.16 + 0.0144
0.174 m/s^2 and then you square root it 0.41 m/s^2

tantheta = Ay/Ax
0.12 m/s^2/0.4 m/s^2
0.3 = 17 degrees

3. The attempt at a solution

The bird's avg acceleration is 0.41 m/s^2 [17 degrees N of E]

I just need to make sure if I did this right, if anyone would clarify this that would be great!!
thanks

2. Mar 4, 2009

### Redbelly98

Staff Emeritus
Re: Physics question, solved but need clarification!!

Welcome to PF

It's mostly correct. You'll want to recheck +/- signs for the velocities. Is Northward or Southward the "positive y" direction?

3. Mar 4, 2009

### RS7

Re: Physics question, solved but need clarification!!

oo then if I messed up on the signs I guess my answer is wrong, and northward is the positive y direction I think

4. Mar 4, 2009

### RS7

Re: Physics question, solved but need clarification!!

Vax/Va = cos31
Vax/4.4 m/s = cos31
3.8 m/s

Vay/Va = sin31
Vay/4.4 m/s = sin31
- 2.3 m/s

Vbx/Vb = cos25
Vbx = 7.8cos25
7.1m/s

Vby/Vb = sin25
Vby= 7.8sin25
3.3m/s

Ay = Vby- Vay / t
3.3 + 2.3/ 8.5 s
0.7 m/s^2

Ax = Vbx- Vax / t
7.1 m/s - 3.8 m/s / 8.5 s
0.4 m/s^2

a^2 = Ax^2 + Ay^2
0.16 + 0.49
0.65 m/s^2 and then you square root it 0.8 m/s^2

tantheta = Ay/Ax
0.7m/s^2/0.4 m/s^2
1.75 = 60 degrees

the bird's avg acceleration is 0.8 m/s^2 [60 degrees N of E]

is this right? :S
I used the signs for the velocities but not quite sure if I did it right...

5. Mar 4, 2009

### Redbelly98

Staff Emeritus
Re: Physics question, solved but need clarification!!

Looks good.