# Motion in physics

1. Oct 10, 2015

### RyanH42

1. The problem statement, all variables and given/known data
I have a picture of the question

2. Relevant equations
I dont know the equation realted to this question

3. The attempt at a solution
I didnt understand the question

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2. Oct 10, 2015

### vela

Staff Emeritus
Why don't you start by identifying what the problem is asking for? What are the definitions for those quantities?

3. Oct 10, 2015

### RyanH42

For a ) I used this $s_a=(d_1/t_1+d_2/t_2)/2$ for b)I did $s_a=(2d/(t_1+t_2))$
It worked but I dont know why ??
For c I am stucked I know the answer of d its zero

4. Oct 10, 2015

### vela

Staff Emeritus
You might find it helpful to explain your reasoning when writing out your solutions. You should definitely do it when posting here rather than expecting us to read your mind. If you're just plugging in numbers to random formulas with no real understanding why, you're setting yourself up for failure in physics.

Your method for (a) looks wrong. Your method for (b) is wrong. The units don't work out. You can't add d/t1 to t2.

5. Oct 10, 2015

### RyanH42

Lets look at then please wait me

6. Oct 10, 2015

### RyanH42

For a) the average speed is $s_a=Δx/Δt$ ok.So half of the time the car goes 55 km/h lets call total trip t and total distance d so the equation will be $55km/h.t_1+90km/h.t_1=d$ , $t_1=t/2$ average speed is total distance divided by time so $d/2t_1= 145t_1/2t_1=72.5 km/h$

For b) So half of the distance is $d/2$ so ;

$d/2=55km/h.t_1$
$d/2=90km/h.t_2$

so $t_1/t_2=1,636$ so $t_1=t_2.1,636$

$s_a=Δx/Δt$

$s_a=d/(t_1+t_2)=180t_2/(t_2+t_2.1.636)$

Last edited: Oct 10, 2015
7. Oct 10, 2015

### RyanH42

Is that wrong ?

8. Oct 10, 2015

### vela

Staff Emeritus
Did you mix up $t_1$ and $t_2$ in the denominator in (b)? (a) looks right.

9. Oct 10, 2015

### RyanH42

I made a wrong in b but I dont know why I check the answer
total time=t1+t2

10. Oct 10, 2015

### RyanH42

Now I fixed

11. Oct 10, 2015

### RyanH42

lets come to see c) which its (savg1+savg2)/2

12. Oct 10, 2015

### vela

Staff Emeritus
The average speed is not the average of the speeds.

13. Oct 10, 2015

### RyanH42

then what it is ?

14. Oct 10, 2015

### vela

Staff Emeritus
Go back to the definition. You used it already.

15. Oct 10, 2015

### RyanH42

$s_a=Δx/Δt$
Δx=2d
Δt=2t
Can you show me how to do that please