How do I find the motion equations from this problem?

[RyanH42]

Homework Statement

I have a picture of the question

Homework Equations

I don't know the equation realted to this question

The Attempt at a Solution

I didnt understand the question

 

[vela]

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

 

[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 don't know why ??
For c I am stucked I know the answer of d its zero

 

[vela]

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.

 

[RyanH42]

Lets look at then please wait me

 

[RyanH42]

For a) the average speed is $s_a=Δx/Δt$ ok.So half of the time the car goes 55 km/h let's 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 $t_1=t_2.1,636$

$s_a=Δx/Δt$

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

 

[RyanH42]

Is that wrong ?

 

[vela]

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

 

[RyanH42]

I made a wrong in b but I don't know why I check the answer
total time=t₁+t₂

 

[RyanH42]

Now I fixed

 

[RyanH42]

lets come to see c) which its (s_avg1+s_avg2)/2

 

[vela]

The average speed is not the average of the speeds.

 

[RyanH42]

then what it is ?

 

[vela]

Go back to the definition. You used it already.

 

[RyanH42]

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