Velocity of car after 180 degree turn

[ujellytek]

Imagine a car is traveling east 60km/h, then after leaving the curve it's velocity is 60km/h west, what's it's change in velocity?

 

[Doc Al]

What do you think?

 

[ujellytek]

What do you think?

I thought it was 0km/h but it was marked wrong, I just want to understand why.

 

[Doc Al]

I thought it was 0km/h but it was marked wrong, I just want to understand why.

Recall that velocity is a vector: direction matters. (If they had asked for the change in speed, then that would be zero.)

 

[ujellytek]

Recall that velocity is a vector: direction matters. (If they had asked for the change in speed, then that would be zero.)

Can you explain how to solve this?

 

[Doc Al]

Choose a sign convention. Let's say east is positive and west is negative.

Given that, what would be the initial and final velocities, including their sign?

 

[ujellytek]

Choose a sign convention. Let's say east is positive and west is negative.

Given that, what would be the initial and final velocities, including their sign?

v1=60 km/h [E] v2=60 km/h [W] ? *delta*v=??

 

[Doc Al]

v1=60 km/h [E] v2=60 km/h [W] ? *delta*v=??

Instead of E & W, use + and -.

v1 = + 60 km/h
v2 = ?

 

[ujellytek]

Instead of E & W, use + and -.

v1 = + 60 km/h
v2 = ?

I have no idea what to do

 

[Doc Al]

I have no idea what to do

If the initial velocity (to the east) is + 60, what must the final velocity (to the west) be?

 

[ujellytek]

If the initial velocity (to the east) is + 60, what must the final velocity (to the west) be?

-120, thanks I get it now

 

[Doc Al]

-120, thanks I get it now

Good. The change in velocity is 120 km/h to the west.

 

[Andrew Mason]

Just a follow up comment on this, it is always a good idea to use vectors. While + and - works for this problem where the velocities are along the same line, using vectors is a better and more general way to solve it.

The question asks for the difference or change in velocity. One must subtract the initial velocity from the final velocity to determine the change in velocity:
$\vec{\Delta{v}} = \vec{v_f} - \vec{v_i}$

If, like me, you find vector subtraction to be a bit confusing, you can easily turn it into a vector addition equation:
$\vec{v_f} = \vec{v_i} + \vec{\Delta{v}}$

You want to find $\vec {\Delta{v}}$. If you draw $\vec{v_f} \text{ and } \vec{v_i}$ tail to tail and then ask your self "what vector added to the initial velocity vector (ie. its tail touching the arrow of the initial velocity vector) results in the final velocity vector?" (ie. its arrow touches the arrow of the final velocity vector) you will have your answer. Since the change in velocity is a vector you must specify its direction as well as its magnitude (ie. its length which represents speed).

AM

 

[NascentOxygen]

With vector problems, ALWAYS draw the triangle or whatever figure.