# Homework Help: Velocity of car after 180 degree turn

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1. Nov 30, 2016

### ujellytek

• HW Template missing as it was moved from another forum
Imagine a car is travelling east 60km/h, then after leaving the curve it's velocity is 60km/h west, what's it's change in velocity?

2. Nov 30, 2016

### Staff: Mentor

What do you think?

3. Nov 30, 2016

### ujellytek

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

4. Nov 30, 2016

### Staff: Mentor

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

5. Nov 30, 2016

### ujellytek

Can you explain how to solve this?

6. Nov 30, 2016

### Staff: Mentor

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?

7. Nov 30, 2016

### ujellytek

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

8. Nov 30, 2016

### Staff: Mentor

Instead of E & W, use + and -.

v1 = + 60 km/h
v2 = ?

9. Nov 30, 2016

### ujellytek

I have no idea what to do

10. Nov 30, 2016

### Staff: Mentor

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

11. Nov 30, 2016

### ujellytek

-120, thanks I get it now

12. Nov 30, 2016

### Staff: Mentor

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

13. Nov 30, 2016

### 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

Last edited: Nov 30, 2016
14. Nov 30, 2016

### Staff: Mentor

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