# Change in kinetic energy

1. Jun 2, 2008

### forty

If an object goes from moving at 10 m/s eastward to moving at 30 m/s southward, how does its kinetic energy change?

If the object is to to change its velocity from east to south it would have stop all of its eastwards motion and then move in the southwards direction?

i have no idea if this is correct but i did the following

.5m(10)^2 + .5m(30)^2

and get 500m (mass is not given in the problem)

Any help would be greatly appreciated!

2. Jun 2, 2008

### konthelion

This object is not moving in a one-dimension but in two-dimension. So, you can't just add/subtract them. You can use the Pythagorean theorem to find the change in velocity.

You don't need the mass. Since you are asked to find the change in kinetic and not the numerical values for the initial and final kinetic energy$$\Delta K=\frac{1}{2}m(v_{f}-f{i})^2$$

3. Jun 3, 2008

### tiny-tim

Hi forty!

No … kinetic energy is a number, not a vector.

What's the KE before? What's the KE after? How do you subtract one from the other?

4. Jun 3, 2008

### forty

so are you saying its just

.5m(30)^2 - .5m(10)^2 = 400m (J)

??

5. Jun 4, 2008

### tiny-tim

Yup!

(of course, the result will be in J only if the mass is in kg)

Sometimes physics really is that easy!