# Homework Help: Magnetic force problem

1. Mar 25, 2006

### Punchlinegirl

A metal ball having net charge 8.9e-6 C is thrown out of a window horizontally at a speed 47 m/s. The window is at a height 89 m above the ground. A uniform horizontal magnetic field of magnitude 0.03 T is perpendicular to the plane of the ball's trajectory. Find the magnitude of the magnetic force acting on the ball just before it hits the ground. Answer in units of N.

I started off by using $$V_y^2= V_y_i^2 + 2a_y (y-y_i)$$
So the vertical component is -$$\sqrt 2gh$$ j.
$$F_b= q v x B = q(v_i - 2 \sqrt 2gh j) x Bk = QvB(-j)-Q \sqrt 2gh B i$$
$$F_b = 8.9e-6 (47)(.03) j + (8.9e-6) \sqrt 2(9.8)(89)(.03)$$
F_b = 1.25e-5 j + 6.44e-5 i
Then I found the magnitude of F_b by squaring both terms and taking the square root to get 6.56e-5 N, but this wasn't right.. can someone help me? Thanks.

2. Mar 26, 2006

### Punchlinegirl

Can someone tell me what I'm doing wrong?