30.15 Calculate the speed of ions that pass undeflected through crossed E and B fields for which E =7.7 kV/m and B = 0.14 T. Ans. 55 km/s

Could you explain on how tan-1 (-3/0) = -90 I am doing vectors problems. thank you.

Solve #3, including: a, b, and c

Children playing in a playground on the flat roof of a city school lose their ball to the parking lot below. One of the teachers kicks the ball back up to the children as shown in the figure below. The playground is 4.80 m above the parking lot, and the school building's vertical wall is h = 6.30 m high, forming a 1.50 m high railing around the playground. The ball is launched at an angle of 0 53.0° above the horizontal at a point d 24.0 m from the base of the building wall. The ball takes 2.20 s to reach a point vertically above the wall. (Due to the nature of this problem, do not use rounded intermediate values in your calculations-including answers submitted in WebAssign.) = = (a) Find the speed (in m/s) at which the ball was launched. x What trigonometric function should you use to find the x-component of the initial velocity? m/s