A charged pendulum bob of mass m and charge q is at angle 60° above the +x-axis as shown. A uniform electric field E acts along +x. When released and it passes the x-axis, its speed is:

Question

G1 Physics · Accepted Answer

Only electric force does work on the smooth horizontal setup. Horizontal displacement along the field is l-l cos60°=l/2. Thus qE(l/2)=(1/2)mv², giving v=√(qEl/m).