A magic medallion is suspended from a string inside a compartment of Hogwarts Express which is running straight westwards on horizontal tracks.

(a) Draw a free-free body diagram for the medallion if the train is moving at constant speed.

(b) Draw a free-body diagram for the medallion if the train is accelerating at a constant rate. Indicate acceleration and the direction of the net force.

(c) Calculate the angle the string makes with the vertical if the train accelerates at a constant rate from rest to 20 m/s in 10 seconds.

(d) If the mass of the medallion is doubled, how will the angle change?