What is the speed of a periodic wave with a frequency of 4.5 hz and a wavelength of 0.9m

Why is there friction between objects

Suppose a wire with a current is pushed upward by a magnetic field. how would the wire move if the direction of the current reversed? explain your answer.

Ahot-air balloon takes off from the ground traveling vertically with a constant upward acceleration of magnitude g/4. after time interval δt, a crew member releases a ballast sandbag from the basket attached to the balloon. how many seconds does it take the sandbag to reach the ground? (express your answer in terms of δt)