a. Standing 23.2 m away from a rock wall, you yell. How much time in seconds will it take you to hear your echo to two significant digits? Make sure to account for the travel from you to the wall and from the wall back to you.

b. Two moons orbit the same planet. The inner moon at an average distance of 1.9110^8m and has an orbital period of 2.1610^5s. Calculate Kepler's constant for this planet and use it with the outer moon's orbital radius of 4.3610^8mto make an estimate of theorbital acceleration of the outer moon in meters per second per second to two significant figures.

c. Lifting a 74-kg barbell and weights from rest up to a speed of 1.0 m/s in 0.50 s, resisted by the combined weight of the barbell and weights, requires what applied force in Nto two significant digits?