Question

Question

1). A modern art sculptor anchors 1 meter of a cast iron pole in the ground and leaves 3 meters of the pole sticking out of the ground at a 60° angle. The owner of the metal shop told him that the pole is rated to withstand up to 9000 Nm of torque before it bends. How massive can the bronze platypus be that the sculptor will hang from the end without damaging the pole? (Ignore the mass of the pole itself)
   a. 450 kg
   b. 300 kg
   c. 600 kg
   d. 30 kg. There’s no way you could make a bronze platypus bigger than that.

2). If a ball is thrown directly upward, what is the instantaneous acceleration of the ball when it reaches the highest point of its trajectory?
   a. No answer text provided.
   b. 9.8 m/s^2
   c. 0 m/s^2
   d. it depends on air resistance

3). A boy standing on a pedestrian bridge 10 meters above the walkway below sees his friend walking below and wants to throw a water balloon at him. The boy angles his throw upward to get more distance, giving the balloon a vertical component of its velocity +10 m/s and horizontal component +10 m/s. Ignoring air resistance, how far horizontally will the balloon travel before hitting the pavement?
   a. between 20 and 30 meters
   b. between 40 and 50 meters
   c. between 10 and 20 meters
   d. between 30 and 40 meters

Answer 1

2.9.81 m/s² acting downward...
When a ball is thrown into the air, neglecting air resistance, the only force acting on it is gravity. We know from Newton's second law that F = ma. Force is directly proportional to acceleration and the constant of proportionality is the mass of the ball. In this sense force is acceleration. At the instantaneous moment the ball is at maximum height, it is not moving, it's velocity is zero, but it's instantaneous acceleration is not zero. We know at the next instantaneous moment the ball begins to move downward, so the instantaneous acceleration at the maximum height cannot be zero, otherwise the ball would not move at all. The answer is the acceleration at the maximum height is the same as the acceleration of gravity which is 9.81 m/s².

The set of equations we use to describe projectile motion is also referred to as constant acceleration equations. The acceleration is constant, i.e. gravity, throughout the entire flight of the ball.