Physics H131: Hints for Problem Set #12

Chapter N4 Problem

• N4R.2: Introduce symbols for the time of the splash and the time when the sound hits you. How can you calculate the first and the difference between the second and the first? Do all calculations symbolically, combining the equations and solving them for the well depth D.

Chapter N5 Problems

• N5T.3: Without the hinge, what would the piano cover do? Think of sliding down an incline.
• N5B.3: What is the condition for the meter stick to balance out?
• N5S.6: Draw a good picture of the situation! You should identify 5 forces. What do you take as the point around which the board would rotate if it started tipping? What is the condition for stability, i.e. that the board doesn't tip when the person stands on it?
• N5S.7: Draw a good picture of the situation! Identify all relevant forces (4 of them if the ladder is approximately massless). What is a convenient choice of the orientation and origin of the reference frame? What are the conditions for static stability? Pay careful attention to the signs and magnitudes of the angles between the force vectors and the r-vectors where they attack! (Check that the resulting signs for the torque components make sense.)
• N5R.1: Make a schematic drawing, showing the coordinate axes, choice of origin, and the three forces acting on your forearm. Write down the condition for static equilibrium. Explain the magnitudes of all numbers which you use.

Chapter N6 Problems

• N6T.7: How large is the normal force in this case? How large a friction force must your tires be able to supply for your car to climb that slope?
• N6B.4: What does the problem statement tell you about the net force? Write down the net force. How does the speed at which you push the box enter? Does it matter? How would things change if you pushed the box at 4 m/s?
• N6B.8: You can assume that the glider starts from rest. This problem is a "constant acceleration" problem. How large is the acceleration?