Physics H131: Hints for Problem Set #11

Chapter N3 Problems

• N3B.4: Identify all interactions acting on the child. What is the child's trajectory? Where does the acceleration vector point? This should tell you where the net force vector points.
• N3S.5: Why does the car come to rest? What happens to its kinetic energy? How can you obtain the friction/drag force from this?
• N3S.9: This problem gives you an expression for the velocity as function of time and asks for the force which causes this kind of motion. Use the kinematic chain to solve the problem.
• N3R.2: The problem tells you how long the rockets burn. If you knew their thrust force, which quantity could you calculate from this? How does this allow you to compute your velocity after the rockets stop firing? What does the given ejection height tell you about this velocity? Combine these conditions, solve them for the thrust force and compute the average acceleration of the seat during the rocket burst.

Chapter N4 Problems

• N4B.2: You can choose the slope or area method, whichever you prefer.
• N4S.7: (a) The exponential function is only defined for unitless arguments! (b) and (c) assume the ball rolls off at t=0 with negligible horizontal and vertical velocity. The problem statement gives you the acceleration on the ball as a function of time after t=0. Use integration to determine the ball's velocity and position as a function of time. What are the integration limits?
• N4R.1: Assume that the interstate is a straight line. Compute the trajectory x(t) of the speeding car. Then compute your own trajectory from the given data on your acceleration. What are the integration limits for the time integration? Be careful to split the integration interval into two intervals, corresponding to the different acceleration laws at early and late times. Where and when do the trajectories x(t) intersect?