PHYSICS 231
1st Midterm, October 17, 1997, 5:30 pm - 6:30 pm
Instructor: P. Q. Hung

Do all problems (20 points each). READ them carefully. You have to EXPLAIN all your answers. DO NOT just write them down. ANSWERS WITHOUT EXPLANATION WILL BE GIVEN NO CREDIT.

Useful formulas:

x=x₀+v_0xt+(1/2)a_xt²

y=y₀+v_0yt+(1/2)a_yt²

v_x=v_0x+a_xt

v_y=v_0y+a_yt

$v_{f}^2 = v_{i}^2 + 2 a \Delta x$

$R = (v_{0}^{2}/g) sin(2\theta_{0})$

$\sum {\vec{F}}_{i} = m \vec{a}$

$f_{s, max} = \mu_s N$

$f_k = \mu_k N$

g=9.8m/s²

I) A Moon rock is thrown upward with velocity 7 m/s. After 7 s, it has a downward velocity of 4 m/s. What is the acceleration due to gravity on the Moon (10 pts)? How high above the starting point did the rock go before it began to fall (10 pts)?

II) A movie stuntman is to run across a rooftop and jump horizontally off it, to land on the roof of the next building (see Figure below). Before he attempts the jump, he wisely asks you to determine whether it is possible. Can he make the jump if his maximum rooftop speed is 4.5 m/s? Explain. An answer with just YES or NO will not be acceptable.

III) The Figure below shows four penguins that are playfully pulled along very slippery (frictionless) ice by a curator. The masses of 3 penguins and the tension in two of the cords are given. Find the penguin mass that is not given (15 pts). Find the acceleration (5 pts).

IV) Two blocks are connected over a pulley as shown in the Figure below. The mass of block A is 10 kg and the coefficient of kinetic friction is 0.20. The angle $\theta$ is 30⁰. Block A slides down the incline at constant speed. What is the mass of block B?