Physics 50 Sample Midterm Exam #2 (25 points) by Todd Sauke Name_____________________________ Question #1. (6 points) At time t = 0 s, a wheel has an angular displacement of zero radians and an angular velocity of +28 rad / s. The wheel has a constant acceleration of –0.36 rad / s2. We want to find the maximum value of the angular displacement, (in radians), for the wheel. (This is how far in radians the wheel turns before the negative acceleration stops the wheel and makes it begin to go backwards.) In the following table of rotational kinematic variables, enter the data, including implied data, for which we know the values. | θ | θ0 | ω | ω0 | α | t |

1 point Write the equation from the study guide that relates the known variables with the target variable. ___________________________________________ 1 point Solve the equation for the target variable and write the maximum value of the angular displacement, (in radians), for the wheel. ____________________________ rad 3 points How many complete revolutions does this correspond to? ____________________________ revolutions 1 point

Question #2. (9 points) In the figure above, a constant external force P = 170 N is applied as shown to a 20 kg box, which is on a rough horizontal surface. The force pushes the box a distance of 8.0 m, in a time interval of 5.7 s, and the speed changes from v1 = 0.3 m/s to v2 = 2.5 m/s. What is the work done by the external force P? ___________________________________ _______ 1 point units ½ point What is the normal force between the box and the horizontal surface?

_______________________________ N 1 point

What is the work done by the normal force? ___________________________________ _______ 1 point units ½ point Write an equation, derived from two adjacent equations in the study guide, relating the total work done on an object to its initial and final velocities.

_______________________________ 1 point

What is the work done by friction? (You do not need to know the coefficient of static friction to figure this out.) ___________________________________ _______ 1 point units ½ point What is the coefficient of kinetic friction between the box and the horizontal surface?

_______________________________ 1 point

What is the power being supplied by the force P? ___________________________________ _______ 1 point units ½ point

Question #3. (2 points) Suppose that a heavy person and a light person are balanced on a teeter-totter made of a plank of

wood. Each person now moves in toward the fulcrum a distance of 25 cms. What effect will this have on the balance of the teeter-totter?

A) The light person's end will go down.

B) The heavy person's end will go down.

C) The teeter-totter will remain in balance.

D) One cannot tell whether either end will rise or fall without knowing the mass of the plank.

E) Only if the plank has significant mass will the light person's end go down. Question #4. (2 points) In the figure below, determine the character of the collision. The masses of the blocks, and the velocities

before and after are given. The collision is: A) characterized by an increase in kinetic energy

B) perfectly elastic

C) partially inelastic

D) completely inelastic

E) not possible because momentum is not conserved.

Question #5. (2 points) Which of the following is an accurate statement?

A) The angular momentum of a moving particle depends on the specific origin with respect to which the angular momentum is calculated.

B) If the torque acting on a particle is zero about an arbitrary origin, then the angular momentum of the particle is also zero about that origin.

C) A particle moving in a straight line with constant speed necessarily has zero angular momentum.

D) If the speed of a particle is constant, then the angular momentum of the particle about any specific origin must also be constant.

E) Consider a planet moving in a circular orbit about a star. Even if the planet is spinning it is not possible for its total angular momentum to be zero.

Question #6. (4 points) A sand mover at a quarry lifts 2,000 kg of sand per minute a vertical distance of 12 meters. The sand is initially at rest and is discharged at the top of the sand mover with a speed of 5 m/s into a loading chute. We want to determine the minimum rate at which power must be supplied to this machine. For the acceleration due to gravity, use g = 9.8 m/s2. What is the change in kinetic energy per kilogram of sand moved? _______________________________ J/kg 1 point

What is the change in potential energy per kilogram of sand lifted? _______________________________ J/kg 1 point

How many kilograms of sand are moved per second? _______________________________ kg/s 1 point At what minimum rate must power be supplied to this machine? _________________________ kW (kilo-Watts)