Physics 131-3 Test 1


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Questions (10 pts. apiece) Answer in complete, well-written sentences WITHIN the spaces provided.

  1. What might cause inherent uncertainties in a length measurement? Explain.

  2. How can you tell from a position versus time graph that your motion is steady (constant velocity)?

  3. Consider the velocity graph below. Which object $A$ or $B$ is faster? Explain. Which object is `ahead'? Explain.

    \includegraphics{f1.eps}

  4. A ball is dropped from rest by one physics student and filmed by another. They obtain a good fit to the data with the equation $y = -(4.72~m/s^2) t^2 + (12.14~m/s) t - (1.31~m)$. What is the physical interpretation of the second coefficient in the equation (i.e., the factor $12.14~m/s$)? Is this result this consistent with the ball being dropped from rest? Explain.

  5. Consider the figure below which shows the velocity and acceleration at the same instant for three particles. (a) For which particle is the speed decreasing? (b) For which particle is the speed constant? Explain your reasoning. Remember that speed is the distance covered divided by the change in time.

    \includegraphics{f2.eps}

Problems. Clearly show all reasoning for full credit. Use a separate sheet to show your work.

1. 15 pts. An earth satellite moves in a circular orbit $3.59\times 10^7~m$ above the Earth's surface with a period of 1 day or $8.64\times 10^4~s$. This is a special orbit called a geosynchronous one. A satellite in this orbit is always at the same point in the sky so it's easy for a ground station to find it. What is the centripetal acceleration of the satellite? How does this compare with the acceleration of gravity on Earth?

2. 15 pts. Nerve impulses typically travel through the body at about 150 miles/hour or about $67~m/s$. Imagine someone drops a brick from one meter above your big toe. You see this and immediately your brain starts the process of moving your big toe out of harms way. Compare the time it takes for the brick to fall with the time it takes for the nerve signal to get from your brain to your toe and start moving it out of the way. Assume you are 1.75 meters tall.

3. 20 pts. You are an engineer determining how quickly a test driver can bring a car to a halt before a traffic light in order to properly time the lights. For the driver to stop her car there are two phases. First, there is a certain reaction time called $t_r$ to begin braking; the car moves at constant velocity during this time. Next, after the driver applies the brakes the car slows down under constant braking deceleration. When the test driver has an initial speed of $22~m/s$ she stops in $57~m$; the combined distance for the two phases. When the test driver has an initial speed of $13~m/s$, she covers $24~m$ at the end of both phases. Assume the reaction time and the acceleration are the same for both trials. What is the reaction time of the driver and the magnitude of the deceleration?

Some constants.


Acceleration of gravity $9.8~m/s^2$ Earth's radius $6.37\times 10^6~m$