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Chapter 2: Q25P (page 84)
A runner starts from rest and in3 sreaches a speed of . Assume that her speed changed at a constant rate (constant net force).
(a) What was her average speed during this 3 s interval?
(b) How far did she go in this 3 s interval?
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Question: the Hall effect can be used to determine the sign of the mobile charges in a particular conducting material. A bar of a new kind of conducting material is connected to a battery as shown in Figure 20.85. In this diagram, the x-axis runs to the right, the y-axis runs up, and the z-axis runs out of the page, toward you. A voltmeter is connected across the bar as shown, with the leads placed directly opposite each other along a vertical line. In order to answer the following question, you should draw a careful diagram of the situation, including all relevant charges, electric fields, magnetic fields, and velocities.
Initially, there is no magnitude filed in the region of the bar. (a) Inside the bar, what is the direction of the electric field due to the charges on the batteries and the surface of the wires and the bar? This is the electric field that drives the current in the bar. (b) If the mobile charges in the bar are positive in what direction do they move when the current runs? (c) If the mobile charges in the bar are negative, in what direction do they move when the current runs? (d) In this situation (zero magnetic fields), what is the sign of the reading on the voltmeter?
Next, large coils (not shown) are moved near the bar. And current runs through the coils, making a magnetic field in the -z direction (into the page). (e) If the mobile charges in the bar are negative, what is the direction of the magnetic force on the mobile charge? (f) If the mobile charges in the bar are negative, which of the following things will happen? (1) Positive charge will accumulate on the top of the bar. (2) The bar will not becomes polarized. (3) Negative charge will accumulate on the left end of the bar. (4) Negative charge will accumulate on the top of the bar. (g) If the mobile charges in the bar are positive, what is the direction of the magnetic force on the mobile charges? (h) If the mobile charges in the bar are positive, which of these things will happen? (1) positive charge will accumulate on the top of the bar. (2) The bar will not becomes polarized. (3) Positive charge will accumulate on the right end of the bar. (4) Negative charge will accumulate on the top of the bar.
You look at the voltmeter and find that the reading on the meter is . (i) What can you conclude from this observation? (Remember that a voltmeter gives a positive reading if the positive lead is attached to the higher potential location.) (1) There is not enough information to figure out the sign of the mobile charges. (2) The mobile charges are negative. (3) The mobile charges are positive.
A steel safe with mass 2200kg falls onto concrete. Just before hitting the concrete its speed is 40m/s , and it smashes without rebounding and ends up being 0.06m shorter than before. What is the approximate magnitude of the force exerted on the safe by the concrete? How does this compare with the gravitational force of the Earth on the safe? Explain your analysis carefully, and justify your estimates on physical grounds.
Question: Because the change of the momentum is equal to the net impulse, the relationship of momentum itself to the net force is somewhat indirect, as can be seen in this question. An object is initially moving in the + x direction with a magnitude of momentum P , with a net force of magnitude F acting on the object in either the + x or - x direction. After a very short time, say whether the magnitude of the momentum increases, decreases, or stays the same in each of the following situations:
a) the net force acts in the + x direction and is F constant.
b) the net force acts in the + x direction and is F increasing.
c) the net force acts in the + x direction and is F decreasing.
d) the net force acts in the - x direction and is F constant.
e) the net force acts in the - x direction and is F increasing.
f) the net force acts in the - x direction and is F decreasing.
Question: 400g of boiling water (temperature 1000 , specific heat 4.2J/K/gare poured into an aluminium pan whose mass is 600g and initial temperature (the specific heat of aluminium is . After a short time, what is the temperature of the water? Explain. What simplifying assumptions did you have to make?
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