Essential University Physics
4th Edition
ISBN: 9780134988559
Author: Wolfson, Richard
Publisher: Pearson Education,
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Textbook Question
Chapter 7.4, Problem 7.4GI
For which of the following systems is (1) mechanical energy conserved and (2) total energy conserved? (a) the system is isolated, and all forces among its constituents are conservative; (b) the system is not isolated, and work is done on it by external forces; (c) the system is isolated, and some forces among its constituents are not conservative
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Chapter 7 Solutions
Essential University Physics
Ch. 7.1 - Suppose it takes the same amount of work to push a...Ch. 7.2 - Gravitational force actually decreases with...Ch. 7.3 - A bowling ball is tied to the end of a long rope...Ch. 7.4 - For which of the following systems is (1)...Ch. 7.5 - A bowling ball is tied to the end of a long rope...Ch. 7.6 - The figure shows the potential energy associated...Ch. 7 - Figure 7.14 shows force vectors at different...Ch. 7 - Is the conservation-of-mechanical-energy principle...Ch. 7 - Why cant we define a potential energy associated...Ch. 7 - Can potential energy be negative? Can kinetic...
Ch. 7 - If the potential energy is zero at a given point,...Ch. 7 - If the difference in potential energy between two...Ch. 7 - If the difference in potential energy between two...Ch. 7 - If conservation of energy is a law of nature, why...Ch. 7 - Determine the work you would have to do to move a...Ch. 7 - Now lake Fig. 7.15 lo lie in a vertical plane, and...Ch. 7 - Rework Example 7.1, now taking the zero of...Ch. 7 - Find the potential energy associated with a 70-kg...Ch. 7 - You fly from Bostons Logan Airport, at sea level,...Ch. 7 - How much energy can be stored in a spring with k =...Ch. 7 - How far would you have to stretch a spring with k...Ch. 7 - A biophysicist grabs the ends of a DNA strand with...Ch. 7 - A skier starts down a frictionless 32 slope. After...Ch. 7 - A 10,000-kg Navy jet lands on an aircraft carrier...Ch. 7 - A 120-g arrow is shot vertically from a bow whose...Ch. 7 - In a railroad yard, a 35,000-kg boxcar moving at...Ch. 7 - You work for a toy company, and youre designing a...Ch. 7 - A 54-kg ice skater pushes off the wall of the...Ch. 7 - Prob. 23ECh. 7 - A particle slides along the frictionless track...Ch. 7 - A particle slides back and forth on a frictionless...Ch. 7 - A particle is trapped in a potential well...Ch. 7 - Example 7.3: A climbing rope is designed to exert...Ch. 7 - Example 7.3: A climbing rope exerts a force given...Ch. 7 - Example 7.3: The force on an electron in an...Ch. 7 - Example 7.3: The potential energy of an electron...Ch. 7 - Prob. 31ECh. 7 - Prob. 32ECh. 7 - Example 7.5: In a railroad switchyard, a rail car...Ch. 7 - Prob. 34ECh. 7 - The reservoir at Northfield Mountain Pumped...Ch. 7 - A carbon monoxide molecule can be modeled as a...Ch. 7 - A more accurate expression for the force law of...Ch. 7 - For small stretches, the Achilles tendon can be...Ch. 7 - A particle moves along the x-axis under the...Ch. 7 - As a highway engineer, youre asked to design a...Ch. 7 - A spring of constant k, compressed a distance x,...Ch. 7 - A child is on a swing whose 3.2-m-long chains make...Ch. 7 - With x x0 = h and a = g, Equation 2.11 gives the...Ch. 7 - The nuchal ligament is a cord-like structure that...Ch. 7 - A 200-g block slides back and forth on a...Ch. 7 - Automotive standards call for bumpers that sustain...Ch. 7 - A block slides on the frictionless loop-the-loop...Ch. 7 - The maximum speed of the pendulum bob in a...Ch. 7 - A mass m is dropped from height h above the top of...Ch. 7 - A particle with total energy 3.5 J is trapped in a...Ch. 7 - (a) Derive an expression for the potential energy...Ch. 7 - In ionic solids such as NaCl (salt), the potential...Ch. 7 - Repeat Exercise 19 for the case when the...Ch. 7 - As an energy-efficiency consultant, youre asked to...Ch. 7 - A spring of constant k = 340 N/m is used to launch...Ch. 7 - A bug slides back and forth in a bowl 15 cm deep,...Ch. 7 - A 190-g block is launched by compressing a spring...Ch. 7 - Prob. 58PCh. 7 - An 840-kg roller-coaster car is launched from a...Ch. 7 - Prob. 60PCh. 7 - A child sleds down a frictionless hill whose...Ch. 7 - A bug lands on top of the frictionless, spherical...Ch. 7 - A particle of mass m is subject to a force...Ch. 7 - A block of weight 4.5 N is launched up a 30...Ch. 7 - Your engineering department is asked to evaluate...Ch. 7 - Your roommate is writing a science fiction novel...Ch. 7 - You have a summer job at your universitys zoology...Ch. 7 - Biomechanical engineers developing artificial...Ch. 7 - Blocks with different masses are pushed against a...Ch. 7 - Nuclear fusion is the process that powers the Sun....Ch. 7 - Nuclear fusion is the process that powers the Sun....Ch. 7 - Nuclear fusion is the process that powers the Sun....Ch. 7 - Nuclear fusion is the process that powers the Sun....
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- If the net work done by external forces on a particle is zero, which of the following statements about the particle must be true? (a) Its velocity is zero. (b) Its velocity is decreased. (c) Its velocity is unchanged. (d) Its speed is unchanged. (e) More information is needed.arrow_forwardRepeat the preceding problem, but this time, suppose that the work done by air resistance cannot be ignored. Let the work done by the air resistance when the skier goes from A to B along the given hilly path be —2000 J. The work done by air resistance is negative since the air resistance acts in the opposite direction to the displacement. Supposing the mass of the skier is 50 kg, what is the speed of the skier at point B ?arrow_forwardConsider a particle on which a force acts that depends on the position of the particle. This force is given by . Find the work done by this force when the particle moves from the origin to a point 5 meters to the right on the x-axis.arrow_forward
- A 4.00-kg particle moves along the x axis. Its position O varies with time according to x = t + 2.0t3, where x is in meters and t is in seconds. Find (a) the kinetic energy of the particle at any time t (b) the acceleration of the particle and the force acting on it at time t, (c) the power being delivered to the particle at time t and (d) the work done on the particle in the interval t = 0 to t = 2.00 s.arrow_forwardA block of mass m = 2.50 kg is pushed a distance d = 2.20 m along a frictionless, horizontal table by a constant applied force of magnitude F = 16.0 N directed at an angle = 25.0 below the horizontal as shown in Figure P6.3. Determine the work done on the block by (a) the applied force, (b) the normal force exerted by the table, (c) the gravitational force, and (d) the net force on the block. Figure P6.3arrow_forwardGive an example of a situation in which there is a force and a displacement, but the force does no work. Explain why it does no work.arrow_forward
- As a young man, Tarzan climbed up a vine to reach his tree house. As he got older, he decided to build and use a staircase instead. Since the work of the gravitational force mg is path Independent, what did the King of the Apes gain in using stairs?arrow_forwardA force F = (6i 2j) N acts on a panicle that under-goes a displacement r = (3i + j) m. Find (a) the work done by the force on the particle and (b) the angle between F and r.arrow_forwardConsider a particle on which several forces act, one of which is known to be constant in time: . As a result, the particle moves along the x-axis from x=0 to x=5 m in some time interval. What is the work done by ?arrow_forward
- A particle moves in the xy plane (Fig. P9.30) from the origin to a point having coordinates x = 7.00 m and y = 4.00 m under the influence of a force given by F=3y2+x. a. What is the work done on the particle by the force F if it moves along path 1 (shown in red)? b. What is the work done on the particle by the force F if it moves along path 2 (shown in blue)? c. What is the work done on the particle by the force F if it moves along path 3 (shown in green)? d. Is the force F conservative or nonconservative? Explain. FIGURE P9.30 In each case, the work is found using the integral of Fdr along the path (Equation 9.21). W=rtrfFdr=rtrf(Fxdx+Fydy+Fzdz) (a) The work done along path 1, we first need to integrate along dr=dxi from (0,0) to (7,0) and then along dr=dyj from (7,0) to (7,4): W1=x=0;y=0x=7;y=0(3y2i+xj)(dxi)+x=7;y=0x=7;y=4(3y2i+xj)(dyj) Performing the dot products, we get W1=x=0;y=0x=7;y=03y2dx+x=7;y=0x=7;y=4xdy Along the first part of this path, y = 0 therefore the first integral equals zero. For the second integral, x is constant and can be pulled out of the integral, and we can evaluate dy. W1=0+x=7;y=0x=7;y=4xdy=xy|x=7;y=0x=7;y=4=28J (b) The work done along path 2 is along dr=dyj from (0,0) to (0,4) and then along dr=dxi from (0,4) to (7,4): W2=x=0;y=0x=0;y=4(3y2i+xj)(dyj)+x=0;y=4x=7;y=4(3y2i+xj)(dyi) Performing the dot product, we get: W2=x=0;y=0x=0;y=4xdy+x=0;y=4x=7;y=43y2dx Along the first part of this path, x = 0. Therefore, the first integral equals zero. For the second integral, y is constant and can be pulled out of the integral, and we can evaluate dx. W2=0+3y2x|x=0;y=4x=7;y=4=336J (c) To find the work along the third path, we first write the expression for the work integral. W=rtrfFdr=rtrf(Fxdx+Fydy+Fzdz)W=rtrf(3y2dx+xdy)(1) At first glance, this appears quite simple, but we cant integrate xdy=xy like we might have above because the value of x changes as we vary y (i.e., x is a function of y.) [In parts (a) and (b), on a straight horizontal or vertical line, only x or y changes]. One approach is to parameterize both x and y as a function of another variable, say t, and write each integral in terms of only x or y. Constraining dr to be along the desired line, we can relate dx and dy: tan=dydxdy=tandxanddx=dytan(2) Now, use equation (2) in (1) to express each integral in terms of only one variable. W=x=0;y=0x=7;y=43y2dx+x=0;y=0x=7;y=4xdyW=y=0y=43y2dytan+x=0x=7xtandx We can determine the tangent of the angle, which is constant (the angle is the angle of the line with respect to the horizontal). tan=4.007.00=0.570 Insert the value of the tangent and solve the integrals. W=30.570y33|y=0y=4+0.570x22|x=0x=7W=112+14=126J (d) Since the work done is not path-independent, this is non-conservative force. Figure P9.30ANSarrow_forwardCite two examples in which a force is exerted on an object without doing any work on the object.arrow_forwardA certain automobile engine delivers 2.24 104 W (30.0 hp) to its wheels when moving at a constant speed of 27.0 m/s ( 60 mi/h). What is the resistive force acting on the automobile at that speed?arrow_forward
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