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# Modern Physics : 8 Questions

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#1. A weight lifter lifts a 250-N set of weights from a ground level to a position over his head, a vertical distance of 2.00 m. How much work does the weight lifter do, assuming he moves the weights at a constant speed?

#2. A 7.80 g bullet moving at 575 m/s penetrates a tree trunk to a depth of 5.50 cm. (a) Use work and energy considerations to find the average frictional force that stops the bullet. (b) Assuming the frictional force is constant, determine how much time elapses between the moment the bullet enters the tree and the moment it stops moving.

#3. Find the height from which you would have to drop a ball so that it would have a speed of 9.0 m/s just before it hits the ground.

#4. A 2 300 kg pile driver is used to drive a steel beam into the ground. The pile driver falls 7.50 m before coming into contact with the top of the beam, and it drives the beam 18.0 cm farther into the ground as it comes to rest. Using energy considerations, calculate the average force the beam exerts on the pile driver while the pile driver is brought to rest.

#5. When a 2.50 kg object is hung vertically on a certain light spring described by Hooke's Law, the spring stretches 2.76 cm. (a) What is the force constant of the spring? (b) If the 2.50 kg object is removed, how far will the spring stretch if a 1.25 kg block is hung on it? (c) How much work must an external agent do to stretch the same spring 8.00 cm from its unstretched position?

#6. Hooke's law describes a certain light spring of unstretched length 35.0 cm. When one end is attached to the top of a door frame and a 7.50 kg object is hung from the other end, the length of the spring is 41.5 cm. (a) Find its spring constant. (b) The load and the spring are taken down. Two people pull in opposite directions on the ends of the springs, each with a force of 190 N. Find the length of the spring in this situation.

#7. A 0.250 kg block along a horizontal track has a speed of 1.50 m/s immediately before colliding with a light spring of force constant 4.60 N/m located at the end of the track. (a) What is the spring's maximum compression if the track is frictionless? (b) If the track is not frictionless, would the spring's maximum compression be greater than, less than, or equal to the value obtained in part (a)?

#8. A certain rain cloud at an altitude of 1.75 km contains 3.20 X 10 kg of water vapor. How long would it take for a 2.70 k-W pump to raise the same amount of water from Earth's surface to the cloud's position?