## 12 February 2013

### Physics Homework Ch5

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1. A weight lifter lifts a 310-N set of weights from ground level to a position over his head, a vertical distance of 1.75 m. How much work does the weight lifter do, assuming he moves the weights at a constant speed?
__________J

2. The record number of boat lifts, including the boat and its ten crew members was achieved by Sami Heinonen and Juha Rasanen of Sweden in 2000. They lifted a total mass of 653.2 kg approximately 4in off the ground a total of 24 times. Estimate the total mechanical work done by the two men in lifting the boat 24 times, assuming they applied the same force to the boat during each lift. (Neglect any work they may have done allowing the boat to drop back to the ground).
_________J

3. A horizontal force of 150 N is used to push a 41.0-kg packing crate a distance of 6.45 m on a rough horizontal surface. If the crate moves at a constant speed, find each of the following.
a). The work done by the 150-N force
_______J
b). the coefficient of kinetic friction between the crate and the surface
_______

4. A sledge loaded with bricks has a total mass of 18.4 kg and is pulled at constant speed by rope inclined at 20.1 degrees above the horizontal. The sledge moves a distance of 20.2 m on a horizontal surface. The coefficient of kinetic friction between the sledge and surface is 0.500
a). What is the tension in the rope
________N
b). How much work is done by the rope on the sledge?
__________KJ
c). What is the mechanical energy lost due to friction
__________KJ

5. A block of mass m= 2.3 kg is pushed a distance of d=4.40 m along a frictionless horizontal table by a constant force of magnitude F= 16.0 N directed at an angle θ = 28° below the horizontal as shown in the figure below.
a). Determine the work done on the block by the applied force.
_______J
b). Determine he work done on the block by the normal force exerted by the table.
_______J
c). Determine the work done on the block by the force of gravity.
_______J
d). Determine the work done by the net force on the block
_______J

6. A mechanism pushes a 3.60 X 10 3 -kg car from rest to a speed of v, doing 4,600 J of work in the process. During this time, the car moves 22.0 m. Neglecting friction between the car and road, find each of the following.
a) the speed v
_________m/s
b). The horizontal force exerted on the car
_________N

7. A 7.00-kg bowling ball moves at 2.05 m/s. How fast must a 2.80-g Ping-Pong ball move so that the two balls have the same kinetic energy?
________m/s

8. A 75-kg base runner begins his slide into second base when he is moving at a speed of 4.4 m/s. The coefficient of friction between his clothes and Earth is 0.70. He slides so that his speed is zero just as he reaches the base.
a) How much mechanical energy is lost due to friction acting on the runner?
__________J
b) How far does he slide?
__________m

9.A 0.30-kg stone is held 1.2 m above the top edge of a water well and then dropped into it. The well has a depth of 4.7 m.
a) Taking Y = 0 at the top edge of the well, what is the gravitational potential energy of the stone-Earth system before the stone is released?
_________J
b) Taking Y = 0 at the tope edge of the well, what is the gravitational potential energy of the stone-Earth system when it reaches the bottom of the well?
_________J
c) What is the change in gravitational potential energy of the system from release to reaching the bottom of the well?
_________J

10. When a 2.50-kg object is hung vertically on certain light spring described by Hooke's law, the spring stretches 3.21 cm.
a). What is the force constant of the spring?
________N/m
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?
________cm
c) How much work must an external agent do to stretch the same spring 8.80 cm from its un-stretched position?

11. A 2,200-kg pile driver is used to drive a steel I-beam into the ground. The pile driver falls 4.40 m before coming into contact with the top of the beam, and it drives the beam 13.8 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.
magnitude ________N
direction (Upward or Downward)?

12. A daredevil on a motorcycle leaves the end of a ramp with a speed of 28.0 m/s as in the figure below. If his speed is 26.4 m/s when he reaches the peak of the path, what is the maximum height that he reaches? Ignore friction and air resistance.
________m

This (above) video is an update to show a "better" way of solving the problem using work-energy analysis / conservation of energy.
The original video (below) is the original post solving the problem using kinematics.

13. A 44-kg pole vaulter running at 12 m/s vaults over the bar. Her speed when she is above the bar is 1.0 m/s. Neglect air resistance, as well as any energy absorbed by the pole, and determine her altitude as she crosses the bar.
_______m

14. A child and sled with a combined mass of 55.0 kg slide down a frictionless slope. If the sled starts from rest and has a speed of 2.10 m/s at the bottom, what is the height of the hill?
__________m

15. A 0.280-kg block along a horizontal track has a speed of 1.40 m/s immediately before colliding with a light spring of force constant 37.2 N/m located at the end of the track.
a) What is the spring's maximum compression if the track is frictionless?
________m
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)?

16. A block of mass m= 4.10 kg is released from rest from point A and slides on the frictionless track shown in the figure below. (Assume ha = 6.9 m.)

a) Determine the block's speed at points B and C
Point B ________ m/s
Point C _________m/s

b) Determine the net work done by the gravitational force on the block as it moved from point A to point C
__________J

17. Tarzan swings on a 30.0-m-long vine initially inclined at an angle of 31.0 ° with the vertical.
a) what is his speed at the bottom of the swing if he starts from rest?
__________m/s
b) What is his speed at the bottom of the swing if he pushes off with a speed of 2.00 m/s?
__________m/s

18. The electrci motor of a model train accelerates the train from rest to 0.760 m/s. in 15.0 ms. The total mass of the train is 530 g. Find the average power delivered to the train during its acceleration.
_________W

1. On #18: how did you know to derive distance as 1/2at?

2. Sorry, I meant distance as 1/2at^2 in the previous comment. Is it because our velocity initial was 0 because the train starts from rest?

1. You are correct. The Distance is always equal to the area under a Velocity verses time diagram. And by using a V vs T diagram you can derive the equation displacement = Vit + 1/2at^2
When the initial velocity is zero, you are only left with the last half of the term (1/2at^2)

3. I somewhat realized how it was derived after I posted, but your response still helped. Thank you so much for making this blog by the way! It is saving me!

1. Amber, You know, I do the tutoring for this class at MWSU. I'm assuming you're a student their. LMK if you need help for anything - Homework/test prep/etc. I've been tutoring three other people who follow this blog as well.

4. Yes I am. That's awesome. What are your hours for tutoring?

1. Almost every Tuesday, Thursday, and Friday from 1:00 to 3:00.
I can actually tutor almost any time. Just email me at my school account and LMK at least three different times that would work for you and I'll try to pick the one that fits my schedule best for that week.

http://www.missouriwestern.edu/cas/Tutors/#TList
the link is to the list of content tutors. Click on my name and it will open my email address.

2. One other thing: I have a Pathophysiology exam every other Thursday (ie. Tomorrow) So, I won't be tutoring, in the CAS, tomorrow.

5. NOTE: I updated the Video for Question number 16. I did this because a VERY SIMILAR question was asked on my physics test. AND if you can follow along with me on this question, then you should be fine on the exam.
--Solving for Velocity by using the Conservation of Energy equation. ---Solving for Work using the Work-energy theorem.

6. super helpful mike keep it up
we have a horrible physic professor and my whole class depends on ur blog to teach
what she is incapable of doing.

1. That's awesome! Where do you go to school?
-- Well, not awesome that your professor sucks, but awesome that you like the blog!

7. This is very helpful! Thank you so much!

1. I really enjoyed making these videos. I'm glad they are helping people!

8. Thank you for your help. I was wondering if you could do a video on this question: Starting from rest, a 4.20-kg block slides 3.00 m down a rough 30.0° incline. The coefficient of kinetic friction between the block and the incline is μk = 0.436. (a) Determine the work done by the force of gravity. (b) Determine the work done by the friction force between block and incline. (c) Determine the work done by the normal force. (d) Qualitatively, how would the answers change if a shorter ramp at a steeper angle were used to span the same vertical height? My profesor gets frazzled by this question every time we ask him for help. He hasn't been able to show us how to do this, but its on my upcoming test.