# Help me with these quite simple problems please

1. Dec 7, 2005

### Trebor

hi

I have exams coming up , and I need help knowing how to solve some questions to review. sorry if theyre quite a few of them, any help would be appreciated. Thanks!
( I have an answer key for these questions, but I do need to know how to solve them on my own because the values will definitely be changed in the real exam, but the same idea and concepts of the questions will be the same)

1.) A 5 pound object is suspended by a string from an overhead support. A horizontal force of 2.7 pounds is applied on the object. The measure of the angle which the string makes with the horizontal is?

2.) A force of 30 newtons is used to raise a 120 newton load using a system of pulleys. The load covered a distance of 1 m for every 5m of rope pulled through the system. the system has an efficency of?

3.) Block A and B rest on a horizontal beam CD which is 5 m long. they weigh 4000 and 2000 pounds respectively and are 1 m apart from each other. If the reaction produced at D (right end of beam) is twice as greast as the reaction at C (Left end of beam), how far from C, will the weight of block A act?

4.) A 2400 watt heater is used to raise the temp of 2 liters of water to Boil point. HOw long will it take for water to boil if the initial temp of water is 40 deg celcius?

5.) Electroplating results in a better quality product by applying a relatively : a.) strong current for short time b.) strong current for long time c.) moderate current for short time d.) moderate current for long time

6.) A certain solar cooker is made of a concave mirrot. TO get the greatest amount of reflected sunlight, whre should the food be placed?
a.) focus of the mirror b.) center of curvature of mirror c.) betweent the focus and center of curvature of mirror d.) abovce center of curvature of mirror

7.) When a large atom such as U 235 (superscript) splits into 2 smaller atoms, then the combined mass of the products splitting will be:
a.) 1/2 original mass b.) same as original mas c.) 1/2 more then original mass d.) 2 x more than original mass

8.) The atomic reaction where 13 (superscript) 6 (subscript) B becomes 12 (superscript) 7 (subscript) C, results in the release of a.) neutron, b.) alpha particle c.)beta part. d.) gamma part.

9.) An electron is travelling at 1% the spped of light. WHat is its kinetic energy in joules?

10.) A beam of electrons is directed between two charged plates ( top plate is positive, bottom plate is negative ). which direction whill the beam curve when it gets between the plates?
a.) up toward top plate b.) right c.) down toward negative plate d.) left

11.) The radiation from a sample of krypton 85 decreases to one third of the original intensity Io in a period of 18 years. What would be the intensity after 18 more years: a.) Io b.) 1/3 Io c.) 1/6 Io d.) 1/9 Io

12.) Given this U 235 (superscript) fission equation,

235 (superscript) 92 (subscript) U + 1 (super) 0 (sub) n --> 140 (super) 54 (sub) Xe + 94 (super) 38 (sub) Sr + (?) 1 (super) 0 (sub) n + energy,

Ce 140 (super) and Sr 94 (super) nuclei are produced and energy is released. HOw many n are given of in the process?

and lastly
13/) a ping pong ball and golf ball are dropped in a vaccuum chamger from the same ht and at same time. when they have fallen halfway they ahve the same velocity. I would have answered acceleration (it is one of the choices) but velocity was the correct answer. why is this? isnt acceleration constant througout when disregarding air resistance?

thanks again any help would be greatly appreciated

2. Dec 7, 2005

### Tide

Since you already "know" the answers you should at the very least show you've made some effort to solve them yourself.

3. Dec 7, 2005

### Trebor

the problem is I dont have any idea how to solve them, since I do not know the equations anymore and dont know the concepts too.

4. Dec 7, 2005

### Tide

Oh, that is a problem! Did you retain any of the principles that your class discussed during the semester?

Generally, these principles will get you through most problems:

(1) In equilibrium, net forces and torques add to zero.

(2) Momentum is conserved.

(3) Energy is conserved.

(4) Heat flows from hot to cold.

(5) Angular momentum is conserved.

There's a couple others but that should help get you started. The management of this site gets upset if members provide solutions to problems before the student shows any of his or her own work.

5. Dec 8, 2005

### Trebor

hi
ok well heres what I have figured out so far
1.) A 5 pound object is suspended by a string from an overhead support. A horizontal force of 2.7 pounds is applied on the object. The measure of the angle which the string makes with the horizontal is?

this one... I think you have to use the x and y components of force... so
Fy = 5 , and Fx = 2.7... then im stuck... do you get the resultant force?

4.) A 2400 watt heater is used to raise the temp of 2 liters of water to Boil point. HOw long will it take for water to boil if the initial temp of water is 40 deg celcius?

Q = mc change in temp. so... Q = (2000 kg)(?) (60).... Im not sure what value to use for c... Also What equation would I use next to solve for time?

13/) a ping pong ball and golf ball are dropped in a vaccuum chamger from the same ht and at same time. when they have fallen halfway they ahve the same velocity. I would have answered acceleration (it is one of the choices) but velocity was the correct answer. why is this? isnt acceleration constant througout when disregarding air resistance?

How come velocity becomes equal by half way? Isnt acceleration equal throughout so shouldnt that be the answer?
Im sorry... the rest I really have no idea on what to do.... I couldnt even guess... I ve tried looking for equations for them over the internet, but I cant seem to find any.

6. Dec 8, 2005

### Tide

1) Draw a free body diagram. The applied horizontal force plus weight will be balanced by the tension in the string which makes an angle given by $\tan^{-1} 5/2.7$ below the horizontal.

4) Use $Q \Delta t$ to find the time to bring the water up to the boiling point. The c in your equation is the specific heat of water. Once the boiling point is reached, it takes additional energy to boil it for which you will use the latent heat of vaporization (no temperature change occurs - only a change of state).

13) I don't know the precise wording of the question so I am not sure what they are looking for but the ping pong ball and the golf ball should have both the same velocity and acceleration if they both start from rest at the same instant and at the same height.