# some 6 confusing problems

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hey guys, i was wondering who is good at science? The reason i am asking is because i got some 6 six questions from TAFE, and i don't have a clue how to do them? So here they are:

1. Use the equation of motion to calculate the speed and distance travelled by an object falling from rest under gravity after 3,6 and 9 seconds. Take g=10 meters per seconds squared.

2. A force of 20 N acts for 8 seconds on a mass of 60Kg. What is the (a) acceleration, (b) final speed, © average speed, (d) distance covered during the 8 seconds.

3. A force of 20 N is exerted on the handle of a door 0.6 meters from the hinge. If the force acts a right angles to the door, what is the moment it exerts?

4. Calculate the kinetic energy of a car of mass 1200Kg travelling at 25 m/s. If it brakes to rest at 40 meters, calculate the braking force.

5. How much heat is released by the combustion of 4Kg of petrol of calorific value 46 MJ/kg?

6. A sonar beam is radiated from a ship and an echo is received after 0.9 seconds. If the speed of sound in the water is 1400 m/s, how far away is the reflecting surface?

These are the questions i have been given for homework, and i don't even know anything. So guys, please be gentle and if u can, can u also show all clear workings out. Thanks

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1. s = ut + ½ at² will give you a value for the distance, when u is the initial velocity (in your case = 0), a is the acceleration due to gravity, and t is the time.

Then use v² = u² + 2as to get an answer for the velocity.

Time does not permit a more detailed answer, or answers to the other questions, but I will get back to this as soon as possible.

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These are the questions i have been given for homework, and i don't even know anything. So guys, please be gentle and if u can, can u also show all clear workings out. Thanks

No, I don't think anyone is going to do your homework for you. You give it a shot, and show your work, and you'll get some help.

deep already gave you two equations that apply to many of your problems. For #3 you need N=rFsin(theta) whenre N is the torque, r is the moment arm, F is force, and theta is the angle between the force and the moment arm.

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2. A force of 20 N acts for 8 seconds on a mass of 60Kg. What is the (a) acceleration, (b) final speed, © average speed, (d) distance covered during the 8 seconds.

a) F= m *a : a = 20/60 =1/3 m/s/s. b) use v = u + at: v = 0 + (1/3)*8. c) use x = (u+v)t : x= (0+ 2.67)*8 = 21.33 m ; avg speed = dist. /time = 21.33/8. d) already found this in q. c, maybe i took a long way.

4. Calculate the kinetic energy of a car of mass 1200Kg travelling at 25 m/s. If it brakes to rest at 40 meters, calculate the braking force.

kinetic energy = 1/2 m*v^2 = 375000 J. i would use x = (u+v)*t to give: 40 = (25+ 0) t, t = 40/25 = 1.6s, The change in momentum of the car is: p=mv = 1200*25 = 30000 kgm/s . F= change in momentum/ time = 30000/1.6 = 18750 N. I suspect they want you to do this problem a different way because they asked you to calculate the Ek but i am doing alot about momentum in physics atm so i would use this way and i cant think of another right now.

5. How much heat is released by the combustion of 4Kg of petrol of calorific value 46 MJ/kg?

if one kg gives 46 MJ (million Joules) , then 4kg of petrol must give how many MJ?

6. A sonar beam is radiated from a ship and an echo is received after 0.9 seconds. If the speed of sound in the water is 1400 m/s, how far away is the reflecting surface?

for this use speed = distance/time. speed = 1400m/s and time = 0.9 s, therefore distance = 1400 *0.9 = 1260 m, however, this is the distance traveled by the sonar beam, not the distance of the ship to the bottom, which is half this distane for your answer.

incase you were unfamiliar, in the motion equations v = final velocity, u = final velocity and x = displacement

sorry if there is any mistakes there, hope it helps

ps. These really aren't confusing problems, you should read more of your text book to understand them.

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