# Can u guys help me with these 6 problems?

1. Apr 30, 2005

### wassam

hey guys, i was wondering who is good at science? The reason i am asking is because i got some 6 six questions from college, and i have some clue on 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, (c) 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. So guys, please be gentle and if u can, can u also show all clear workings out. Thanks

Last edited: Apr 30, 2005
2. Apr 30, 2005

### Data

well, what have you covered in class? For example, you will have covered Newton's second law. Do you know what it is? What relationships do you have concerning the kinetic energy of an object? Take a look at your textbook!

A little bit of reading should give you all that you need to know to do those questions. Most people here won't just do them for you. If you need help, we will be more than happy to offer it - but we do mean "help," not complete solutions.

3. Apr 30, 2005

### wassam

the thing is i have done science a long time ago, and i have been given these 6 six questions, which would help me get skills recognition for science. I dont exactly remember everything, hence asking for your help guys :(

4. Apr 30, 2005

### Data

5. Apr 30, 2005

### wassam

anyone else want to help please :)

6. Apr 30, 2005

### whozum

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.

Use
$$x = v_0t + \frac{1}{2} gt^2$$ Initial velocity is zero.

The rest are similar or follow fromthis, and newtons second law. Looking up the equations will take no more than one minute.

7. Apr 30, 2005

### bahamut

v(t)=gt;(1)
s(t)=(1/2)gt^2;(2)
g=10m/s^2;
so v(3)=30m/s,v(6)=60m/s,v(9)=90m/s;
and s(3)=45m,s(6)=180m,s(9)=405m.

a=F/m=20/60=1/3 m/s;
v(final)=at=8/3 m/s;
V(average)=(1/2)v(final)=4/3 m/s;
S=v(average)*t=32/3 m;

M=F*L*cosa=20*0.6=12 N*m

fs=(delta)E=(1/2)mv^2;
so f=1200*25^2/2/40=?(you caculate it yourself)

E(max)=M*r=4*46=184 MJ;
E(heat)=E(max)*R (R is a ratio concerning this problem,you will find it in an engineering reference book,I don't know how to speak it in english)

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?
2S=Vt=1400*0.9=1260 m;
so S=630 m.