







StefanBoltzmann Law 
20091228 

From florence: Hi
I have a college tutor and we are stumped with this question.
Assuming a filament in a 100 W light bulb acts like a perfect blackbody, what is the temperature of the hottest portion of the filament if it has a surface area of 6.3 X 10^5 m^2? The StefanBoltzmann constant is 5.67 X 104 W/(m^2 X K^2.\
Thank you for your help. If you can only answer one question per day please answer this one and guide me through a solution. Answered by Janice Cotcher. 





Finding Specific Heat of a Substance 
20091226 

From erin: Hi
2 kg metal requires 1.00 X 10^4 J of heat to raise its temperature from 20 degrees C to 40 degrees C. What is the specific heat capacity of the metal? Answered by Janice Cotcher. 





Initial Velocity of a Car Plunging Off a Cliff 
20090607 

From Mimi: A car fails to take a sharp turn on a flat road and plunges off a cliff. The car lands 65m horizontally and 43m vertically from where it left the edge of the cliff. How fast was the car travelling? Answered by Stephen La Rocque. 





Find the resultant of this displacement pair 
20090222 

From katydidit: Find the resultant of this displacement pair:
500 miles at 75 degrees east of north and
1500 miles at 20 degrees west of south.
How do I graph this and how do I solve this problem? Answered by Penny Nom. 





A falling block 
20081018 

From KOBINA: A 50 kg block being held at rest 20m above the ground is released. the block falls (no friction). how fast is the block traveling (in m/sec) when it has lost 40% of its original potential energy. Answered by Stephen La Rocque. 





Balancing on a fulcrum (net torque and equilbrium) 
20080115 

From Eric: If I have a 4" bar that has no weight value with a 275lb. weight on one side
and a 125lb. weight on the other side what would be the folcrum point? Please
keep the answer very simple. Also is there a fixed equation to figure folcrum points?
Is there an equation for if the 4" bar had a weight value? Answered by Stephen La Rocque. 





Constantly accelerated motion 
20071029 

From Priyanka: A, B and C are three points lying in that order on a straight line. A body is
projected from B towards A with speed 3 m/s. The body experiences an
acceleration of 1 m/s^2 towards C. If BC =20 m, find the time taken to reach C and
the distance travelled by the body from the moment of projection until it reaches C. Answered by Stephen La Rocque. 





Average speeds 
20070622 

From Dee: A guy goes 3.1m a second for 29 seconds in a golf cart. He then walks 1.5 meters a second. How long does he have to walk to get the average speed of 2.1m a second? Answered by Stephen La Rocque. 





A couch sliding off a truck 
20070430 

From William: A couch with a mass of 1 X 10^2kg is placed on an adjustable ramp connected to a truck. As one end of the ramp is raised, the couch begins to move downward. If the couch slides down the ramp with an acceleration of .70 meters per second when the ramp angle is 25 degrees, what is the coefficient of kinetic friction between the ramp and couch?
I drew a force diagram and if I did it correctly I identified the forces involved as "mg" (mass x gravity), "Fn" (normal force) and the "Ff" (frictional force). I know that we have the couch sliding down the ramp a .70 m/s but I don't think this a force and I'm not sure how this info fits into the problem. I know that the formula for calculating the coefficient of friction is Ff/Fn. Based upon the force diagram I drew and calculated Fn to be 1082N. I can't seem to get past this point. How do I determine what the frictional force is? Answered by Stephen La Rocque. 





Rotorride 
20070307 

From Jessie: In a "Rotorride" at a carnival, people are rotated in a cylindrically walled "room". The room radius is 4.6m and the rotation frequency is 0.550 revolutions per second when the floor drops out. What is the minimum coefficient of static friction so that people will not slip down? People on this ride say they were "pressed against the wall". Is there really an outward force pressing them against the wall? If so, what is its source? If not, what is the proper description of their situation (besides scary)? Answered by Stephen La Rocque. 





A gymnast is swinging on a high bar. 
20070214 

From William: A gymnast is swinging on a high bar. The distance between his waist (center of mass) and the bar is 1.1 m. If his speed at the top of his swing is zero and his gain in speed is due entirely to his change in gravitational potential energy, find his speed at the bottom of his swing. Answered by Stephen La Rocque. 





Driving on a banked roadway 
20070205 

From William: A race track is shaped like an inverted cone. Race cars drive around this track in a horizontal path. If a drivers speed is 34 m/s, at what distance from the tip of the cone should he drive his car if there is no friction?
THANKS IN ADVANCE FOR ANY ASSISTANCE YOU CAN PROVIDE!
Sincerely, William Bush Answered by Stephen La Rocque. 





Static electricity on an airplane 
20001112 

From John: What discharges an airplane of static electricity in a electric storm when it is flying. I assume the charge is on the out side of the plane but when does it get discharged, or does it occur at landing? Answered by Ted Mathie. 





Calculer la hauteur de chute de cette pierre 
20001216 

From Marcus: Une pierre tombe au fond d'un puit, ŕ 5 secondes on entend un son (de la pierre). Calculer la hauteur de chute de cette pierre en tenant compte de la vitesse du son = 340m.s^{1} Il faut savoir que pour la chute libre v= g.t ; h = 1/2 g.t^{2} ; v = 2g.h g intensité du champs de pesanteur = 10 ; h hauteur de chute. Answered by Claude tardif. 

