darkbob5150

Sounds like you need Newton's first law of motion: A body will continue in a state of rest or uniform motion in a straight line unless acted on by an unbalanced external force. When an unbalanced force acts on a body it will cause it to accelerate in the direction of the applied force. The acceleration lasts for as long as the force is applied. [The important thing about forces are that they are vectors, that means that every force must have both a magnitude (how hard you push it) and a direction (what direction you push it in).] So forces tend to change the direction something travels in and/or change the objects speed. I'm not too sure what you mean by your questions......an object travelling in a diagonal line is not under the influence of an unbalanced force (unless it's speed is changing). Do you mean it's moving under gravity? An undefined force that is changing it's magnitude / direction would cause the object to change it's speed / direction accordingly and travel in an undefined path. Here's an example that might help...? If you throw a ball diagonally up into the air (like you were throwing it to someone a few metres away) then think about what that would do. The gravitational force acts straight down with a magnitude of: Mass of the ball x Acceleration due to Gravity (from Newton's 2nd law F = Ma). So the vertical (up and down) motion of the ball is subject to an acceleration and will therefore cause the ball to slow down when heading for the sky, stop and then speed up towards the ground. The force due to gravity is constant, so the speed of the ball is always changing by a constant amount. The horizontal motion has no gravitational force acting on it, so the ball's direction / speed parallel to the ground does not change - your friend catches it with the same horizontal speed as you threw it with. This technique (resolving components of motion) allows you to resolve an objects motion into directions at right angles to each other to make a problem simpler. Here you can make the horizontal motion separate to the vertical motion so the horizontal motion is just a constant motion in a straight line problem, and the vertical motion is just acceleration in one dimension.

Thought you might like a little relief after discussing all the really complicated problems on this topic! If you looked at a realitivistic dice travelling really fast in some frame with it's '3 face' facing in the direction of motion (along the x axis say), what would it look like to a (distant) observer in the same frame who's looking up at the dice ( so looking at the underside or '6 face' ) as it shoots past (at right angles to) them. By 'see' I mean you take a photograph of the dice by having a photographic plate along the x axis that receives parallel light from the dice - this way you don't have to worry about a lens or crap like that. Think about the effect of light travel time and what rays arrive when. And remember that opposite sides on a dice add up to 7...

sorry, just read the numbers in your reply properly. Yeah, you had it right - didn't need all that boring crap I wrote, but it explains a little bit as to why it's right! Soz

Ok, the equation for kinetic energy - as Dave said - gives you the energy that the object has, at a particular time, because it is moving. You would use the KE in a question when there are no external force acting because energy would then be a conserved quantity - the amount of energy (kinetic + potential) would remain constant allowing you to work nice little things out. Because this question is about average velocities it means that the velocity changes (accelerates) as the object moves which would indicate that there are external (unbalenced) forces acting. External forces mean that you can't apply the conservation of energy because the force will either add energy to the system (your moving object) like pushing it to make it go faster, or take it away like friction. If you take the extra energy that the force introduces to the system into account then you can use the energy formula: Work done on an object is really just the change in energy of that object due to the applied force. It only depends on the initial and final states which is a useful and important feature. In this case (if you take the setup I had earlier where the object starts at rest, constantly accelerates and reaches a top speed of 2m/s after 10m) you can take the work done on the object as the change in KE ( = Final - Initial = 1/2 x m x 2^2 - 0 ) and you get the same answer as before for the work. Divide by the time taken to get the power.

I think you might be right to be suspicious...unless by external torque they mean that the TOTAL torque acting on the wheel is 50Nm. The total torque acting on the wheel SHOULD be given by Applied Torque - Frictional Torque. The Applied Torque is 50Nm like they say, the Frictional Torque can be worked out and should be I x Pi / 6. The Total Torque is then equal to I x (angular acceleration) as you said. That comes out to give an I of 13.6 kgm^2. Not sure - might just be a dodgy question/answer! Let me know if you work it out...