# Search results

1. ### Momentum vs. Kinetic energy

I can see from the formulae that a bullet will have greater KE than the gun even though the mometum of the kickback is equal to that of the bullet. I also know that a bullet will do a lot more damage to a wall than the backfire of the gun. Whats behind this? Is it simply the concentration of...
2. ### Relative velocity of perpendicular vectors

It didn't make any sense what I was describing there did it. What I was trying to describe was something more like the relative velocity of the river with respect to someone jumping over the river but the resultant vector in that case would be the opposite direction. What I'm really wondering is...
3. ### Conservation of momentum with initial velocity of 0

I'm having a bit of trouble understanding the concepts of momentum conservation. Lets say I'm standing on a frictionless surface and I throw a 3kg brick horizontally with a velocity of 4m/s. In this scenario I can see that I'm going to be repelled backwards at a velocity of 12/my mass in kg but...
4. ### Relative velocity of perpendicular vectors

Using this diagram as an example http://img25.imageshack.us/img25/231/diagramig.jpg [Broken] would I be right in assuming that the relative velocity of the boat with respect to the velocity of the current is the speed at which its moving away from say a stone floating down the river? In other...
5. ### Simple pulley systems

I'm fairly fascinated by the idea of mechanical advantage using pulley systems. Using this pulley system http://www.swe.org/iac/images/plly_071.jpg [Broken] as an example I read that this provides a mechanical advantage of 4. If I'm only putting say 20N force in and the block weights 80N where...
6. ### Calculating average speeds when distance is unknown

I found a brilliant website with physics questions + solutions here http://www.solvephysics.com/problems_kinematics.shtml [Broken] the question I'm on they only tell you the speed travelled in fractions of the total distance but don't tell you the total distance. I looked at the solution but...
7. ### Horizontal component of projectile motion

Thanks. I got a question on a test a while ago where I knew the velocity a ball was thrown horizontally off a roof and knew how far the ball went before it landed but I couldn't do the question because I was convinced horizontal velocity wasn't constant. Ah well now I know.
8. ### Horizontal component of projectile motion

Unless there wind resistance do you always treat the horizontal velocity of a projectile as constant?
9. ### Average velocity from multiple displacement/velocities

Heres the problem: "A car travels at a constant speed of 60kph for 30km, 40kph for another 30km and 50kph for the final 30km. What is the average speed of the car." I know that in this case I can just add the velocities and divide by 3 since the displacements are equal and get 50kph as the...
10. ### Rearranging the kinematic equations

Thanks a lot. So it appears its better to look at it like doing the same operation on both sides than moving something from one side of the equation to the other. Yeah its gotten very tough already. Its basically just this rearranging equations part I have trouble with but that seems to be one...
11. ### Rearranging the kinematic equations

Alright heres the steps v^2 = u^2 + 2as 1.) Moved the v^2 over and got 0 = u^2 - u^2 + 2as 2.) Moved the s and got -s = u^2 - v^2 + 2a 3.) Changed all the signs and got s = v^2 - u^2 - 2a I never fully learned algebra which seems to be the source of 90% of the problems I have with physics. Like...
12. ### Rearranging the kinematic equations

The equations for linear motion I've memorized are: 1.) v = u + at 2.) v^2 = u^2 + 2as 3.) s = ut + 1/2(at^2) v = final velocity, u = initial velocity, a = acceleration, t = time, s = displacement In an example my physics teacher uses the equation x = x0 + (v^2-u^2)/2a I'm assuming he...
13. ### Galileos law of free fall

Ah right. Thanks.
14. ### Galileos law of free fall

Thanks that explains where they got 16 but why .5?
15. ### Galileos law of free fall

I'm reading a maths book called Thomas Calculus and in their method for getting the average speed of an object when only the height its dropped from is known is this formula here which they call Galileos law: y = 16t^2 y being the distance travelled after time. What I don't get is where they...