# Recent content by jmb88korean

1. ### Theoretical Question: Mass of Earth

I was just curious about this. Are humans counted in the mass of the Earth? If so as the population increases does the masss of the Earth also increase? I think it wouldn't due to humans technecally being made from the Earth. We are carbon, nitrogen, hydrogen, and oxygen from the Earth so when...
2. ### Dynamics and statics

First break this into three parts of acceleration, constant speed, and decceleration. Change the 60km/hr into meters per second to get uniform units. Knowing the final velocity, the acceleration and the initial velocity of zero, use vf= v(initial) + at to find the time for the first part. Do the...
3. ### Friction problem

I believe the above work is correct ecept that I don't believe its possible to have a negative coeffient of friction. Since friction itself is a retarding force, its inherently negative, thus it would look like (-2.45m)= -F thus the acceleration become positive.
4. ### Initial Velocity of a gun!Help please

In this case, since the gun is told to be at a certain height, all the energy is potential, but its spring potential energy. Use E=(.5)kx^2 with k being the constant and x being the distance the spring is compressed. All this energy becomes linetic at the point of release so set it equal to the...
5. ### Basic kinematics

if distance vs time gives you velocity, doing distance vs time gives you velocity, which vs. time gives you acceleration. slope of second graph is acceleration
6. ### Forces and friction :-s

Knowing both mass and acceleration you should find fnet using F=ma. This value plus the frictional force must equal the applied force so simply use the mass to find the normal force to get frictional force.
7. ### Theoretical Range of a Rocket

If you know the final distance in the y direction (60 meters), the acceleration(gravity), and the time, which is half time due to max height being at half the distance/time, you can find initial y velocity. Using this and the angle, find the intial x velocity.
8. ### More Work

Its like punching a piece of paper. The paper works on you but you still break through. Relative to the paper, since the fist broke through, work is negative.
9. ### Easy Resultant Velocity Problem

If he moves 10m/s and is slowed by 2.5m/s than how fast is the duck moving? Its the difference between the values. If confused draw vectors. That is it unless angles are involved.
10. ### Kinetic Energy

Say KE=1 originally. It would become 2.25 if increased by a factor of 2.25. With the Equation KE=.5mv^2, you can isolate the v^2 to get v^2= KE/(.5m). Just replace the values and you would get that answer. 2.25/.5= 4.5 which becomes the square root of 4.5 after you square root velocity
11. ### Friction and the Coefficents of Friction question

Floor is horizantal so normal force equals gravitational force. To break frictional force, applied force must equal it.
12. ### Potential and Kinetic Energy Problem

Using the known distance, gravity, and the fact that its velocity is zero at its highest point you can find its final velocity
13. ### Forces with an angle

Gravitational force equals the normal force on horizantal surfaces. Use that and the fact that it took 29N to break friction. At that point the applied force and frictional force is equal so frictional force is 29N. use that to find the coefficient. Same for the other one except with 23N