physics help?

Question

ok i just need some answers but if you can explain that would be great!

1.how fast would a 4kg mass have to be going to get to a height of 20 meters?

2.a car can start from rest and accelerate to a speed of 65miles per hour. there is nothing pushing it from outside where does the energy come from?what type of potential energy did the car start with-

3.an object is in the top of a 100meter building. what are the values for kinetic energy and potential energy at the various heights if its total energy is 500joules?
at the top of the building?
1/4 way down the face of the building?
1/2 way down the face of the building?
3/4 way down the face of the building?
at the bottom right befor it hits the ground



thanks these are the only ones i dont get help

Answer 1

To have enough kinetic energy to get to that height....

Set KE = 1/2 mv^2 equal to potential energy PE = mgh

Solve for v. Notice that the mass doesn't matter.

It came from the gas. There is electromagnetic potential energy in the chemical bonds of the gasoline that is liberated when you burn it.

Use the techniques from problem 1 to do problem 3.

Answer 2

Well..... At a height of 20 meters all of its kinetic energy would have been converted to potential energy. So
mv²/2 = mgh or
v² = 2gh
v = √(2*9.8*20) = √39.2 = 6.26 m/s

The automobile converts it's fuel (chemical energy) into mechanical energy and tries to turn its wheels which exerts a force against the ground. Then, in accordance with Newtons 3'rd Law, the ground exerts an equal, but opposite, force against the wheels and the car moves forwards.

The *total* mechanical energy of the object stays the same. That is, the sum of it's potential and kinetic energies is constant. At the top of the building it has 500J of potential energy and 0 kinetic energy. 1/4 of the way down, the acceleration of gravity has converted 1/4 of its potential energy into kinetic energy, so it has
(3/4)*500 = 375J of potential energy and
500 - 375 = 125J of kinetic energy. You work the rest of them out the same way.

HTH ☺

Doug