Michael's Blog

Earth’s Precession

mperlstein — Tue, 20 Apr 2010 01:50:47 +0000

As I understand it, the Earth’s precession is a gradual shifting of the tilt of the axis about which the Earth spins. This happens because the gravitational pull of both the sun and moon exert an external torque on the earth because it does not have a perfectly uniform shape and density, which causes the direction of the spin, and therefore the axis, to shift.

Moment of Inertia

mperlstein — Fri, 05 Mar 2010 04:31:46 +0000

Moment of inertia is the rotational parallel to mass in a linear system; it is a measure of an object’s resistance to changes in its rotational rate and determines the relationship between angular momentum and angular velocity. The integral of r^2 dm is equal to the moment of inertia ()

Greenhouse Gasses

mperlstein — Tue, 16 Feb 2010 03:44:30 +0000

‘Heat-trapping’, as I understand it, refers to radiation in the form of ultra-violet rays. These rays that are produced by the sun travel through earth’s atmosphere and hit earth’s surface. Some of these waves are absorbed, but many of them are reflected back towards the atmosphere. Here, the ‘heat trapping gasses’ reflect the waves back towards earth. This is possible because the individual molecules are so large that they are actually able to block the waves and re-direct them. These energy waves are associated with heat because when they are absorbed my molecules on earth, the absorbing molecules begin to vibrate faster, which is what produces the sensation of ‘heat’. The energy that is ‘absorbed’ is causing the electrons of that molecule to go to a higher energy level. Because light is energy, it has the effect described a few sentences ago of causing increased vibration of the molecule=heat.

mperlstein — Wed, 10 Feb 2010 06:47:33 +0000

On your blogs, please make a clear and concise statement detailing the distinctions between kinetic energy and momentum.

The first of two distinctions between kinetic energy and momentum is that momentum is a vector quantity while kinetic energy is a scalar quantity. That means that the momentum of two objects in a collision can either partially or fully cancel each other, while the energy adds up. Momentum is directional, and energy is not.

The second distinction is how they are proportional to velocity. Momentum is directly proportional to velocity, while energy is proportional to it’s square. This means that an object moving 2x the velocity of another object will have 2x the momentum, but 4x the energy. This allows for more work to be done by a lighter object moving at a higher velocity than a heavy object moving more slowly, even though they may have the same momentum.

-momentum is proportianal to velocity, kinetic is to the square of velocity

An Asteroid at the Edge of the Universe

mperlstein — Thu, 21 Jan 2010 04:13:06 +0000

These equations are not appropriate for attempting the ‘asteroid at the edge of the universe’ problem, because both equations require a measured value of the force of gravity. The edge of the universe is so far away from earth that the pull of gravity would be essentially non-existent on an asteroid out there. Therefore, both equations would read that the potential energy of the asteroid would be 0. Additionally, both equations require a distance from the asteroid to the earth, and since the edge of the universe is expanding at the speed of light, this value would also be impossible to obtain.

Impulse on a Falling Egg

mperlstein — Thu, 14 Jan 2010 04:30:17 +0000

The impulse of a particle is equal to the change in the momentum of the particle. Momentum equals mass times velocity, and therefore the impulse measures how quickly the velocity changes in relation to the mass of the object. Clearly this has large implications in our egg-drop project. We are essentially being asked to find how to create the slowest change in velocity, so that the momentum changes slowly and the egg does not break from the force of the impact. The force is the change in momentum in relation to time, or how long it takes the momentum to change. Having the egg drop onto a pillow, for example, would be ideal. The downy softness would ease the egg into it’s stop (extending the stopping time), and therefore reducing the force with which it hits the ground. So, there are two ways to go about this project in relation to impulse. Slow the time it takes the mass to stop (cushioning), or reduce the velocity from which it needs to stop (air resistance).

Mechanical vs. Non-mechanical Energy

mperlstein — Sat, 09 Jan 2010 22:51:55 +0000

Mechanical Energy is the sum of the potential and kinetic energies of a system (Emech=K+U), according to Serway. I also looked it up online however, and found a description of mechanical energy as energy in a system that you can see, which i like to think of as energy that causes movement in a system of more than one component, such as pushing down on a lever (the system is the arm and lever). It would follow then that non-mechanical energy is energy in a system that has only one component, such as an electron.

Question 6 and 7: Centripetal Force

mperlstein — Sun, 22 Nov 2009 19:09:14 +0000

6) I would not accept this statement, because the mere fact that they are in orbit means that they are not outside of the pull of gravity. Being ‘in orbit’ means that an object is traveling with a centripetal force (in this case gravity) acting upon the object and pulling it to the center of the circle that the path is a section of. This is what keeps it in a curvilinear path instead of the velocity (which is directional tangent to its point on the curvilinear path at any moment) making the object move in a straight line and therefore ‘out of orbit’. Therefore, the astronauts have weight because they are still in range of the gravitational pull.

7) The rotating cylinders would produce an effective simulation of gravity because of the centripetal force that would be produced by their rotation at a constant rate of acceleration (constant velocity is impossible because of the nature of non-linear motion). The rotation of the cylinder would mean that anything that has any contact (direct or indirect) with the shell of the circle would have the centripetal force acting upon it in the direction of the center of the the cylinder. For example, if a floor of the colony bisected the cylinder into two smaller cylinders, and a person was standing at any point on that floor, then the person would be experiencing the centripetal force because the floor is connected to the walls, and friction transfers the force from the floor to the person’s feet, producing an effect similar to gravity of holding the person on the floor.

(This answer makes little sense to me because in this scenario it seems like the force would be acting upon the person ‘sideways’ towards the center, and not ‘downwards’ like gravity. It seems to me that in order to produce a gravitational effect the floors would have to be concentric to the center and the outer walls… like tubes surrounding the axis that the cylinder rotates on.)

Force Questions due 11/2

mperlstein — Sat, 31 Oct 2009 20:12:11 +0000

A) The external forces acting on the ball are gravity, which pulls the ball towards the earth, and the force that the hand is exerting on the ball, which holds the ball in place so it doesn’t fall towards the earth.

B) Gravity is the force acting on the ball as it is falling. The reaction force is the gravitational force that the ball is exerting on the earth.

5) You could shake your hands to get the water off. This exemplifies Newton’s 1st law, because the water droplets begin moving with your hand, but once your hand stops they continue to move because of their inertia (they continue with a constant velocity in the same direction that your hand was moving) and are therefore propelled off of your hand.

10)

A) The “force of the throw” must exceed the gravitational force, otherwise the ball would never leave the girl’s hand. Because the force of the throw exceeds the gravitational force, the ball moves upwards in the direction the girl throws it.

B) The force of the throw is equal to the gravitational force at the apex of the flight of the ball, at the moment when there is no movement. The force of the throw decreases as the ball moves higher, because the gravitational force is constant, while the force of the throw was only one initial ‘burst’ of force. Therefore, the force of the throw decreases until at the apex it is equal to the gravitational force, and after this the gravitational force exceeds the force of the throw.

C) The force of the throw is however much force the child threw the ball with.

D) The ball moves away from the child’s hand because the amount of force that she threw the ball with was greater than the gravitational force pulling the ball downwards towards the earth.

Newton’s Laws and Free Body Force Diagrams

mperlstein — Thu, 29 Oct 2009 03:58:44 +0000

Newton’s Laws:

1) When viewed from a stationary point, an object in motion will stay in motion with a constant velocity, and an object at rest will stay at rest, IF there are no external forces acting upon the object.

2) When viewed from a stationary point, the acceleration of an object is directly proportional to the force acting upon it, and inversely proportional to its mass. Force= mass*acceleration

3) For every action there is an equal and opposite reaction.

Free Body Force Diagram: A diagram that displays ALL forces (physical and field) that are acting on an object.