kinetic energy

[1123581321]

Hi,

 

Wondering why in the formula K = 1/2 m v (squared) for kinetic energy, there is a half.

[Mr Skeptic]

It comes out very nicely if you know calculus (actually a lot of things in physics do). It's the same reason for the (1/2) in the equation that relates distance and acceleration. I think it's also the same reason for the (1/2) in the area of a triangle.

[swansont]

Kinetic energy changes because you do work on an object, changing its energy.

 

[math]W = \Delta{KE} = \int F {dx}[/math]

 

[math]F = ma = m\frac{dv}{dt} = m\frac{dv}{dx}\frac{dx}{dt} = mv\frac{dv}{dx}[/math]

 

[math]W = \Delta{KE} = \int mv {dv} = \frac{1}{2}mv^2[/math]

 

(assuming you started at rest)

 

As Mr Skeptic has noted, you need to know calculus to see why it shows up.

[PaulS1950]

How is mass calculated from weight? Isn't it weght x (1/2 gravity)?

[Newbies_Kid]

Mass in Kg while weight in Newton

F=MA

9.81 N = 1kg x 9.81 m/s^2

[PaulS1950]

Mass in Kg while weight in Newton

F=MA

9.81 N = 1kg x 9.81 m/s^2

 

OK, can you show an example that an old American can understand using pounds for weight?

[Sisyphus]

1 pound of force (weight) is equal to 1 pound of mass times standard gravity (~32ft/s^2). So basically, a 1 pound mass will weigh very close to 1 pound on the Earth's surface.

 

The fact that "pound" is a measure of both mass and weight is one of the many reasons SI units are more practical than English units. "Pound-force" is sometimes used to specify weight rather than mass.

[Mr Skeptic]

Because in SI units people never use a measure of mass as a measure of force. How many kilograms do you weigh?

[Newbies_Kid]

OK, can you show an example that an old American can understand using pounds for weight?

 

pounds is a unit for mass.. while weight is force. As gravity acceleration is constant at sea level, may be using pounds for weight should be no problem in our daily life. " Give me 2 newtons of sugar please?" sounds weird isn't it

[swansont]

pounds is a unit for mass.. while weight is force. As gravity acceleration is constant at sea level, may be using pounds for weight should be no problem in our daily life. " Give me 2 newtons of sugar please?" sounds weird isn't it

 

Pounds is a unit for force. The English unit for mass is a slug. Pounds-mass is a contrived unit to make the math and unit analysis easier for people using those units (mainly engineers).

[PaulS1950]

E= MV^2 /2

M = ? if weight is .25 Lbs.

Trying to convert pounts of weight to pounds of mass M = P x (G/2) ?

Is that formula correct?