Why does energy behave the way it does?

[scilearner]

Hello everyone,

 

May be the question is when something has energy and there is no resistance why does it move? Thanks

[Physicsfan]

consider a stationary object such that the vector sum of all forces acting on it is zero, or in other words there is no force on the body at all.

This body has no potential energy.

Now if a force F (A push) is applied on the body for a time t seconds, then the velocity obtained by the body would be obtained from the equation [imath]v=at[/imath].

Now the energy gained by the body(Kinetic energy) is the same as the work done by the force on the body, because energy is transferred to the body.

(law of conservation of energy).

Now we know that work is [imath]F S[/imath]

now we know [imath]F=ma[/imath], but we do not know the magnitude of the displacement.This can be obtained from the equation [imath]v^2=u^2+2aS[/imath] or [imath]v^2=2aS[/imath].

"play" with the equation and you get [imath]S = v^2 /2a[/imath]

now multiply [imath]ma[/imath] to the expression and you get [imath]K = 1/2mv^2 [/imath] which as you know is the formula for kinetic energy.

[scilearner]

Thanks Physics fan I always like when equations start to make sense My question is however more on let's say I push a box on the floor, it gains energy and moves. Why does an object decide to change in position when energy is gained. Is this something we know because of observation. I think this question is more philosophical.

[scilearner]

Hello everyone,

 

I don't know I have got really confused. Now if an object has energy, it can move. Is their any scientific explanation in molecular level or something why does energy make an object move. Also how do net forces work. If their is a box at rest and two people are pushing it with equal force in opposite directions. The object stays at rest. My question is what happens to the energy provided by two people. Does it cancel (how?), or does the object keep gaining energy without moving? I don't understand how forces give energy in a certain direction, what determines the direction? Thanks!!

Edited May 25, 2010 by scilearner

[swansont]

Energy is a bookkeeping concept, as are most physics terms. We observe that things behave a certain way, and come up with equations that describe this. The equations we use to describe motion indicate that there is a quantity that does not change (energy), because the equations do not change over time. (Other continuous symmetries give rise to the momentum conservation laws)

 

We have forces, which cause acceleration. It turns out that we can look at force as a function of position, as well as time, and that information is useful to us. I don't know how practical it is to think of things in terms of "energy permits motion" vs. "motion exists because of forces, and energy helps us keep track of it." It's sort of a chicken vs egg situation.

[scilearner]

Energy is a bookkeeping concept, as are most physics terms. We observe that things behave a certain way, and come up with equations that describe this. The equations we use to describe motion indicate that there is a quantity that does not change (energy), because the equations do not change over time. (Other continuous symmetries give rise to the momentum conservation laws)

 

We have forces, which cause acceleration. It turns out that we can look at force as a function of position, as well as time, and that information is useful to us. I don't know how practical it is to think of things in terms of "energy permits motion" vs. "motion exists because of forces, and energy helps us keep track of it." It's sort of a chicken vs egg situation.

 

Thanks swansoft Can you also help me with my last question. If their is a box at rest and two people are pushing it with equal force in opposite directions. The object stays at rest. My question is what happens to the energy provided by two people. Does it cancel (how?), or does the object keep gaining energy without moving? I don't understand how forces give energy in a certain direction, what determines the direction?

[Physicsfan]

My question is however more on let's say I push a box on the floor, it gains energy and moves. Why does an object decide to change in position when energy is gained. Is this something we know because of observation. I think this question is more philosophical.

 

A body does not move just because it has energy.Energy is scalar quantity,

it does not have direction.A body moves when a force is applied because force produces a change in the momentum in the body,and momentum is a vector quantity.Force is the rate at which a body gains momentum.

[swansont]

If there is no displacement, there is no work, and no transfer of energy. The two people will experience an increase in temperature; they will shed energy as heat transferred to the surroundings.

 

(BTW, it's swansont)

[michel123456]

Motion is an energy reservoir.

[scilearner]

A body does not move just because it has energy.Energy is scalar quantity,

it does not have direction.A body moves when a force is applied because force produces a change in the momentum in the body,and momentum is a vector quantity.Force is the rate at which a body gains momentum.

 

Ok you are right but what I was meaning to say was an object moves if it has kinetic energy. I mean if force is the rate at which a body gains momentum, that means force gives energy to an object. Once an object has kinetic energy it keeps moving, so force is not needed for movement, are you saying it is needed to start the process, but when it starts doesn't it do by transferring energy. Anyway may be you are right but I'm very confused at the moment. I have another question.

 

Let's say a box was moving forward at constant speed, it must have some energy, then a force in opposite direction acts on it to make the object stop, how has the negative force removed energy from the object, has it been transferred to heat? Also if I keep applying this force, the object would eventually accelerate the opposite side, how come now the negative force is giving energy to object and before it was taking away. Thanks

[swansont]

Let's say a box was moving forward at constant speed, it must have some energy, then a force in opposite direction acts on it to make the object stop, how has the negative force removed energy from the object, has it been transferred to heat? Also if I keep applying this force, the object would eventually accelerate the opposite side, how come now the negative force is giving energy to object and before it was taking away. Thanks

 

It depends on the details of the force. It could be compressing a spring. It could be turning a generator which generates electrical energy. It could be sliding uphill and gaining gravitational potential energy.

 

If the motion is 1-D, the add/subtract energy depends on which frame you are in. If we view this in the frame that the box starts in, it is originally at rest, and always speeding up (in the negative direction). In another frame, it is moving faster to start with, and never comes to rest.