can someone help me with this hypothetical physics/science question..?

Question

Okay. So imagine someone is sitting in a car and they have a dog on their lap. They are driving up to a weigh station to be weighed. Right at the exact moment that the weight reader person takes the reading, the driver throws the dog up into the air. (so the weight measurement is taken when the dog is in mid-air) SO, would the truck weigh the same or less than if the driver just kept the dog on his lap?

Its got me befuddled because isnt there a law of physics about how energy is neither created nor destroyed? So my thought was that the weight of the dog would be transferred to the driver as he tosses him up in the air. (the dogs weight force goes DOWN when he goes UP) But then part of me thinks that maybe if the dog is mid-air right when the weight measurement is taken, the weight has temporarily been suspended.

Argh... its driving me crazy! What would the correct answer be?

Answer 1

Okay, there are two things to consider here. The first is that mass and weight are two different things; weight is the force of a mass acting under gravity. So that's what the weigh station is weighing - the downward force of the truck & all its contents.

When you throw a dog up into the air you're using a force to do it, and according to Mr. Newton an equal and opposite force will therefore be acting on you (ie, you push the dog up with 1 newton of force and 1 newton of force will act on you in response pushing you down). So if you were weighed right when you were throwing the dog you'd weigh a little bit more because of this downward force on you and the truck.

Now, because you can't go "down" because the ground is in the way, this temporary force of throwing the dog would be resisted by the ground (or the car's shock absorbers and stuff) so the truck would effectively be "bounced" back from the ground. So your weight on the scale might depend on what stage of this "bouncing" your truck was at... if it was later in the bounce I guess you could weigh a bit less because the truck would be bouncing up and counteracting gravity a bit. If the dog flew high enough I guess this bouncing would eventually stop and then you'd weigh a bit less (because the dog floating in the air doesn't contribute anymore because he's not putting any force on you, the truck, or the weigh scale).

Where does the energy go? For the dog, it goes into his elevated position (which is a higher energy state because he has more "potential energy"). For you, it goes into bouncing your truck against the ground, so it dissipates as vibrations in the ground, vibrations in the air, vibrations in the truck, heat (which is molecular vibrations), some wear on your shock absorbers (metal grinding off, whatever), etc. Of course, the energy originally comes from your muscles which comes from food you ate which ultimately comes from the sun by plants doing photosynthesis, so energy is conserved here.

The mass remains constant; even if you go out into space where there is no gravity (well, much less than on earth anyway) your mass is the same.

Mass affects how hard it is to move something, so if you took a 1 ton box up into space and it was moving at high speed towards you it would still hurt you because it has a lot of mass even though it weighs nothing. So on the weight scale here, even though the weight changes depending on when you measure it, the total mass of the truck and driver and dog stays the same.

Answer 2

you're not creating or destroying energy. You're creating potential energy by the dog being suspended above the person's lap a distance. When the dog falls back down it will create the downward force used to propel the dog up. Energy and weight are extremely different. The car will weigh less. The person seated would exert a force down to push the dog up. This creates potential energy from kinetic energy. The person would temporarily create a heavier car in doing so, though it probably would be negligible compared to the weight of a vehicle. The same would happen when the dog was falling back.

Answer 3

This has little to do with conservation of energy. You're throwing a dog, not poofing the dog out of existance.

Your car will measure more the instant the dog is thrown, because the amount of force required to throw the dog up in earth's gravity is more than the amount of gravitational force the dog exerts when it is at rest in the car, at rest on the ground (it has to be, or the dog wouldn't accelerate upwards).

You might be forgetting that a weight scale measures force, not mass. The mass of the car with the dog flying about is the same, but the force changes the instant the dog is thrown, plus the force on the scale will be less while the dog is airborne.

Answer 4

It weighs less while the dog is in the air, but while the driver is tossing him up and catching him, the momentum of the dog is changing, exerting more downward force on the scale. So the truck weighs more while the driver is tossing him up and catching him, and less while the dog is in mid-air.

Get it?

Answer 5

If you eliminate the surroundings from the earth (so there is not any terminal pace) then considering it's less difficult. Initially the gravity will pull the character downwards simply adore it does at the floor, however as you get towards the center of the earth the drive of gravity pulling downwards will weaken till you get to the center of the earth in which there shall be no total final result of gravity. Afterwards you are going to start to sluggish as you come back to the skin at the different aspect and gravity will get more potent. Theoretically you are going to pop out at the different aspect at precisely the pace you entered.

Answer 6

It's the car minus the dog. While he in the air, he's not a part of the system that includes the car

Answer 7

The dog's weight will not be measured by the scale because the dog is temporally weightless as it is in free fall.