Motion

[Qfwfq]

But you and the boat will contain the same amount of momentum.

:doh:

 

What you mean to say is that (if there are no other forces on either body!) the rate of change in momentum is the same for each. Of course, if you are pushing the truck without your feet slipping on the ground, this holds concerning the truck and Earth. Try measuring the change in Earth's velocity now...

[arkain101]

my bad?

 

I will have to look into that.

 

Momentum is conserved right?

 

Is it equall split if a force is applied in free floating space between two objects?

[pgrmdave]

In the vacuum of space, pushing the truck WOULD move it... Hence the idea of cancelled forces is again negated.

 

Not necessarily - even in a vacuum inertia exists.

[Qfwfq]

Each of the two bodies pushes the other. The total is zero, but this concerns the composite system, not just the truck. If it is on perfectly flat ground and frictionless, a small force will give it a small acceleration.

 

Try thinking about this one: if he brakes are on full, which force is cancelling out that of who is trying to push the truck?

[Mohit Pandey]

Each of the two bodies pushes the other. The total is zero, but this concerns the composite system, not just the truck. If it is on perfectly flat ground and frictionless, a small force will give it a small acceleration.

 

Try thinking about this one: if he brakes are on full, which force is cancelling out that of who is trying to push the truck?

I think the answer is mechanical force is cancelling it.

[Qfwfq]

Which mechanical force?

[Sleeth]

I think the answer is mechanical force is cancelling it.

 

Which mechanical force?

 

I think he means the machanical force of the brakes, only possible due to the force of friction though!

[Qfwfq]

OK I'll let the cat out of the bag...

 

It would be the brakes if you consider the wheels (and all that rotates with them) as a separate thing from the truck. If you don't, then it's the friction force of the ground on the tyres.

 

Now, for the force your hands apply to the truck, there is the equal and opposite one applied by the truck on your hands (or onto you if you prefer). What is it balanced by?

[Mohit Pandey]

OK I'll let the cat out of the bag...

 

It would be the brakes if you consider the wheels (and all that rotates with them) as a separate thing from the truck. If you don't, then it's the friction force of the ground on the tyres.

 

Now, for the force your hands apply to the truck, there is the equal and opposite one applied by the truck on your hands (or onto you if you prefer). What is it balanced by?

The equal and opposite force applied by the truck on my hands is balanced by my force applied to the truck. Is the answer correct?:hihi:

[Southtown]

Not really. The forces are both still there, they're just absorbed by the friction of the opposite object. The truck absorbs your force unnoticably while you feel the truck's force in your arms, legs, and *back, that is until one object's friction gets overwhelmed by the other's force.

 

* ouch

[Mohit Pandey]

My next question :naughty:

 

Tom, Jack and John were riding in a motorcar that was moving with a high velocity on an expressway when an insect hit the windshield and got struck on the windscreen. Tom and Jack started pondering over the situation.

 

Tom suggested that the insect suffered a greater change in momentum as compared to the change in momentum in the motorcar because the change in velocity of the insect was much more than that of the motorcar.

 

Jack said that since the motorcar was moving with a larger velocity, it exerted a larger force on the insect. And as a result the insect died.

 

John while putting an entirely different explanation said that both the motorcar and the insect experienced the same force and a change in their momentum.

Comment on these suggestions.

[Qfwfq]

John is perfectly right. What needs to be cleared is that they undergo different changes in velocity due to having different masses.

 

The force of the truck onto you is balanced by the force of the ground on your shoes. (Note, also for what I said above about the truck's wheels, this doesn't consider the vertical component of weight.) As Southtown says, the force also goes through your arms, back and legs, you can consider these as being different parts pushing each other. Now, if you consider truck and you, all the way from tyres to shoes, as a composite "thing" then it is applying a force to the ground through your shoes and an opposite on to the same ground through the tyres, these balance each other. The ground, by the third law, is applying a (horizontal) force through the tyres and an opposite one through the shoes, these also balance each other.

 

What you need to distinguish is between the two opposite forces on some body (which may balance each other) and the two opposite ones that two bodies are applying to each other by the third law. A force applied to a body might not be balanced by other forces, in this case the body accelerates and the "equal and opposite reaction" is applied by that body on whatever is pushing it and is called inertia.

[Jay-qu]

Sounds suspiciously like a homework problem..

 

John is correct. The force are equal and opposite, but because of the larger mass of the car, it experiences less change in velocity than the insect.

[Mohit Pandey]

Horse pulls a cart. According to the third law of the motion, the cart exerts an equal and opposite force on the force. The cart does not move until force exerted by the horse overcomes the force of friction.

 

My question is that when the cart had moved, does it applies the same force on the horse which it was applying before moving on the horse. If yes, does this not affect the horse or the motion of cart. Why does not the force applied by the cart stops the horse? According to the third law of the motion, as friction force should be equal to the force applied by the horse, how does the horses applies more force than it? :hihi:

 

:xx: Keep smiling and asking, it makes people wonder what you're up to!;)

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[Southtown]

When the friction of the cart is overcome, any of the horse's leftover force is absorbed by the ground. The ground then pushes back, and hence the motion of the horse and cart.

 

The horse does not have to work any harder once the cart is moving, unless she wants to go faster. Any increase in force beyond the cart's friction accelerates the horse and cart, because the ground's force increases with the horse's.

[arkain101]

Compare the situation to a horse and cart in space

 

and a horse and cart on the road.

 

In space theres no friction so no forces can be made into motion.

 

On the road the horse has the friction and the cart does not.

 

The Forces are equal on both horse and cart in all situations, but the one who "wins" and gets its way of motion is the one who has the most friction. So if the cart puts its breaks on and stops the horse friction force has overcome the horses pulling force, but if we add another horse, the friction is overcome.

 

I can give you a really good example later. But try to know that it isnt about motion, since motion is relative to your observation. Its about force.

[Qfwfq]

My question is that when the cart had moved, does it applies the same force on the horse which it was applying before moving on the horse. If yes, does this not affect the horse or the motion of cart. Why does not the force applied by the cart stops the horse? According to the third law of the motion, as friction force should be equal to the force applied by the horse, how does the horses applies more force than it?

It isn't easy to follow what you are asking, but I'll just try to give an outline.

 

The two forces are always equal and opposite. When the force applied by the horses overcomes friction, the cart will be accelerating and therefore the opposite force (cart-->horse) is due to friction plus inertia. If you throw a stone, you feel it pressing your hand more than just the stone's weight.