1. don't you think if hydraulic brakes can can stop a heavy vehicle by applying just little force , then why we can't produce more energy by applying little force.
suppose a hydraulic piston has been fitted on inside the road so that passing vehicles weight exerts a force on the hydraulic piston and we can can convert the up and down movement of the piston by using suitable,into the rotatory motion and connect it to the dynmo don't you think it would be useful???
4 producing more energy by applying little force.............

2.

3. Originally Posted by jayshankar
don't you think if hydraulic brakes can can stop a heavy vehicle by applying just little force , then why we can't produce more energy by applying little force.
suppose a hydraulic piston has been fitted on inside the road so that passing vehicles weight exerts a force on the hydraulic piston and we can can convert the up and down movement of the piston by using suitable,into the rotatory motion and connect it to the dynmo don't you think it would be useful???
4 producing more energy by applying little force.............
Energy is force times distance. Hydraulics work by changing a small force applied over a large distance. (Your foot depressing the brake petal), into a much larger force applied over a smaller distance. (the brake pads/shoes pressing against the disk/drum). The energy of the vehicle is then dissipated away as heat caused by the friction generated. The energy output of your foot is not directly transformed into energy that stops the truck. IOW, hydraulics can multiply force, but not energy.

A better example would be a hydraulic jack. You can use one to lift a car by the force exerted by your arm on the jack handle. However to do this, your arm exerts a few pounds of force downward for the distance of the handle's travel, while the piston of the jack lifting the car only rises by a small fraction of that distance. You are exchanging a small force over a large distance for a large force over a small distance. The energy expended by your arm pushing down through its downward stroke will be equal to the energy it takes for the piston to lift the car the small distance that it does. (You'll actually expend more energy because some of it will be lost through friction etc. before it get to lifting the car)

4. Originally Posted by jayshankar
don't you think if hydraulic brakes can can stop a heavy vehicle by applying just little force , then why we can't produce more energy by applying little force.
suppose a hydraulic piston has been fitted on inside the road so that passing vehicles weight exerts a force on the hydraulic piston and we can can convert the up and down movement of the piston by using suitable,into the rotatory motion and connect it to the dynmo don't you think it would be useful???
4 producing more energy by applying little force.............
Everything that Janus has told you is correct.

In addition, hydraulic brakes apply a huge force to the brake pads.

What happens is that your foot pedal applies the force of your leg to a piston of small diameter that pushes on the brake fluid. That moderate force, over a piston of small area creates a large pressure. Reacting to that pressure near the brake is a piston of larger diameter. The high pressure acting on a large area produces a large force, which moves the brake pads a small but crucial distance to create a large frictional force on the rotor or brake drum. That frictional force is what slows the wheels.

Energy and work performed are as noted, but are not significant issues in braking a vehicle. What happens is that kinetic energy of the vehicle is converted to heat in the brake drum or rotor by friction with the pads. The friction is a function of the force applied by the hydraulics.

5. As DrRocket said, it's the friction that stops the vehicle. I feel I should add that it'd be easy to increase the friction (and therefore stop the car faster) without changing the force by making the pads sticky or rough or something similar. Of course, there are practical reasons why they don't do such things.

6. Originally Posted by MagiMaster
As DrRocket said, it's the friction that stops the vehicle. I feel I should add that it'd be easy to increase the friction (and therefore stop the car faster) without changing the force by making the pads sticky or rough or something similar. Of course, there are practical reasons why they don't do such things.
That helps up until you hit the limit of the frictional force between the tires and the road, then you just skid. it is helpful to be able to control the force to stay in the regime in which the friction is substantial, but insufficient to lock up the wheels -- hence the prevalence of ABS brakes.

It could also be kinda hard on the rotors or break drum. Shoe brakes stop you just fine when the rivets are exposed and dsc brakes work when the pad is worn away and the metal substrate bears on the rotor (lots of friction) -- but it really tears up the machinery. Noisy too.

7. Ah right. I forgot about the tires. The real world gets so complicated.

8. All this is true but misses the actual question asked, can a piston in the road be harnessed to produce energy?
No, if you think about its impractical for many reasons which I'll leave to you hydraulic engineers to ponder.

9. It actually has been proposed. Piezoelectric energy harvesting from roadways has also been suggested. Of course, since nothing's for free, the energy harvested would come from the engines of the vehicles traveling over it, so there would be an immeaurably small decrease in mileage per gallon (or kwh) for the vehicles.

10. Originally Posted by MeteorWayne
It actually has been proposed. Piezoelectric energy harvesting from roadways has also been suggested. Of course, since nothing's for free, the energy harvested would come from the engines of the vehicles traveling over it, so there would be an immeaurably small decrease in mileage per gallon (or kwh) for the vehicles.
Or maybe just a decrease in the heat lost to the road through the usual interaction between the tires and the road, so that electric eneergy rather than thermal energy is the end result of the normal loss processes.

However, the electricity generated in that scenario would peak during peak traffic hours, when people are on the road, which is not exactly an optimal fit with demand. It would also be difficult to maintain and probably quite expensive.

11. Correc Dr. R ,highways already cost millions per mile-the above scheme would be astronomical in cost-nature nver gives something for nothing

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