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Forces - Hydraulics

This GCSE Physics quiz challenges you on forces and hydraulics. Pressure can be transmitted in all directions through all fluids. It is transmitted particularly efficiently through liquids. Although their particles are further apart than solids, they are still close enough together for liquids to be non-compressible. That means that when pressure is applied to the surface of a liquid its volume remains the same. If you fill a flexible container (for example, a balloon) with water and squeeze it, it will change shape but not volume. Areas that are not supported will bulge out, wherever they are showing that the pressure is transmitted in all directions through liquids.

This means that liquids can be used to transmit pressure over long distances and round corners by using tubes in a hydraulic system.

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Even better than this, hydraulic systems can be used to magnify small forces to make them very large. They are used in a wide range of machines like car braking systems, cranes and diggers.

These systems are extremely powerful. You may have felt the power of hydraulic systems at school. A common experiment is to half fill a large syringe with water and connect it via a length of plastic tubing to a smaller syringe, also half-filled with water. One person holds the larger syringe and a second person holds the smaller one. They then have a competition to try to push the plungers to try to fill the other person's syringe. The person with the smaller syringe always wins easily (either that or the tube comes off one of the syringes because the pressure is so high!) since the force they use is magnified.

This idea is used for car braking systems. In car braking systems, the syringes are replaced by sealed metal cylinders filled with a special fluid (brake fluid). The pipes that are used to join the cylinders are very strong and fixed very firmly to the cylinders. When the brake pedal is pushed, it operates something called a servo which magnifies the force from the driver's leg. The servo is connected to the brake master cylinder which is the equivalent to the small syringe. The brake pipes connect the master cylinder to 4 separate slave cylinders. These are the equivalent of the large syringe. As the plunger of the master cylinder is pushed in, the pressure is transferred through all of the pipes to the slave cylinders. The slave cylinders have a larger cross-sectional area than the master cylinder which magnifies the pressure. This pushes the brake pads against the brake disc (or brake drum on some cars), converting the kinetic energy into heat energy and slowing or stopping the car.

Pressure is defined as the force acting per unit area. It is therefore calculated by dividing the force that is creating the pressure by the area over which it is acting. When the force is in newtons and the area in square metres, the pressure will be in pascals (Pa). The unit is named for the French mathematician, Blaise Pascal, who also carried out scientific research into hydraulics. He is said to have invented the syringe, although more primitive versions had been used by the Romans and an Iraqi/Egyptian physician. It was Pascal who discovered that liquids transmit pressure in all directions and that knowledge has become known as Pascal's law. He invented the hydraulic press, a very powerful machine that can be used to cut and shape even metals. He showed that the pressure increases with the depth of a liquid and not the weight, as was thought before his work. Pascal was amazingly talented and his name has been given to many other scientific and mathematical things such as Pascal's triangle, a computer programming language and more. He was only 39 when he died.

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  1. Different cross-sectional areas on the effort and load sides of a hydraulic system enables it to be used as a what?
    For a given pressure, if it acts over a larger area, the force will be greater
  2. What is the pressure of a system which experiences a force of 100 N over an area of 10 m2?
    100 divided by 10
  3. Which of the following can benefit from the use of hydraulics?
    Diggers and brakes are two common GCSE examples of the use of hydraulic systems
  4. What is the formula used to calculate pressure in different parts of a hydraulic system?
    Pressure is a measure of the force per unit area
  5. How is pressure distributed in a liquid?
    This is Pascal's law
  6. What fact can be said of liquids?
    Using a liquid in a hydraulic system is a good idea, as the liquid transfers the pressure from one side of the system to the other. Changes in the area of a section of the system can allow the force to be multiplied
  7. If a system has an area of 2 m2 at its effort side and 5 m2 at its load side, how much more force does the load side experience?
    Simple proportion
  8. A 2 cm x 5 cm rectangle within a hydraulic system experiences a force of 50 N. What is the pressure within the system?
    Always check units given in the question. It is common for measurements to be given in cm2 when a formula requires them to be in m2. 10 cm2 = 0.001 m2
  9. What are the units of pressure?
    One pascal is a force of one newton acting over one metre squared so they are quite small units
  10. What is the length of a rectangle within a hydraulic system having a width of 5 m that is subjected to a pressure of 15 Pa and a force of 15 N?
    Rearrange the pressure equation to work out the area. You know the length of one side of the rectangle and so you can work out the length of the other

Author: Martin Moore

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