# Units of Pressure

#### Students’ Learning Outcomes

·         Define the term pressure.
·         Identify the units of pressure.
·         Explain hydraulics and hydraulic system by giving examples.

#### Information for Teachers

·         A force is push or pull.
·         Force can be classified into contact or non-contact forces.
·         Other examples of forces:
1.       Squeezing.
2.       Twisting.
3.       Stretching.
4.       Folding.
·         When a force acts on an object it exerts pressure. The pressure acts at a right-angle to the object itself, and its strength depends on the amount of force and the area over which it is applied.
·         When we apply a force on a body then according to conditions it has two effects like:-
o   When force applied on small area, it gives high pressure e.g. force applied on the sharp edge of a knife produces a high pressure due to which cut things easily.

o   When force on a large area is applied it gives low pressure, e.g. camels have wide feet to spread their weight on the sand which lowers the pressure and allows them to walk on sand without sinking in sand.
o   Pressure is the force exerted on a unit area. Mathematically, it is represented as P=F/A, where F=force, A=area and P=pressure.
o   Pressure is measured in Pascal’s (Pa). This is the S l unit of pressure. It is a pressure of one newton per meter square
o   Kilopascal (k P a) is 1000 pa.
o   Pressure is everywhere. It operates many machines and affects our weather. Solids liquids and gases all apply pressure to the surfaces they touch.
o   Pneumatic machines are driven by the pressure of gases, usually air. Unlike liquid, air can be compressed into a smaller space and this increases its pressure.
o   “Hydraulics” is the study in which we deal with mechanical properties of liquids.
o   Hydraulic system is used in car braking system. A small piston connected with one end of a tube is pressed with a small force by pressing on the brake pedal; consequently the large piston connected to the other end of the tube is pushed out with a much larger force which pushes the brake brushes. This results in stopping the car ultimately.

# Concept Map

#### Material / Resources

Balloon, water, common pin, tread, 2 syringes, water, rubber tube 2*10cm, 2 cylinders, 1 big and 1 small, 2 cork pistons according to the sizes of cylinder, toy car or any other thing

#### Worm up activity

·         Perform a following activity involving the students for giving them the concept of gas pressure.

#### Activity 1

·         Fill the balloon with water.
·         Tie the upper end of balloon.
·         Water exerts pressure downward so the balloons look like a drop.
·         Make several holes on the balloon’s surface. The water will start dropping down.
·         When the balloon is squeezed gently, the water starts to drop down from every hole equally and more speedily.
·         After the demonstration tell the students that when we apply a small amount of force to squeeze the balloon, it increases the pressure on water and ultimately water comes down from the holes with more speed. Now announce that we will learn about pressure, its units & its functions in the form of hydraulics.

#### Development

Tell the definition of pressure to the students first as force acting per unit area is called pressure. Tell the factors upon which pressure depends.  Write the following formula & units as:- P= F/A (N/m2

#### Activity 1

·         Ask the students a day before to bring clay and a coin. Divided the students into 5 or 6 groups. Ask the students to press the coin flat and then edgewise. Now ask the following questions from the students.

#### Activity 2

·         Clear the student’s concept by performing the following activity under his/her assistance.
·         Divide the students in pairs
·
·         Instruct o fill the syringes about half with water.
·         Put the hollow rubber tube from both sides at nozzles of the syringes as shown in the figure.
·         Ask the students to push the plunger of one syringe inward, the pressure exerted through it will push the plunger of other syringe outward direction.
·         Now as a student to push both the plungers inward with the same force. There will be no movement in the plungers because the amount of pressure on both sides is equal.

#### Activity 3

·         Perform the following activity to give the concept of hydraulics.
·         Instruct the students to attach the cylinder according to the figure shown above.
·         Instruct the students to place a toy car on the piston of larger cylinder.
·         Add some weight of 50 gm. or more on the piston of smaller cylinder. The student will observe the changes. Now add more weight on the piston.
·         As well as the weight increases on the small piston, the amount of pressure exerted by it will increase on the larger piston, so the car will rise up.
·         Now tell the students that hydraulics lift at service stations work in the same manner.
·         Ask the students to identify more hydraulic systems which can used in

#### Sum up / Conclusion

·         The hydraulic system works out on the principle of 2nd activity.
·         The pressure transmits equally in all directions.
·         Amount of pressure increases with the depth of the liquid.
·         Larger weights can be lifted by applying smaller force using hydraulic systems.

#### Assessment

Q1. At service station why hydraulic lifts are used?
(Expected response: To lift the car so that it can be washed properly)
Q2. Why people with high heel shoes are not allowed on jogging tracks?
(Expected response: Because high heels exert more pressure than flat shoes which make
The ground uneven)
Q3. When will the pressure be more, while standing on the mattress or when lying on the
Mattress
(Expected response: Pressure will be more when one stands on the mattress)