cahier physique - physics workbook

PART I: INTRODUCTION : how much pull for how much area (one LAb)
TRY without hints please.

1) You can push a drawing pin into a piece of wood - but you cannot push your finger into the wood even if you exert a larger force. Why ? What is the difference between a sharp knife and a blunt knife? Why a lady wearing high heel can damage a wooden floor badly and not a man (heavier) wearing snickers ?

pressure is weight per unit area.
High heels exert a great pressure on the ground because the weight
is distributed over a really small area. You can try an experiment with
a brick or a block of wood. place a piece of clay and find out which face will crush the clay the most.

2) How much pull or push per unit area is called the pressure.
P = force (in newtons) / area (in squares meters)
or P = F (N) / A (m²)
The unit for pressure is therefore ____ / _____ or ___ / ____
this unit is also called pascal or Pa.

3) an elephant weighting 40, 000 N (W) stands on one foot of area 1000 cm² . We want to find out what pressure is exerted on the ground.
(that is the force per square meters)
A) just to give you an idea of how heavy is an elephant, find out the mass of the
elephant in kg, in tons and its weight in pounds.
hint: - weight (N) = mass (kg) x 9.8 m/s/s or W = m g solve for m ; 1,000 kg = 1 ton ; 1kg = 2.2 pounds

B) convert area= 1000 cm² to m²
hint: 1 cm = 0.01m (or 10^-2) so 1 cm² = 0.0001 m²(or 10^-4)

C) Find the pressure exerted by the elephant.
hint: P = weight (N) / area (m² )

4) Consider a girl weighing 400 N standing on one "stiletto' heel of area 1 cm2.
A) Obviously she is less heavy than the elephant. How many times less ?
hint: find the ratio between her weight and the elephant's
B) Find the area of the heel in m². (use B previous question)

C) What is the pressure exerted by the girl ? (P = weight / area).
Is it less or more than the pressure exerted by the elephant ?
If they both taking a walk on a beach, which one will sink farther into the ground ?
why do camels have large feet ?

5) Did you try to lay on a bed of nails ? Why is this possible ?
watch this video on U-tube. You may want to ask your instructor to do the same thing..

6) Because I don't want to do this experiment, I will use a balloon instead. the balloon
will pop with one nail. Will it pop with a bed of nails ?

7) A) The bottom of 10 shoe is about 11 inches long and 4 inches wide, so the total area of such shoes is _________ in². The pressure of a 180 pound (lb) man exerts on the ground is P = ___ / ____ = ___ lb/in² .

8) What pressure is exerted on the snow by a 180 lb skier if his skis are 6ft long and 5 in wide ? (in lb/in²)

9) A boy and a sled have a combined mass of 38kg. The runners of the sled are 1.60m long and 1.2cm wide.
A) find the weight in newtons (N) of the system (boy + sled)
hint: weight = mass x 9.8
B) Find the total area A of the runners in m². (2 of them!)
hint: first convert cm to m.

C) Find the pressure exerted on the snow in N/m2. (that is in pascals)
hint: P (pa) = W (N) / A (m²)

D) We can express the pressure in another unit called the atmosphere (atm).
1 atmosphere = 10⁵ pascals. Convert the pressure found in C) in atm.
1 atmosphere is about the pressure exerted by the air on the ground (the atmosphere) at sea level (temperature 20C).

MINI LAB foot pressure

10) Which of the following exerts the most pressure while resting on the floor ?
A) a 25N box with 1.5 m sides
B) a 15N cylinder with a base radius of 1.0 m
C) a 25N box with 2.0 m sides
D) a 25 N cylinder with a base radius of 1.0 m ?
hint: area of a circle = pi x (radius)² pi = 3.14... (use your calculator)

PART II: FLUIDS AT REST : PASCAL's PRINCIPLE

A change in pressure applied to an enclose fluid is transmitted
undiminished to every point of the fluid and to the wall of the container.

For example an egg is a container with fluid inside. If you press
on the egg from every where, the pressure will be transmitted.
Try this experiment: place an egg in a plastic bag and try to crush it.

Any change in the pressure of a fluid is transmitted uniformly in all directions
throughout the fluids. But pressure is force per unit area.
Imagine you fill a small glass bottle with water.
If you exert a pressure at the top, the same pressure will be exerted on the
bottom. But the pressure is force per unit area. So the same force per unit
area will be applied to the bottom of the bottle. But because the area of the
bottom is larger, so is the force ! and pop goes the bottom !

force2 = force1 x area2/area1
or force2/area2 = force1/area 1
or pressure1 = pressure2

Using pascal's principle you can blow up the bottom of a bottle.
try this at home (you need a lucozade type bottle)

do not try this at home:
http://www.teachertube.com/view_video.php?viewkey=79f13c66d289839bbf02

do this experiment and get extra credits.

Cut six pieces of glass tubing 3 cm long. Cut small holes in a tennis ball just big enough to insert the glass tubes. Place the holes at top and bottom, right and left, nearest you and farthest from you. Put the tennis ball and glass tubes in a bucket of water. Squeeze out all the air so the tennis ball is full of water. Take out the tennis ball and hold it in your hand with your fingers around it but not over the glass tubes. Squeeze the tennis ball. The same amount of water squirts out through the glass tubes in all directions.
SOURCE

DId you know?
The Heimlich maneuverer is a sharp pressure applied to the abdomen of a person to dislodge a foreign object caught is the throat, It is an application of Pascals principle. Why? extra credits if you explain why.

This principle is used in hydraulic machines. The idea is always to magnify your force.
see blow. Using that principle you can lift car.

The pressure (P1 ) exerted by the force of the small piston is transmitted throughout the fluid and results in a multiplied force on the larger piston.
The formula to used is: F2/F1 = A2/A1. If the area is multiplied by 9, so is the force.

LAB2 hydraulic system \

1) A force of 10N (F1) is applied to a circular piston with an area of 2cm² (F2) in a hydraulic jack. The output piston for the jack has an area of 100 cm² (A2).
A) What is the force exerted on the output piston by the fluid ? (F2)
hint: you can find by how much the area is multiplied to find out by how much the force is multiplied or you can use F2/F1 = A2/A1 to solve for F2.
B) What is the pressure in pascals in the fluid ?
hint: pressure = force / area = F1/A1 . convert cm² to m² first. 1 cm² = 0.0001
C) Fill the blank : The output force (F2) is _____times larger than the input force (F1).
hint : find F2/F1 .
The large area (A2) is ________ times larger then the small area (A1).
hint: find A2/A1
This number is called the mechanical advantage of the machine:
by how much the force is multiplied = F2/F1

2) Dentist's chairs are examples of hydraulic-lift systems. If a chair weighs 1600N (F2) and rests on a piston with a cross-sectional area of 1440 cm² (A2).
A) what force (F1) must be applied to the smaller piston with a cross area of 72cm² to lift the chair ?
hint: you can plug the numbers in the formula F2/F1 = A2/A1 and solve for F1. you don't need to convert.
B) The output force F2 is ____ times larger than the input force F1. (find F2/F1)
The mechanical advantage of the chair is ________.

3) the load bearing piston in a certain hydraulic system has an area 50 times as large as the input piston. If the larger piston supports a load of 6000N (F2), how large a force must be applied to the input piston? (Find F1).
hint: A2/A1 = 50 so F2/F1 = 50, solve for F1

4) A force of 10N (F1) is applied to a circular piston with an area of 2cm² in a hydraulic jack. The output piston for the jack has an area of 100 cm².

5) Suppose that the input piston of hydraulic jack has a diameter of 2cm and the load piston has a diameter of 25cm. The jack is being used to lift a car with a mass of 1400kg.
A) What are the areas of the input (A1) and load (A2) pistons in square centimeters ?
hint: area = pi x (diameter/2)²
B) What is the ratio of the area of the load piston to the area of the input area ?
hint: find A2/A1
C) What is the weight of the car ? (F2)
hint: weight = mass(kg) x 9.8
D) What force must be applied to the input piston to support the car ?
hint: A2/A1 = F2/F1 solve for F1

6) A hydraulic jack used to lift cars is called a three-ton jack. The large piston is 22mm in diameter , the small one 6.3mm . Assume that you want to lift a 3 tons car.
A) Find the force exerted on the small piston to lift the car. Try on you own. you can . If it is too hard, follow the steps. (refer to the previous problem)
step1) Find the small area A1. The area of a circle = Pi x (diameter /2 )² Pi=3.14 ..
step2) Find the large area A2.
step3) convert 3 tons in kg. that will give you the mass of the car. (M)
step4) Find the weight of the car , call it F2. weight = mass (kg) x 9.8
step5) You are now ready to find F1. F2/F1 = A2/A1 . solve for F1/
B) convert F1 and F2 in pounds to give you an idea of the power of this machine.
hint: 1 newton = 0.22 pounds.
F1 = mass (kg) x 9.8 = ________ N
F2 = ________N

7) In a car lift, compressed air exerts a force (F1) on a piston with a radius of 5cm. (1) This pressure is transmitted to a second piston with a radius of 15cm (2).
A) The diameter of the output piston is ________ times larger than the input piston.
B) So the area A1 of the output piston is _________ times larger than the input piston A2. (square the previous ratio)
C) so the output force F2 is ____________ times larger than the input force F1.
D) So the mechanical advantage is ___________.

8) If the machine multiplies your effort, there is a price to pay. Let's say you are lifting a car using a hydraulic jack. (see below). The car is the load and you are pushing on the pump. Imagine the situation. you are changing your tire. What is the price to pay ?
After a while,you will be exhausted. not because the effort is large but because ...

let's say the mechanical advantage is 10. The load (F2) is ________ times larger than the effort (F1) and the distance you need to pump will be __________ times larger
than the distance the car is raised.