cahier physique - physics workbook

INTRODUCTION -
In this lab, using ratios, you will find out that we are the right size. A scaled up or scaled down human being is not possible. Unless you change the shape of the human being. A gazelle scaled up can't be unless you change its shape, to become for example an elephant. But this won't be a gazelle anymore.

PROCEDURE: you need a TI. (source: hands-on physics activities - Jossey-Bass edition)

Imagine 3 men. Minuscule Mike (17cm long), Standard Stan (170cm) and, towering Tom (1700cm).

FILL THE BLANKS:
Mike and Tom are scale models of Stan. Same shape but scaled up or down. The scaling is by 10 for Tom. That means that his arm will be 10 times longer, the leg 10 times longer, every bones 10 times longer. That means also that the diameter of the bones like the femur will also be 10 times larger.
Any linear dimension (side) will be 10 times stan's corresponding dimensions.
The scaling is by 1/10 for Tom. All linear dimension is divided by 10. Complete the table and we will discuss the results. Understand that the cross-sectional area of our femurs support our weight. REmember what your learnt before :
If you scale up a shape by 10, the lengths are multiplied by __________ (10? 100? 1000?), the areas are multiplied by (10 ? 100 ? 1000?)
and the volumes are multiplied by (10? 100 ? 1000?). Also, as the weight (volume) increases, the ratio surface/weight _____________ !!!
(hint: a mouse has more surface area per weight than an elephant)

	Mike	Stan	Tom
Height h	17cm	170cm	1700cm
diameter of femur d (see picture below) hint: diameter is a linear dimension. multiply or divide by 10	____	4cm	____
radius of femur r hint: radius = diameter /2	____	2cm	_____
cross sectional support area of femur A hint: area = pi x (radius)² = 3.14 r²	____	12.6 cm²	_______
volume of entire body V (use Stan's volume to find Mike's and Tom's) hint: remember volume is multiplied/divided by ........	____	60,000 cm³	______
Mass of entire body hint: mass is like volume. it is multiplied/divided by .........	____	60kg	_____
weight = mass(kg) x 9.8 hint: multiply the mass by 9.8 from above row	____	______N	_______
pressure on bones = weight/support area hint: divide the weight by the cross sectional area of femur since it is the area of femur that supports our weight.	____	47N/cm²	____
surface area of the body. use Stan's to find Tom's and Mike's hint:areas are multiplied/divided by .......	____	17,000cm²	_____
surface area / volume hint: divide the surface area of the body by the volume.	____	_______	____

ANALYSIS
1) Observe the table. Tom is scaled up by a factor of ____.
It means all his lengths are multiplied by _____. But because the areas are found by multiplying length by length, when compared to Stan's, all his areas (area of body and area of femur) are multiplied by ______. His body volume is Stan's body volume multiplied by
__________.

2) Le'ts look at the pressure the weight exert on the areas of the femur supporting the weight . The pressure is the force per unit area. More pressure, more damage to a supporting pole. Compare the pressure for the 3 persons. The weight of Tom exerts a pressure ________ times Stan's/
A pressure of 400 N is like 40kg/squared cm or almost 90 pounds/cm² !!!
What do you think will happen to Tom ?
If you were to build a giant, what changes would you make so the bones don't break ?

3) Let's look at the surface/volume ratio. THat is the surface of skin (border between inside and outside) available per unit of volume.
As the size of the person decreases , from Tom to Mike, the ratio _________. As an animal 's size decreases, its surface ___________ when compared
to the weight. If the surface increases, it means more exchange with the outside world. So there will be more _________ loss. how many times more ? _______ (compare the ratio surface/volume)
That means that Mike needs to eat _______ times more than Stan to survive.

4) When an animal or object fall 2 forces act on it. The pull due to gravity proportional to the weight.
The drag due to air proportional to the surface of the body and speed.
What would Stan, Tom, and Mike look after falling 900 meters?
hint: as the surface increases, drag increases. As the weight increases, gravity increases

5)A mouse can fall from a 10-story building and walk away. True or false ?

6) If you were to scale up a gazelle by a factor of 2, can you just multiply all the lengths ?
What do you have to do for the animal to survive ?

On the op, the femur of a gazelle,
in the middle the femur of a gazelle scaled up.
The bones of such an animal would collapse.
Bottom the femur of an elephant.
To get a larger animal than the gazelle, you need to change
its __________. So of course, it is not a gazelle anymore !

7) Why do you think the same quantity of water will evaporate faster when placed in a large plate that when placed in a glass ?

8) Some really heavy dinosaurs had spikes on their back. Why ?

9) A cube is 1cm x 1cm x 1cm
Its volume is _______ cm³, its total surface area is _________cm². the ratio surface area/volume is S /V = _________
a cube is now 2cm x 2cm x 2cm, the ratio surface area/volume is ____________
a cube is 3cm x 3cm x 3cm the ratio surface area / volume is ______________
As you increase the side of a cube (one linear dimension), the ratio S/V _______________.
volume is proportional to mass (kg).
so smaller objects have larger __________ ____________ per unit volume.
or smaller objects have larger ___________ _____________ per unit mass. (per kg) (same answer)

10) why steel wool (used to scrub the plates) rust very fast ?

11) why thein French fries cook faster than fat fries?