# Physics 212, Quiz #2a: Dec 12, 1997

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A hydraulic device consists of a U-shaped tube. The left-hand column is a cylinder 0.5 m in diameter, and the right-hand column is a cylinder 5.0 m in diameter. The entire tub is filled with oil (density 800 kg/m^3). The upper end of each column is covered with a movable piston, which has negligible mass. The entire apparatus is at rest, with both pistons at the same height.

Question 1: What is the area of the left-hand piston? What is the area of the right-hand piston?

```

Left-hand  piston: Area = pi*(0.25m)^2 =  0.196 m^2
Right-hand piston: Area = pi*(2.5m)^2  = 19.6 m^2

```

Question 2: Buffalo Bills lineman Ted Washington (mass = 160 kg) steps on the left-hand piston. A Mack Truck (mass = 10000 kg) drives onto the right-hand piston. What is the force exerted by Mr. Washington on the left-hand piston? What is the pressure he places on the piston? What is the force exerted by the truck on the right-hand piston? What is the pressure it places on the piston?

```
Answer: Force of Mr. Washington = (mass)*(g) = (160 kg)*(9.8 m/s^2)
= 1570 N down

Pressure of Mr. W on left-hand piston
= force/area = (1570 N)/(0.196 m^2)
= 8000 N/m^2

Force of truck          = (mass)*(g) = (10000 kg)*(9.8 m/s^2)
= 98,000 N down

Pressure of truck on right-hand piston
= force/area = (98,000 N)/(1.96 m^2)
= 5000 N/m^2

```

Question 3: What is the net force on the right-hand piston? Does the Mack truck move up, down, or stay still?

```
Answer: The force on the left-hand piston is transmitted by the oil
to the right-hand piston, and multiplied by the ratio of the
piston areas.  It is exerted _upwards_ on the right piston,
because the oil exerts pressure on the bottom of the piston.

area right
Force on right piston = (force on left piston)*(----------)
area left

19.6 m^2
= (1568 N)*(---------)
0.196 m^2

= (1568 N)*(100) = 156,800 N  up

Net force on right-hand piston = (force of truck on top) [down] +
(force transmitted on bottom) [up]

= (98,000 N down) + (156,800 N up)

= 58,800 N up

So the truck moves up.

```