u3f – l4 1.solve these problems 1.a teacher standing on a 6in by 8in block of wood exerts a...

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U3f – L4 1. Solve these problems 1. A teacher standing on a 6in by 8in block of wood exerts a pressure of 4 psi. How much does the teacher weigh? 2. A basketball originally has 10 psi of pressure and a volume of 50 mL. If the volume is decreased to 10 mL, what is the resulting pressure? 2. Take out handout from Friday. We will talk about the solutions in 10 minutes. March 28, 2011 DRILL

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U3f – L4

1. Solve these problems

1. A teacher standing on a 6in by 8in block of wood exerts a pressure of 4 psi. How much does the teacher weigh?

2. A basketball originally has 10 psi of pressure and a volume of 50 mL. If the volume is decreased to 10 mL, what is the resulting pressure?

2. Take out handout from Friday. We will talk about the solutions in 10 minutes.

March 28, 2011

DRILL

U3f – L4

A teacher standing on a 6in by 8in block of wood exerts a pressure of 4 psi. How much does the teacher weigh?

Drill #1

Step 1: Write given information

P = 4 psi

F = ??

A = ?

Step 2: Write the formula and solve

A = L*W=6 in *8 in = 48 in2

P = F/A

4psi = F/48in2

F = 4psi *48in2

F=192 lbs

U3f – L4

A basketball originally has 10 psi of pressure and a volume of 50 mL. If the volume is decreased to 10 mL, what is the resulting pressure?

Drill #2

Step 1: Write given information

P1 = 10psi

V1 = 50 mL

P2 = ??

V2 = 10mL

Step 2: Write the formula and solve

P1*V1=P2*V2

10psi * 50 mL = P2*10mL

500 psi*mL = P2*10mL

P2 =500 psi*mL/(10mL)

P2=50psi

U3f – L4

A gas system has initial pressure and volume of 80 psi and 6 L. If the volume changes to 10 L, what will the resultant pressure be in psi?

Worksheet #4

Step 1: Write given information

P1 = 80 psi

V1 = 6 L

P2 = ?

V2 = 10 L

Step 2: Write the formula and solve

P1V1 = P2V2

(80 psi) x (6 L) = P2 x (10 L)

480 psi*L = P2 x (10 L)

10 L 10 L

P2 = 48 psi

U3f – L4

A sample of hydrogen gas is compressed from 300 cm3 to 2.5 cm3. Its pressure is now 5 psi. What was the original pressure of the helium?

Worksheet #5

Step 1: Write given information

P1 = ?

V1 = 300 cm3

P2 = 5 psi

V2 = 2.5 cm3

Step 2: Write the formula and solve

P1V1 = P2V2

P1 x (300 cm3) = 5 psi x (2.5 cm3)

P1 x (300 cm3) = 12.5 psi*cm3

300 cm3 300 cm3

P1 = 0.0417 psi

U3f – L3

The surface of a concrete block that touches the ground is 0.75 ft2. What is the pressure of the concrete block on the surface of the ground if the block weighs 50 lbs.

Worksheet #6

Step 1: Write given information

P = ?

A = 0.75 ft2

F = 50 lbs

Step 2: Write the formula and solve

P=F/A

P=50lbs/0.75 ft2 P=66.7lbs/ft2

U3f – L3

1. The diagram below illustrates how hydraulic brakes in a car work. The pedal must be pressed with a force of 8 lbs. The surface area of the piston connected to the pedal is 1 square inch. If the surface area of the piston connected to the other end of the brake line is 1.5 square inch, what is the force applied to that piston?

P1 = P2

Worksheet #7

P1 = F1 / A1 P2 = F2 / A2

U3f – L3

Worksheet #7

Solution:Step 1: Write given information

P1 = ? P2 = ?

F1 = 8 lbs. F2 = ?

A1 = 1 in2 A2 = 1.5 in2

Step 2: Write the formula and solve for the unknowns

P1 = F1 / A1

P1 = (8 lbs) / (1 in2)

P1 = 8 psi

P1 = P2 = 8 psi

F2 = 8 psi x 1.5 in2 = 12 lbs.P2 = F2 / A2 F2 = P2 x A2

U3f – L4

A bicycle tire is holding air at 85 psi of pressure. The valve of the tube lets air in and out. If the cross sectional area of the valve is 0.035 in2, how much force is required to add more air to the tube?

Worksheet #8

Step 1: Write given information

P = 85 psi

A = 0.035 in2

F = ?

Step 2: Write the formula and solve

P=F/A

85psi=F/0.035 in2 85psi*0.035 in2 =F

F =2.975 lbs

U3f – L4

A bicycle pump can be modeled like the diagram in number 7 above. Suppose the tube has 85 psi of pressure inside. To add more air to the tube, how much force is required to push the handle end of a bike pump if its cross-sectional area is 1 in2

Worksheet #9

P1 = F1 / A1 P2 = F2 / A2

P1=P2

U3f – L4

Worksheet #9

Solution:Step 1: Write given information

P1 = 85psi P2 = 85psi

F1 = 2.975 lbs. F2 = ?

A1 = 0.035 in2 A2 = 1 in2

Step 2: Write the formula and solve for the unknowns

P2 = F2 / A2

85psi= F2 / (1 in2)

F2= 85 psi* 1 in2

F2 = 85 lbs

U3f – L4

If we doubled the amount of force used in number 9 above, what must we change the area of the pump to be in order to maintain the same pressure

Worksheet #10

Step 1: Write given information

P = 85 psi

A = ???

F = 170 lbs

Step 2: Write the formula and solve

P=F/A

85psi=170lbs/A 85psi*A=170 lbs

A=170lbs/85 psi

A=2in2

Quiz on Wednesday or Thursday.

Know the definitions.

Understand the problems

U3f – L4