Class Notes 11-30 ps

Hand back Papers

Grades

Solving Problems through calculations

Steps to solve problems

1) Identify GIVENS, and what is WANTED2) DRAW pictures and LABEL values3) IDENTIFY the EQUATION that connects

givens and what is wanted4) CONVERT units, if needed5) PLUG in values, including constants6) Solve, DO the MATH7) CHECK answer, units

Watch out for the hidden values!

Goal of session

• Answer the questions placed around the room• Students may precede in any order• SHOW WORK!! For each problem state:

– Givens– What is wanted– Equation– Conversions done– Values plugged into equation– Answer with units

• F = ma • GPE = mgh• p=mv• P= W/t• W = Fd• a = (vf -vi)/t• KE = ½mv2 • MA = Fo / fi

Remember…

That I used 9.8 as the acceleration due to gravity and a

rounded some of the answers

Question #1

• A force of 6.2 x 105 N is needed to blast off a 5.0 x 103 kg rocket from rest. What is the rocket’s average acceleration?

• Answer: 124 m/s2

Question #2

• How much does a 59.0 kg woman weigh on Earth?

• Answer 578.2 N

Question #3

• A coin falls from rest from the balcony of a building. If it takes 2.7 s for it to hit the ground, what is the final velocity of the coin?

• Answer : 26.5 m/s

Question #4

• An object has a kinetic energy of 810 J after falling a certain distance. If the mass of the object is 20 kg, what is the speed of the object at this time?

• Answer: 9 m/s

Question #5

• The brakes on a car exert a frictional force of 6000N in getting the car to stop. If the work done by the brakes is 120,000 J in coming to a stop, how many meters did the car travel after the driver applied the brakes?

• Answer: 20 m

Question #6

• An object weighing 75 N is dropped from the top of a building and falls a distance of 28 m to the ground. How much work does gravity do on the object from the time it is dropped to the time it hits the ground?

• Answer: 2100 J

Question #7

• A weightlifter presses a 400 N weight 0.5 m over his head in 2 seconds. What is the power of the weightlifter?

• Answer: 100 W

Question #8

• Calculate the gravitational potential energy relative to the ground when an 82 kg person climbs to the top of a 2.0 m stepladder.

• Answer: 1607 J

Question #9

• A crane has a mechanical advantage of 27. An input force of 8650 N is used by the crane to lift a pile of steel girders. What is the weight of the girders?

• Answer: 233550 N

Question #11

• A commercial plane travels at a velocity of 234 m/s. The plane has 253 people seated onboard. If the average person on the plane has a mass of 68,000 g, what is the momentum of the passengers on the plane?

• Answer 4,026,000 kg m/s

Question #12

• A school bus can accelerate from a complete stop at 1.3 m/s2. How long will it take the bus to reach a velocity of 12.1 m/s?

• Answer: 9.31 s

Question #13

• A cat sits on top of a fence that is 2.0 m high. The cat has a GPE of 88.9 kJ . What is the mass of the cat?

• Answer: 1,742,000 J

Old Homework

• 858,859 111-125?

• Homework to be handed in tomorrow!– Page 858: 101-103, 106-108– Page 857:96-98– Page 852: 6-9

Scientific Notation

• A way to write very large or very small

• Convert number to a specific form (set-up)

A number between 1-10 x 10 to an exponent

Positive exponents for large #

Negative exponents for small #

Examples

• 2,407,000 =

Examples

• 0.000305=

Try

• 6,907,800,000

• Answer

• 6.9078 x109

Try

• 0.000 000 080 07

• Answer:

• 8.007 x 10-8

Conservation of energy equation

• GPEi + KEi = GPEf = KEf

• The total amount of energy is constant, but the amount in each form may change

• If there is a gain in KE, there is a loss in GPE

Cliff Diving