In order to be accepted into college, you will more than likely need to take an entrance exam, depending on the school that you wish to attend. One entrance exam that you will need to take is the ACT exam. The ACT exam measures high-school students on what they have learned during high school and determines if they are ready to enter college-level classes. The ACT also gives you an idea as to which college-level classes you are prepared to take.
The ACT exam consists of four sections:
There is also a Writing Section on the ACT exam that is optional.
To prepare for the ACT exam, it is best to take multiple ACT practice tests to see which areas you will need more work on as well as what you can expect on the actual exam. Our ACT Science Practice is similar to those science questions you will be asked on the actual ACT exam.
The Science section of the ACT exam includes 40 questions and is timed for 35 minutes. The Science section measures the interpretation, analysis, evaluation, reasoning, and problem-solving skills that are required in the natural sciences. The Science section includes questions about:
- Interpretation of Data
- Scientific Investigation
- Evaluation of Models, Inferences, and Experimental Results
Wind can provide a renewable source of energy. The energy of the wind is actually solar energy, as the sun warms the Earth’s surface by varying amounts at different locations. This creates differential pressures as the warm air expands, and initiates air motions. High altitude airflows are similar to ocean currents, but near the surface, winds are affected by surface features. Wind turbines capture this energy with a set of rotors that are set into rotation by the wind. The rotors are made of lightweight fiberglass or carbon fiber and are held aloft on a tall tower. It is important to hold the blades high above the ground to avoid wind shear, a difference in airflow at different points along the rotor blades which can damage them. The blades rotate at about 40 rpm. Through a gearbox, they rotate a driveshaft at about 1500 rpm. The shaft, in turn, drives a generator.
The power P available from moving air is proportional to the cube of the wind velocity: P = 1/2 pAv^3
where A is the cross section covered by the blades, p is the air density, and v is the air velocity. As the air passes through the rotor, it slows down. The turbine cannot take all the energy from the air, since then it would stop dead behind the rotor. Theoretically, the maximum efficiency that can be achieved is 59%. Figures A and B show a power curve for a 600 kilowatt (kW) wind turbine. To avoid damage from excessive winds, starting with wind speeds of 15 m/sec, the blades are adjusted to limit the power to 600 kW. For winds above 25 m/sec, the turbine is shut down. One drawback of wind turbines has been the noise they make, but modern designs with slow rotating blades are fairly quiet. Figure C shows the noise spectrum of a large turbine. The x-axis shows the frequency of sound in Hertz, and the y-axis shows the level of sound at each frequency. The total noise is 50 decibels (dB), which is less than the noise in a typical office.
One drawback of wind turbines has been the noise they make, but modern designs with slow rotating blades are fairly quiet. Figure C shows the noise spectrum of a large turbine. The x-axis shows the frequency of sound in Hertz, and the y-axis shows the level of sound at each frequency. The total noise is 50 decibels (dB), which is less than the noise in a typical office.
What wind velocity provides the maximum efficiency for this turbine?
A. 5 m/s
B. 7 m/s
C. 10 m/s
D. 12 m/s
C: The efficiency curve in part B of the figure has a clear maximum value at a wind velocity of 10 m/s.