C902 Middle School Science: Content Knowledge
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Free C902 Middle School Science: Content Knowledge Questions
A student pushes a 20 kg crate across the floor with 50 N force. The acceleration is 2 m/s². What is the friction force?
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10 N
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40 N
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50 N
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100 N
Explanation
To determine the friction force, we apply Newton’s second law, F_net = ma, where F_net is the net force acting on the object, m is the mass, and a is the acceleration. The applied force is 50 N, mass is 20 kg, and acceleration is 2 m/s². The net force needed to accelerate the crate is ( F_net = 20 * 2 = 40 N. Since friction opposes motion, the friction force is the difference between the applied force and the net force: 50 - 40 = 10 N.
Correct Answer:
10 N
Red sunset over Nairobi is caused by:
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Refraction
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Scattering of blue light
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Reflection
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Absorption of red light
Explanation
A red sunset occurs because of the scattering of blue light. During sunset, sunlight passes through a larger thickness of the atmosphere, which scatters shorter wavelengths (blue and violet) out of the line of sight. The longer wavelengths, such as red and orange, are less scattered and dominate the color we see. This phenomenon is called Rayleigh scattering and explains why sunsets appear red or orange rather than blue.
Correct Answer:
Scattering of blue light
A magnet is brought near an iron nail. The nail becomes temporarily magnetized due to:
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Magnetic induction
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Electric current
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Friction
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Gravity
Explanation
When a magnet is brought near an iron nail, the nail becomes temporarily magnetized through magnetic induction. The magnetic field of the magnet causes the domains in the iron nail to align in the same direction, producing temporary magnetism. This effect lasts only while the magnet is near or until the domains return to random alignment. Electric current, friction, and gravity are not responsible for this temporary magnetization.
Correct Answer:
Magnetic induction
Seasons on Earth are caused by:
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Changes in distance from the Sun
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Earth’s 23.5° axial tilt and revolution
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Moon phases
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Precession of the equinoxes
Explanation
Seasons result from Earth’s 23.5° axial tilt combined with its revolution around the Sun. This tilt causes varying angles of sunlight and day lengths throughout the year, leading to seasonal changes. Changes in distance from the Sun have minimal effect on seasons, moon phases affect tides but not seasons, and precession of the equinoxes is a slow wobble in Earth’s axis that affects the timing of seasons over thousands of years, not their annual occurrence.
Correct Answer:
Earth’s 23.5° axial tilt and revolution
In a Punnett square for two heterozygous parents (Aa × Aa), the phenotypic ratio is:
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1:1
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3:1
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1:2:1
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9:3:3:1
Explanation
When two heterozygous parents (Aa × Aa) are crossed, the genotypes of the offspring are AA, Aa, Aa, and aa. This produces a phenotypic ratio of 3 dominant traits to 1 recessive trait (3:1) because three of the offspring display the dominant phenotype (AA and Aa) and one displays the recessive phenotype (aa). The 1:2:1 ratio refers to genotypes, not phenotypes.
Correct Answer:
3:1
A pea plant with purple flowers (Pp) is crossed with a white-flowered plant (pp). What is the probability of offspring with white flowers?
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0%
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25%
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50%
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75%
Explanation
When crossing a heterozygous purple-flowered pea plant (Pp) with a homozygous recessive white-flowered plant (pp), a Punnett square shows the possible genotypes of the offspring: Pp and pp. There is a 50% chance the offspring will inherit the pp genotype, resulting in white flowers, and a 50% chance for Pp, resulting in purple flowers.
Correct Answer:
50%
Which process allows organisms to maintain a stable internal environment despite external changes?
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Evolution
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Homeostasis
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Photosynthesis
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Mutation
Explanation
Homeostasis is the process by which organisms regulate their internal environment to maintain stability despite changes in external conditions. This includes controlling temperature, pH, water balance, and other physiological variables. Evolution is a long-term change in populations, photosynthesis is the process of converting light energy to chemical energy, and mutation is a change in DNA, none of which directly maintain internal stability.
Correct Answer:
Homeostasis
The primary source of energy for Earth’s climate system is:
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Geothermal heat
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The Sun
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Tidal forces
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Radioactive decay
Explanation
The Sun is the main source of energy driving Earth’s climate system. Solar radiation provides the energy that warms the atmosphere, oceans, and land, influencing weather patterns, ocean currents, and the water cycle. While geothermal heat, tidal forces, and radioactive decay contribute small amounts of energy, they are negligible compared to the vast influence of solar energy on Earth's climate.
Correct Answer:
The Sun
Which weather instrument measures air pressure?
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Anemometer
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Barometer
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Hygrometer
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Psychrometer
Explanation
A barometer measures air pressure, which is essential for forecasting weather and understanding atmospheric conditions. An anemometer measures wind speed, a hygrometer measures humidity, and a psychrometer is used to determine relative humidity using wet- and dry-bulb thermometers.
Correct Answer:
Barometer
In a parallel circuit, total resistance when two 6 Ω resistors are added is:
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12 Ω
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3 Ω
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6 Ω
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36 Ω
Explanation
In a parallel circuit, the total resistance is calculated using the formula: 1 / R_total = 1 / R1 + 1 / R2. For two 6 Ω resistors: 1 / R_total = 1/6 + 1/6 = 2/6 = 1/3. Therefore, R_total = 3 Ω. This shows that adding resistors in parallel decreases the overall resistance of the circuit compared to individual resistances.
Correct Answer:
3 Ω
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