Topic Quiz: Physical Science

Evidence: The passage states that EM waves "do not need a medium (matter) to travel through, so they can cross the vacuum of space. That is how energy from the Sun reaches Earth."

Step by step:

1. Conduction needs particles touching each other; a vacuum has almost no particles.
2. Convection needs a moving liquid or gas; there is none to carry heat across space.
3. Radiation is energy carried by electromagnetic waves, which need no matter.
4. So sunlight and infrared reach Earth by radiation, exactly as the passage says.

The answer is "They do not need a medium (matter) to travel through, so they can cross the vacuum of space."

Tip: When a question asks you to "explain" with the passage, find the sentence that gives a cause, not one that is merely true. All EM waves having the same speed is true, but it does not explain how they cross a vacuum.

Evidence: The graph shows that at every force, the 2 kg cart's acceleration is half the 1 kg cart's: at 2 N, 2 m/s² (1 kg) vs. 1 m/s² (2 kg); at 4 N, 4 m/s² vs. 2 m/s². The passage states the law: "the acceleration of an object equals the net force on it divided by its mass."

Step by step:

1. Fix the force and compare: at 4 N, the 1 kg cart accelerates at 4 m/s² and the 2 kg cart at 2 m/s².
2. Doubling the mass (1 kg → 2 kg) halved the acceleration (4 → 2 m/s²). The same pattern holds at 1, 2 and 3 N.
3. Check with a = \dfrac{F}{m}: \dfrac{4}{1} = 4 and \dfrac{4}{2} = 2. ✓
4. Both lines rise, so force does affect acceleration, which rules out a "no effect" choice.

The answer is For the same net force, doubling the cart's mass cut its acceleration in half.

Tip: To compare two groups on one graph, hold one variable steady (pick one x-value) and read both lines at that point.

Evidence: The passage states Newton's third law: "when one object pushes or pulls on a second object, the second object pushes or pulls back on the first with a force of equal size in the opposite direction."

Step by step:

1. The action is: rope pulls on sled, 150 N forward.
2. Swap the two objects: the reaction is sled pulls on rope.
3. Same size, opposite direction: 150 N backward.
4. Friction (40 N) acts on the sled too, but it comes from the snow and is a different size. Gravity and the normal force are vertical and also act on the sled. None of these is the partner of the rope's pull.

The reaction force is the sled pulling backward on the rope with 150 N.

Check: Action-reaction forces never cancel each other, because they act on different objects. That is why the sled can still accelerate.

Tip: To find a reaction force, write A pushes/pulls on B and simply flip it to B pushes/pulls on A, keeping the same size.

Evidence: The stem says that after the solid dissolves, "the pack feels very cold." A temperature drop in the pack means thermal energy is leaving its surroundings (your skin, the ankle) and going into the pack.

Step by step:

1. An exothermic process releases thermal energy to its surroundings, which feel warmer (like a hand warmer or a campfire).
2. An endothermic process absorbs thermal energy from its surroundings, which feel colder.
3. The cold pack takes in thermal energy from the ankle, so the ankle cools. The process is endothermic.
4. "Cold" does not flow out of the pack. Thermal energy always moves from warmer objects (the ankle) to cooler ones (the pack).

The answer is The process is endothermic: it absorbs thermal energy from the surroundings, such as the ankle.

Check: The swelling goes down because the ankle loses thermal energy to the pack, exactly what an endothermic process predicts.

Tip: Exo = exit (energy leaves, surroundings warm up). Endo = enter (energy goes in, surroundings cool down).

Evidence: The passage gives the car's mass ("500 kg"), says it starts "at rest at point A," and explains that "friction with the track and air resistance turn a small part of it into thermal energy and sound along the way." The diagram shows A at 40 m and B at 0 m.

Step by step:

1. Total energy at A (all potential, since the car is at rest): PE = 500 \times 9.8 \times 40 = 196{,}000 J.
2. At B the height is 0 m, so potential energy is 0. If no energy were transformed by friction, kinetic energy would be 196,000 J.
3. Measured kinetic energy is 180,000 J. Difference: 196{,}000 - 180{,}000 = 16{,}000 J.
4. Energy cannot be destroyed, so the missing 16,000 J became thermal energy and sound, exactly as the passage describes.

The answer is About 16,000 J was changed into thermal energy and sound by friction and air resistance.

Tip: If energy seems to be "missing," it was transformed, usually into heat. Any choice saying energy was destroyed or created breaks the law of conservation of energy.

Evidence: The graph shows that 100 g of water can dissolve 238 g of sugar at 40 °C and 287 g at 60 °C. The passage says a saturated solution leaves "any extra sugar" undissolved as solid, and an unsaturated solution "holds less than that amount."

Step by step:

1. At 40 °C: the maximum is 238 g. Since 250 > 238, only 238 g dissolves, and 250 - 238 = 12 g stays solid. The solution is saturated.
2. At 60 °C: the maximum rises to 287 g. Since 250 < 287, all 250 g dissolves. The solution is now unsaturated: 287 - 250 = 37 g more could still dissolve.

The answer is At 40 °C the solution is saturated with about 12 g of sugar left undissolved; at 60 °C all the sugar dissolves and the solution is unsaturated.

Check: Heating raises solubility (the curve goes up), so the leftover crystals at 40 °C should dissolve when the mixture is heated, which matches.

Tip: Compare the amount added with the solubility at each temperature. Added more than the maximum → saturated with leftover solid. Added less → unsaturated.

Evidence: The pH scale is logarithmic: each change of one pH unit is a tenfold change in acidity, and lower pH means more acidic. Pure water is pH 7.

Step by step:

1. Write 1,000 as a power of ten: 1{,}000 = 10 \times 10 \times 10 = 10^3.
2. Three factors of 10 means a change of 3 pH units.
3. More acidic means a lower pH, so subtract: 7 - 3 = 4.

The lake sample has a pH of 4.

Check: Count down: pH 6 is 10 times more acidic than 7, pH 5 is 100 times, pH 4 is 1,000 times. ✓

Tip: To go from "times more acidic" to a pH change, count the zeros: 10 → 1 unit, 100 → 2 units, 1,000 → 3 units. Then move down the scale for more acidic.

Evidence: The passage gives the speed of EM waves as "about 3.0 \times 10^8 m/s" and the wave equation v = f\lambda. The stem gives \lambda = 0.10 m.

Step by step:

1. Rearrange for frequency: f = \dfrac{v}{\lambda}.
2. Substitute: f = \frac{3.0 \times 10^8\text{ m/s}}{0.10\text{ m}}
3. Dividing by 0.10 is the same as multiplying by 10, which raises the exponent by one: f = 3.0 \times 10^9\text{ Hz}

The frequency is 3.0 \times 10^{9} Hz (3 GHz).

Check: This is close to the Wi-Fi router's 5.0 \times 10^9 Hz in the table, and both are microwaves, so the size makes sense. Also 3.0 \times 10^9 \times 0.10 = 3.0 \times 10^8. ✓

Tip: Units guide the setup: (m/s) ÷ m = 1/s = Hz. If your units do not come out as hertz, you have set up the division the wrong way.