LET Elementary Physics Reviewer

Step 1 – Recall the formula: Work (W) = Force (F) × displacement (d). Step 2 – Identify the displacement: the wall did not move, so d = 0 m. Step 3 – Substitute: W = F × 0 = 0 joules. Step 4 – Conclude: No matter how large the force, if the object does not move, physics work is zero. Step 5 – Why the others are wrong: Option A confuses effort with physics work; Option C (negative work) applies when force and motion are opposite, not when there is no motion; Option D is not the work formula.

Step 1 – Use the lifting-work formula: W = m × g × h. Step 2 – Identify values: m = 5 kg, g = 10 m/s², h = 2 m. Step 3 – Substitute: W = 5 × 10 × 2 = 100 joules. Step 4 – The unit is the joule (J), which equals one newton-meter. Step 5 – Why others are wrong: 10 J comes from multiplying only m × h (forgetting g); 25 J has no correct derivation; 50 J results from using g = 5 instead of 10.

Step 1 – Formula for gravitational PE: PE = m × g × h. Step 2 – Values: m = 10 kg, g = 10 m/s², h = 3 m. Step 3 – Multiply: PE = 10 × 10 × 3 = 300 joules. Step 4 – The answer is in joules because 1 J = 1 kg·m²/s². Step 5 – Why others are wrong: 30 J omits g (only m × h); 13 J incorrectly adds the numbers; 100 J uses only m × g without h.

Step 1 – Formula: Power (P) = Work (W) ÷ Time (t). Step 2 – Values: W = 600 J, t = 30 s. Step 3 – Divide: P = 600 ÷ 30 = 20 watts. Step 4 – The watt (W) is the SI unit of power; 1 watt = 1 joule per second. Step 5 – Why others are wrong: 18,000 W comes from multiplying instead of dividing; 2 W divides by 300 instead of 30; 570 W subtracts time from work.

Step 1 – The Law of Conservation of Energy states energy cannot be created or destroyed, only transformed. Step 2 – At the table's edge, the ball has maximum gravitational PE (it is high up) and zero KE (not yet moving downward). Step 3 – As it falls, height decreases so PE decreases; speed increases so KE increases. Step 4 – At the floor just before impact, PE ≈ 0 and KE is maximum; total energy (PE + KE) remains the same throughout. Step 5 – Option D reverses the conversion; Options A and B contradict the conservation law.

Step 1 – Temperature measures the average kinetic energy of particles; it is an intensive property (does not depend on amount). Step 2 – Both the cup and pot are at 100°C, so their temperatures are equal. Step 3 – Heat (thermal energy) is an extensive property; it depends on the number of particles present. Step 4 – The pot has far more water molecules storing energy, so it contains more total heat. Step 5 – Options A and B confuse the two concepts; Option D is a classic misconception the LET frequently tests.

Step 1 – Conduction requires direct contact between particles; impossible in a vacuum. Step 2 – Convection requires a fluid (liquid or gas) to carry heat; space is a vacuum with no fluid. Step 3 – Radiation transfers heat through electromagnetic waves; these waves need no medium and travel through empty space. Step 4 – The Sun emits infrared (and other) electromagnetic radiation that crosses 150 million km of vacuum to warm Earth. Step 5 – This is the LET's most-tested heat transfer distinction: only radiation works in a vacuum.

Step 1 – Wave speed formula: v = f × λ (frequency times wavelength). Step 2 – Values: f = 5 Hz, λ = 6 m. Step 3 – Multiply: v = 5 × 6 = 30 m/s. Step 4 – The unit hertz means 'per second,' so Hz × m = m/s, which is the correct unit for speed. Step 5 – 1.2 m/s results from dividing λ by f; 11 m/s comes from adding instead of multiplying; 0.83 m/s comes from dividing f by λ.

Step 1 – Lightning produces both light and sound (thunder) at nearly the same instant and location. Step 2 – Light is an electromagnetic wave that travels at about 300,000,000 m/s in air. Step 3 – Sound is a mechanical wave that travels at only about 340 m/s in air. Step 4 – Because light travels about 880,000 times faster than sound, it reaches your eyes almost instantly, while sound takes several seconds to cover the same distance. Step 5 – You can estimate the storm's distance: count the seconds between flash and thunder, then divide by 3 to get approximate kilometers.

Step 1: Understand that matter contains positive charges (protons) and negative charges (electrons). Step 2: Protons are fixed inside the nucleus and cannot move between objects. Only electrons can be transferred. Step 3: When the balloon is rubbed on hair, friction causes electrons to move from the hair to the balloon. Step 4: The balloon gains electrons and becomes negatively charged; the hair loses electrons and becomes positively charged. Step 5: Friction never creates charges — it only transfers them. This is the Law of Conservation of Charge. Option A is wrong because charge cannot be created. Option C is wrong because protons do not move. Option D is wrong because the balloon gains, not loses, charge.

Step 1: The wall is initially neutral — it has equal positive and negative charges. Step 2: The negatively charged balloon repels electrons in the wall's surface, pushing them away. Step 3: This leaves the wall's surface near the balloon with a net positive charge — without any transfer of electrons. Step 4: Unlike charges attract, so the balloon sticks to the wall. Step 5: This redistribution of charge without contact is called induction. Conduction requires touching; friction requires rubbing; grounding removes charge to the Earth.