Physics questions asked by real students

Real physics questions asked by students, solved step by step Every question below was submitted by a real student and answered step by step.

Average Speed for 2 Hours at 60 and 3 Hours at 40 km/h

Equal-time segments require weighting by time: distance = 2(60) + 3(40) = 240 km over 5 h, giving 48 km/h, not the naive arithmetic mean of 50 km/h.

Average Speed of a 10 km Out-and-Back Trip in 20 Minutes

A car drives 10 km from A and immediately returns, taking 20 minutes total. Total path is 20 km and 20 min = 1/3 h, so the average speed is 20 / (1/3) = 60 km/h.

Average Speed of a Jogger Including a 5-Minute Rest

A jogger runs 3.0 km in 10 min, rests 5 min, then runs 1500 m in 5 min. Total 4500 m over 20 min gives an average speed of 225 m/min, since rest time still counts.

Average Speed on the Last 20 km of a 120 km Trip

A 120 km trip averages 60 km/h, so it lasts 2 h. The first 30 km at 40 km/h take 0.75 h and 70 km at 70 km/h take 1 h, leaving 20 km in 0.25 h = 80 km/h.

Average Speed: Half a Trip at 50, Half at 60 km/h

The average speed is 54.5 km/h, not 55. Equal distances at different speeds call for the harmonic mean 2*v1*v2/(v1 + v2) = 2(50)(60)/110 = 600/11 km/h.

Average Speed with a Quarter at 20 and Three Quarters at 40

Split the road into fractions of s: time = (s/4)/20 + (3s/4)/40 = s/32, so the average speed is exactly 32 km/h, well below the naive mean of 35 km/h.

Compare Average Speeds on AB and BC of 12 km Each

A car passes A, B, C spaced 12 km apart, taking 20 min for AB and 30 min for BC. Then v(AB) = 36 km/h, v(BC) = 24 km/h and v(AC) = 28.8 km/h, so AB is fastest.

Convert 45 mph to m/s With Dimensional Analysis

45 mi/h equals 20.1168 m/s, about 20.12 m/s. Chain two conversion factors — 1 mile = 1609.344 m and 1 hour = 3600 s — so the unwanted units cancel on paper.

Distance an Airbus A320 Flies During a 500 ms Blink

At 1040 km/h a plane moves 1040000/3600 = 288.9 m/s. Over a 500 ms blink that is 288.9 x 0.5 = 144.4 m, so the closest listed value is 144 m.

Find AB When Two Walkers Arrive 5.5 Minutes Apart

Two walkers leave A at 1.5 m/s and 2.0 m/s; the faster arrives 5.5 min = 330 s earlier. Solving s/1.5 - s/2 = 330 gives s/6 = 330, so AB = 1980 m.

Solve x'' = -0.005 sin(x): The Nonlinear Pendulum

The first integral is (1/2)(x')^2 - 0.005cos(x) = C, and the exact solution needs elliptic integrals. Small angles give x = C1cos(wt) + C2sin(wt) with w = 0.0707.

Find F_B and Its Angle So the Resultant Points Along +y

F_B = 1300 N at 62.2 degrees. Forcing the horizontal components to cancel and the vertical components to total 1500 N turns one vector condition into two scalar equations.

Rule of Mixtures: Composite Stiffness and Strength

E11 = 225.4 GPa and sigma11 = 2428.1 MPa. The rule of mixtures weights fibre and matrix properties by volume fraction: 0.66(340) + 0.34(3.05) for stiffness.

Speed of an Object Covering 180 km in 6 Hours: v = s/t

An object in uniform straight-line motion covers 180 km in 6 h, so v = s/t = 180/6 = 30 km/h. Unit conversion shows this equals about 8.33 m/s, not 30 m/s.

Solve a Convection-Plus-Radiation Balance for the Temperature

The correct root is about 947 K, not the 1729 K a linear estimate suggests. The T⁴ radiation term is far from negligible, so the quartic must be solved numerically.

Time to Walk 780 m at a Constant 2 m/s Using t = s/v

A pedestrian walks 780 m at a constant 2 m/s. Rearranging v = s/t gives t = 780/2 = 390 s, and 390 s converts to exactly 6 minutes 30 seconds.