Trigonometry · real student question

Craft M climbs at 600 km/h along a path making 60 degrees with the horizontal. After 1.8 minutes, is M higher or lower than craft N, which is at an altitude of 10 km?

Question

Craft MM climbs at 600600 km/h along a path making an angle of 6060^{\circ} with the horizontal. At the same moment craft NN is at an altitude of 1010 km.

After 1.81.8 minutes, which craft is higher?

Step-by-step solution

  1. Take only the vertical part of the velocity. The 600600 km/h is speed along the slanted path, not upward speed. Height depends on the vertical component, which in a right triangle with the path as hypotenuse is the side opposite the 6060^{\circ} angle:

    vy=600sin60v_y=600\sin 60^{\circ}

    Using 600600 directly would overstate the altitude by about 15%15\%.

  2. Evaluate the sine exactly.

    sin60=32  vy=60032=3003 km/h\sin 60^{\circ}=\frac{\sqrt3}{2}\ \Longrightarrow\ v_y=600\cdot\frac{\sqrt3}{2}=300\sqrt3\ \text{km/h}

    Keeping 3\sqrt3 exact avoids rounding drift; numerically 3003=519.615300\sqrt3=519.615 km/h.

  3. Convert the time to match the speed units. The speed is per hour but the time is in minutes:

    1.8 min=1.860=0.03 h1.8\ \text{min}=\frac{1.8}{60}=0.03\ \text{h}

  4. Multiply to get the height climbed.

    hM=vyt=3003×0.03=93 km15.588 kmh_M=v_y t=300\sqrt3\times 0.03=9\sqrt3\ \text{km}\approx 15.588\ \text{km}

  5. Compare with the other craft. hN=10h_N=10 km, and

    15.588>1015.588>10

    so MM is higher.

  6. Check the estimate a second way. In 0.030.03 h at 600600 km/h the craft covers 1818 km along the path; the vertical share is 18sin60=18(0.8660)=15.58818\sin60^{\circ}=18(0.8660)=15.588 km ✓ - the same figure by a different route, and comfortably above 1010 km.

Answer

hM=9315.59 km>10 km, so M is higherh_M=9\sqrt3\approx 15.59\ \text{km}>10\ \text{km},\ \text{so }M\ \text{is higher}

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