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Algebra Level 2

At what time after 7 : 00 7:00 will the minute hand overtake the hour hand?

7 : 40 3 7 7:40\frac{3}{7} 7 : 44 6 17 7:44\frac{6}{17} 7 : 38 2 11 7:38\frac{2}{11} 7 : 36 5 11 7:36\frac{5}{11}

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2 solutions

If the minute hand moves a distance of x x , the hour hand moves a distance of x 12 \dfrac{x}{12} . From the figure,

x = 35 + x 12 x=35+\dfrac{x}{12} \color{#D61F06}\implies x = 38 2 11 x=38 \frac{2}{11}

Thus, the minute hand will overtake the hour hand at 7 : 38 2 11 \boxed{7:38 \frac{2}{11}} .

Maximos Stratis
Nov 7, 2017

Let ω h \omega_h and ω m \omega_m be the angular frequency of the hour hand and the minute hand respectively.
The hour hand completes one full revolution ( 2 π 2\pi rad) every 12h=12*60m. So: ω h = 2 π 12 60 = π 360 r a d m \omega_h=\frac{2\pi}{12*60}=\frac{\pi}{360}\frac{rad}{m}
The minute hand complete one full revolution ( 2 π 2\pi rad) every 60m. So: ω m = 2 π 60 = π 30 r a d m \omega_m=\frac{2\pi}{60}=\frac{\pi}{30}\frac{rad}{m}
Now, we let θ h \theta_h be the angle (counting clockwise) between the vertical position and the hour hand and θ m \theta_m the angle between the vertical position and the minute hand. If we consider t = 0 t=0 the moment at which the clock shows 7:00 o'clock then the starting angles are: θ h ( 0 ) = 7 12 2 π = 7 π 6 r a d \theta_h(0)=\frac{7}{12}2\pi=\frac{7\pi}{6}rad and θ m ( 0 ) = 0 r a d \theta_m(0)=0rad . Therefore we have:
θ h ( t ) = θ h ( 0 ) + ω h t = 7 π 6 + π 360 t \theta_h(t)=\theta_h(0)+\omega_h*t=\frac{7\pi}{6}+\frac{\pi}{360}t
θ m ( t ) = θ m ( 0 ) + ω m t = π 30 t \theta_m(t)=\theta_m(0)+\omega_m*t=\frac{\pi}{30}t
We want to find the moment at which the two hands are on top of one another. That is the moment at which θ m = θ h \theta_m=\theta_h . So:
π 30 t = 7 π 6 + π 360 t \frac{\pi}{30}t=\frac{7\pi}{6}+\frac{\pi}{360}t
Working out some tedious algebra we conclude that:
t = 7 60 11 = 38 + 2 11 m t=\frac{7*60}{11}=38+\frac{2}{11}m
So the answer is: 7 : 38 2 11 \boxed{7:38\frac{2}{11}}


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