Mathematical question
The diagram shows the angles formed by the hands of an analog clock showing a time of 2:20 Clock angle problems are a type of mathematical problem which involve finding the angle between the hands of an analog clock .
Clock angle problems relate two different measurements: angles and time . The angle is typically measured in degrees from the mark of number 12 clockwise. The time is usually based on a 12-hour clock .
A method to solve such problems is to consider the rate of change of the angle in degrees per minute. The hour hand of a normal 12-hour analogue clock turns 360° in 12 hours (720 minutes) or 0.5° per minute. The minute hand rotates through 360° in 60 minutes or 6° per minute.[ 1]
Equation for the angle of the hour hand [ edit ] θ hr = 0.5 ∘ × M Σ = 0.5 ∘ × ( 60 × H + M ) {\displaystyle \theta _{\text{hr}}=0.5^{\circ }\times M_{\Sigma }=0.5^{\circ }\times (60\times H+M)} where:
θ is the angle in degrees of the hand measured clockwise from the 12 H is the hour. M is the minutes past the hour. M Σ is the number of minutes since 12 o'clock. M Σ = ( 60 × H + M ) {\displaystyle M_{\Sigma }=(60\times H+M)} Equation for the angle of the minute hand [ edit ] θ min. = 6 ∘ × M {\displaystyle \theta _{\text{min.}}=6^{\circ }\times M} where:
θ is the angle in degrees of the hand measured clockwise from the 12 o'clock position. M is the minute. The time is 5:24. The angle in degrees of the hour hand is:
θ hr = 0.5 ∘ × ( 60 × 5 + 24 ) = 162 ∘ {\displaystyle \theta _{\text{hr}}=0.5^{\circ }\times (60\times 5+24)=162^{\circ }} The angle in degrees of the minute hand is:
θ min. = 6 ∘ × 24 = 144 ∘ {\displaystyle \theta _{\text{min.}}=6^{\circ }\times 24=144^{\circ }} Equation for the angle between the hands [ edit ] The angle between the hands can be found using the following formula:
Δ θ = | θ hr − θ min. | = | 0.5 ∘ × ( 60 × H + M ) − 6 ∘ × M | = | 0.5 ∘ × ( 60 × H + M ) − 0.5 ∘ × 12 × M | = | 0.5 ∘ × ( 60 × H − 11 × M ) | {\displaystyle {\begin{aligned}\Delta \theta &=\vert \theta _{\text{hr}}-\theta _{\text{min.}}\vert \\&=\vert 0.5^{\circ }\times (60\times H+M)-6^{\circ }\times M\vert \\&=\vert 0.5^{\circ }\times (60\times H+M)-0.5^{\circ }\times 12\times M\vert \\&=\vert 0.5^{\circ }\times (60\times H-11\times M)\vert \\\end{aligned}}} where
H is the hour M is the minute If the angle is greater than 180 degrees then subtract it from 360 degrees.
The time is 2:20.
Δ θ = | 0.5 ∘ × ( 60 × 2 − 11 × 20 ) | = | 0.5 ∘ × ( 120 − 220 ) | = 50 ∘ {\displaystyle {\begin{aligned}\Delta \theta &=\vert 0.5^{\circ }\times (60\times 2-11\times 20)\vert \\&=\vert 0.5^{\circ }\times (120-220)\vert \\&=50^{\circ }\end{aligned}}} The time is 10:16.
Δ θ = | 0.5 ∘ × ( 60 × 10 − 11 × 16 ) | = | 0.5 ∘ × ( 600 − 176 ) | = 212 ∘ ( > 180 ∘ ) = 360 ∘ − 212 ∘ = 148 ∘ {\displaystyle {\begin{aligned}\Delta \theta &=\vert 0.5^{\circ }\times (60\times 10-11\times 16)\vert \\&=\vert 0.5^{\circ }\times (600-176)\vert \\&=212^{\circ }\ \ (>180^{\circ })\\&=360^{\circ }-212^{\circ }\\&=148^{\circ }\end{aligned}}} When are the hour and minute hands of a clock superimposed?[ edit ] In this graphical solution, T denotes time in hours; P , hands' positions; and θ , hands' angles in degrees. The red (thick solid) line denotes the hour hand; the blue (thin solid) lines denote the minute hand. Their intersections (red squares) are when they align. Additionally, orange circles (dash-dot line) are when hands are in opposition, and pink triangles (dashed line) are when they are perpendicular. In the SVG file , hover over the graph to show positions of the hands on a clock face. The hour and minute hands are superimposed only when their angle is the same.
θ min = θ hr ⇒ 6 ∘ × M = 0.5 ∘ × ( 60 × H + M ) ⇒ 12 × M = 60 × H + M ⇒ 11 × M = 60 × H ⇒ M = 60 11 × H ⇒ M = 5. 45 ¯ × H {\displaystyle {\begin{aligned}\theta _{\text{min}}&=\theta _{\text{hr}}\\\Rightarrow 6^{\circ }\times M&=0.5^{\circ }\times (60\times H+M)\\\Rightarrow 12\times M&=60\times H+M\\\Rightarrow 11\times M&=60\times H\\\Rightarrow M&={\frac {60}{11}}\times H\\\Rightarrow M&=5.{\overline {45}}\times H\end{aligned}}} H is an integer in the range 0–11. This gives times of: 0:00, 1:05.45 , 2:10.90 , 3:16.36 , 4:21.81 , 5:27.27 . 6:32.72 , 7:38.18 , 8:43.63 , 9:49.09 , 10:54.54 , and 12:00. (0.45 minutes are exactly 27.27 seconds.)
^ Elgin, Dave (2007). "Angles on the Clock Face". Mathematics in School . 36 (5). The Mathematical Association: 4–5. JSTOR 30216063 .