At what time after
7
:
0
0
will the minute hand overtake the hour hand?
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Let
ω
h
and
ω
m
be the angular frequency of the hour hand and the minute hand respectively.
The hour hand completes one full revolution (
2
π
rad) every 12h=12*60m. So:
ω
h
=
1
2
∗
6
0
2
π
=
3
6
0
π
m
r
a
d
The minute hand complete one full revolution (
2
π
rad) every 60m. So:
ω
m
=
6
0
2
π
=
3
0
π
m
r
a
d
Now, we let
θ
h
be the angle (counting clockwise) between the vertical position and the hour hand and
θ
m
the angle between the vertical position and the minute hand. If we consider
t
=
0
the moment at which the clock shows 7:00 o'clock then the starting angles are:
θ
h
(
0
)
=
1
2
7
2
π
=
6
7
π
r
a
d
and
θ
m
(
0
)
=
0
r
a
d
. Therefore we have:
θ
h
(
t
)
=
θ
h
(
0
)
+
ω
h
∗
t
=
6
7
π
+
3
6
0
π
t
θ
m
(
t
)
=
θ
m
(
0
)
+
ω
m
∗
t
=
3
0
π
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
. So:
3
0
π
t
=
6
7
π
+
3
6
0
π
t
Working out some tedious algebra we conclude that:
t
=
1
1
7
∗
6
0
=
3
8
+
1
1
2
m
So the answer is:
7
:
3
8
1
1
2
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x = 3 5 + 1 2 x ⟹ x = 3 8 1 1 2
Thus, the minute hand will overtake the hour hand at 7 : 3 8 1 1 2 .