The expression for the frictional force between the sled and the ground is:

where

is the coefficient of friction, m is the mass of the object and

is the gravitational acceleration.
The friction force in our problem is F=80.85 N. The mass of the object is m=15 kg. Re-arranging the formula, we can find the value of k:
Answer:
0.1667Hz
Explanation:
by explanation frequency is given by 1/time
after carrying out the calculation of time we can easily get frequency.
time=speed/distance
=18/3
=6s
but frequency=1/time
hence=1/6
=0.1667hertz
Explanation : If a bar magnet is suspended at the center on a string and allow to swing freely, then the magnet will rotate so that it will line with the pole of the earth .
So, we can say that when the magnet suspended freely by the string, then the magnet will rotate and stop in north and south direction . The north pole of the magnet will stop in the south direction of the earth and the south pole of the magnet will stop in the north direction of the earth.
Answer:
Galileo Galilei developed the first ever telescope