a stone is thrown vertically downwards with a speed of 10 m/s from a bridge. acceleration under gravity (9.8 m/s squared). the stone strikes the water 1.8 s later. from what height above the water was the stone thrown? assume 2 significant digits.

3 answers

To solve this problem, we can use the equation of motion for vertical motion:

s = ut + 1/2 * at^2

where:
s = height
u = initial velocity
a = acceleration
t = time

In this case, the stone is thrown downwards, so the initial velocity u is -10 m/s (negative sign indicates the downward direction), acceleration a is -9.8 m/s^2 (negative sign indicates acceleration due to gravity), and the time t is 1.8 s.

Substituting these values into the equation:

s = -10 * 1.8 + 1/2 * (-9.8) * (1.8)^2

s = -18 - 8.82

s ≈ -26.82

Since the height cannot be negative, we take the magnitude of s:

height = |-26.82| ≈ 26.82 meters.

Therefore, the stone was thrown from a height of approximately 26.82 meters above the water.
a 55kg person is skating on the Rideadu canal. she reaches a maximum speed and then glides to a stop in 100 m. the coefficient between her skates and the ice is 0.010. what is the work done by kinetic friction?
To calculate the work done by kinetic friction, we can use the formula:

Work = Force * Distance

The force of kinetic friction can be calculated using the equation:

Force of Kinetic Friction = coefficient of friction * Normal Force

The normal force is equal to the weight of the person, which can be calculated as:

Weight = mass * acceleration due to gravity

Given:
Mass (m) = 55 kg
Coefficient of friction (μ) = 0.010
Distance (d) = 100 m
Acceleration due to gravity (g) = 9.8 m/s^2

First, calculate the weight:

Weight = mass * acceleration due to gravity
Weight = 55 kg * 9.8 m/s^2
Weight = 539 N

Next, calculate the force of kinetic friction:

Force of Kinetic Friction = coefficient of friction * Normal Force
Force of Kinetic Friction = 0.010 * 539 N
Force of Kinetic Friction = 5.39 N

Finally, calculate the work done:

Work = Force * Distance
Work = 5.39 N * 100 m
Work = 539 J

Therefore, the work done by kinetic friction is 539 Joules.