A 58.0-kg projectile is fired at an angle of 30.0° above the horizontal with an initial speed of 140 m/s from the top of a cliff
1 answer:
(a)
The total mechanical energy of the projectile at the beginning is the sum of the initial kinetic energy (K) and potential energy (U):
The initial kinetic energy is:
where m = 58.0 kg is the mass of the projectile and is the initial speed. Substituting,
The initial potential energy is given by
where g=9.8 m/s^2 is the gravitational acceleration and h=132 m is the height of the cliff. Substituting,
So, the initial mechanical energy is
(b)
We need to calculate the total mechanical energy of the projectile when it reaches its maximum height of y=336 m, where it is travelling at a speed of v=99.2 m/s.
The kinetic energy is
while the potential energy is
So, the mechanical energy is
And the work done by friction is equal to the difference between the initial mechanical energy of the projectile, and the new mechanical energy:
And the work is negative because air friction is opposite to the direction of motion of the projectile.
(c) 88.1 m/s
The work done by air friction when the projectile goes down is one and a half times (which means 1.5 times) the work done when it is going up, so:
When the projectile hits the ground, its potential energy is zero, because the heigth is zero: h=0, U=0. So, the projectile has only kinetic energy:
E = K
The final mechanical energy of the projectile will be the mechanical energy at the point of maximum height plus the work done by friction:
And this is only kinetic energy:
So, we can solve to find the final speed:
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Answer:
The minimum angle at which the box starts to slip (rounded to the next whole number) is α=19°
Explanation:
In order to solve this problem we must start by drawing a sketch of the problem and its corresponding fre body diagram (See picture attached).
So, when we are talking about friction, there are two types of friction coefficients. Static and kinetic. Static friction happens when the box is not moving no matter what force you apply to it. You get to a certain force that is greater than the static friction and the box starts moving, it is then when the kinetic friction comes into play (kinetic friction is generally smaller than static friction). So in order to solve this problem, we must find an angle such that the static friction is the same as the force applie by gravity on the box. For it to be easier to analyze, we must incline the axis of coordinates, just as shown on the picture attached.
After doing an analysis of the free-body diagram, we can build our set of equations by using Newton's thrid law:
we can see there are only two forces in x, which are the weight on x and the static friction, so:
when solving for the static friction we get:
We know the weight is found by multiplying the mass by the acceleration of gravity, so:
W=mg
and:
we can substitute this on our sum of forces equation:
the static friction will depend on the normal force applied by the plane on the box, static friction is found by using the following equation:
so we can substitute this on our equation:
but we don't know what the normal force is, so we need to find it by doing a sum of forces in y.
In the y direction we got two forces as well, the normal force and the force due to gravity, so we get:
when solving for N we get:
When seeing the free-body diagram we can determine that:
so we can substitute that in the sum of y-forces equation, so we get:
we can go ahead and substitute this equation in the sum of forces in x equation so we get:
we can divide both sides of the equation into mg so we get:
as you may see, the angle doesn't depend on the mass of the box, only on the static coefficient of friction. When solving for we get:
when simplifying this we get that:
now we can solve for the angle so we get:
and we can substitute the given value so we get:
which yields:
α=19.29°
which rounds to:
α=19°
