Work = (force) x (distance
28.4 joules = (force) x (4 meters)
Divide each side by (4 meters) :
Force = (28.4 joules) / (4 meters)
Force = 7.1 Newtons
Ek = 6KJ.
In physics, the kinetic energy of a body or object is the one that owns due to its movement and is given by the equation 
, where m is the mass of the object in kilograms and v is the velocity in m/s.
An object that it has a mass of 30 kilograms and moves with a velocity of 20m/s, its kinetic energy is given by:
When the projectile is at its maximum height above ground, it's at the point
of changing from rising to falling. At that exact point, its vertical speed is zero,
so the 14 m/s must be all horizontal velocity. That's not going to change.
Since we need to consider changes in vertical speed now, we need to make
some assumption about where this is all happening, so that we know the
acceleration of gravity. I'll assume that it's all happening on or near the Earth,
and the acceleration of gravity is 9.8 m/s².
I'm also going to neglect air resistance.
a). 1.2 sec before it reaches its maximum height, the projectile is rising
at a vertical speed of (1.2 x 9.8) = 11.76 m/s.
The magnitude of its velocity is
the square root of (14² + 11.76²) = 18.28 m/s, directed about 40° above horizontal.
b). 1.2 sec after it reaches its maximum height, the projectile is falling
at a vertical speed of (1.2 x 9.8) = 11.76 m/s.
The magnitude of its velocity is
the square root of (14² + 11.76²) = 18.28 m/s, directed about 40° below horizontal.
===========================
In 1.2 second before or after zero vertical speed, an object in free fall moves
(1/2) (g) (t²) = (4.9) (1.2²) = 7.06 meters .
c). & d).
1.2 seconds before it reaches maximum height, the projectile is located at
x = -14 m
y = -7.06 m
e). & f).
1.2 seconds after it reaches maximum height, the projectile is located at
x = +14 m
y = -7.06 m .
I hope you recognize that 6 answers, plus a little bit of explanation,
all for 5 points, ain't too shabby. You made out well.
(A) 
The energy stored by the system is given by
where
P is the power provided
t is the time elapsed
In this case, we have
P = 60 kW = 60,000 W is the power
t = 7 is the time
Therefore, the energy stored by the system is
(B) 4830 rad/s
The rotational energy of the wheel is given by
(1)
where
is the moment of inertia
is the angular velocity
The moment of inertia of the wheel is
where M is the mass and R the radius of the wheel.
We also know that the energy provided is
So we can rearrange eq.(1) to find the angular velocity:
(C) 
The centripetal acceleration of a point on the edge is given by
where
is the angular velocity
R = 0.12 m is the radius of the wheel
Substituting, we find
When an object moves its length contracts in the direction of motion. The faster it moves the shorter it gets in the direction of motion.
The object in this question moves and then stops moving. So it's length first contracts and then expands to its original length when the motion stops.
The speed doesn't have to be anywhere near the speed of light. When the object moves its length contracts no matter how fast or slow it's moving.
