A projectile is shot from the edge of a cliff 80 m above ground level with an initial speed of 60 m/sec at an angle of 30° with
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a) Time at which velocity is +20.0 m/s: 2.04 s
b) Time at which velocity is -20.0 m/s: 6.12 s
c) Time at which the displacement is zero: t = 0 and t = 8.16 s
d) Time at which the velocity is zero: t = 4.08 s
e) i) ii) iii) The acceleration of the boulder is always downward
f) See graphs in attachment
Explanation:
a)
The motion of the boulder is a uniformly accelerated motion, with constant acceleration
downward (acceleration due to gravity). So, we can use the following suvat equation:
where:
v is the velocity at time t
u = 40.0 m/s is the initial velocity
a=g=-9.8 m/s^2 is the acceleration
We want to find the time t at which the velocity is
v = 20.0 m/s
Therefore,
b)
In this case, we want to find the time t at which the boulder is moving at 20.0 m/s downward, so when
v = -20.0 m/s
(the negative sign means downward)
We use again the suvat equation
And substituting
u = +40.0 m/s
a=g=-9.8 m/s^2
We find the corresponding time t:
c)
To solve this part, we can use the following suvat equation:
where
s is the displacement
u = +40.0 m/s is the initial velocity
is the acceleration
t is the time
We want to find the time t at which the displacement is zero, so when
s = 0
SUbstituting into the equation and solving for t,
which gives two solutions:
t = 0 (initial instant)
which is the instant at which the boulder passes again through the initial position, but moving downward.
d)
To solve this part, we can use again the suvat equation
where
u = +40.0 m/s is the initial velocity
is the acceleration
We want to find the time t at which the velocity is zero, so when
v = 0
Substituting and solving for t, we find:
e)
In order to evaluate the acceleration of the boulder, let's consider the forces acting on it.
If we neglect air resistance, there is only one force acting on the boulder: the force of gravity, acting downward, with magnitude
where m is the mass of the boulder and the acceleration of gravity.
According to Newton's second law, the net force on the boulder is equal to the product between its mass and its acceleration:
Combining the two equations, we get
So, the acceleration of the boulder is downward at any point of the motion, no matter where the boulder is (because the force of gravity is constant during the motion).
f)
Find the three graphs in attachment:
- Position-time graph: the position of the boulder initially increases as the boulder goes upward; however, the slope of the curve decreases as the boulder goes higher (because the velocity decreases). The boulder reaches its maximum height at t = 4.08 s (when velocity is zero), then it starts going downward, until reaching its initial position at t = 8.16 s
- Velocity-time graph: the initial velocity is +40 m/s; then it decreases linearly (because the acceleration is constant), and becomes zero when t = 4.08 s. Then the velocity becomes negative (because the boulder is now moving downward) and its magnitude increases.
- Acceleration-time graph: the acceleration is constant and it is , so this graph is a straight horizontal line.
Learn more about accelerated motion:
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