Transcript Solving Projectile problems

Why is it so hard to
get rubbish in the bin?
Projectile motion

Remember
Newton’s 1st law of motion
– If there is no force acting on an object
there is no acceleration, so the object
will be stationary or will move at a
constant velocity.
Definition of a projectile

Any object moving
through air without
its own source of
power and only
under the influence
of gravity…
eg: a golf ball after
being hit.
Compare a ball rolling off a desk
to one being dropped


The balls land at the
same time because
the force of gravity is
the same on both.
The rolled ball
continues moving
forward at the same
speed as on the desk:
there is no horizontal
force to slow it down.
Key physics ideas for projectiles
Divide the story into horizontal and
vertical directions.
 There are no horizontal forces acting so
the velocity is constant and we use the
equation v = d/t.
 The only vertical force is gravity so we
know the acceleration is 9.8ms-2.
 Time of flight is the only thing common to
the vertical and horizontal movements.
 Use the five equations of motion.

Solving Projectile problems: the
first type…
Facts about simple
vertical projectiles
 Throwing something
straight up or down
 Acceleration is
-9.8ms-2 on the way
up and +9.8ms-2 on
the way down
 Velocity is 0ms-1 at
the top of the motion
Q: A tennis player throws up a ball at 5.8ms1 for a serve. What is the maximum height
the ball reaches?







Fill in the information
you know
Vi =
Vf =
a=
d=
Choose an equation of
motion
Substitute and solve
Use the problem
and common sense
 Vi = 5.8 ms-1
 Vf = 0 ms-1
 a = -10 ms-2
d = ?
 Vf2 = Vi2 +2ad
0 = 5.82 + 2(-10)d
d = 1.7m

Q: A tennis ball is thrown up at 3.9ms-1. How
long does it take to reach its maximum height?
Q: A rock is dropped off a cliff 75m high. How
long does it take to reach the bottom of the
cliff? What is its speed after 2s?




Vi = 3.9 ms-1, Vf =
0ms-1,
a = -10 ms-2, t = ?
Vf = Vi + at
0 = 3.9 +-10t
t = 0.398s




d = 75 m, Vi = 0 ms-1, a =
-10 ms-2
d = Vit + 1/2at2
75 = 1/2 (-10)t2
t = 3.9s
Vf = Vi + at = 0 +10x2
= 20ms-1
Solving projectile problems: the second
type

A

B

Range
Half projectiles
At position A the
velocity is only
horizontal
At position B the
horizontal velocity is
the same but there is
a vertical component
Q: A person fires a gun horizontally from the
top of a cliff 70m high. The bullet travels out
at 100ms-1. Two possible Q…
What is the time to the
bottom?
Since time is the dame
vertically or
horizontally, use either
vertical or horizontal,
eg: vertically,
 Vi = 0 ms-1, a = 10
ms-2, d = 70 m
 d = Vit + 1/2at2
 70 = 0 + ½(10) t2
 t = 3.7s
What is the range?
Must use the
horizontal
information,
Vi = 100 ms-1, a = 0
ms-2, t = 3.7 s
d = vt = 100(3.7)
= 370m
Q: A duck shooter fires a bullet horizontally at
80ms-1 to hit a duck. They miss, and the bullet
comes down 1.8m to hit the ground. Find the time
for the bullet to hit the ground and the distance
away from the shooter that it lands.
Q: A food crate is dropped from a plane
flying horizontally at 160ms -1. If the crate
takes 12s to hit the ground, from how high
was it dropped. How far does the crate
travel horizontally from the point where it
was dropped?