Practice Problem: If the acceleration is 2.0 m/s² instead of 4.0 m/s. where is the cyclist, and how fast is he moving, 5.0 s after he passes the signpost if xo = 5.0 m and vox 15 m/s? Answers: 105 m, 25 m/s. =

Glencoe Physics: Principles and Problems, Student Edition
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ISBN:9780078807213
Author:Paul W. Zitzewitz
Publisher:Paul W. Zitzewitz
Chapter3: Accelerated Motion
Section3.3: Free Fall
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OSAGE
O
EXAMPLE 2.8 Constant acceleration on a motorcycle
Here we will look at a more complex problem for which we will use all three of our primary constant-
acceleration equations. A motorcyclist heading east through a small Iowa town accelerates after he passes
a signpost at x = 0 marking the city limits. His acceleration is constant: ax =
4.0 m/s². At time t = 0, he
is 5.0 m east of the signpost and has a velocity of vox = 15 m/s. (a) Find his position and velocity at time
1 = 2.0 s. (b) Where is the motorcyclist when his velocity is 25 m/s?
SOLUTION
SET UP Figure 2.21 shows our diagram. The problem tells us that
x = 0 at the signpost, so that is the origin of coordinates. We point the
x axis east, in the direction of motion. The (constant) acceleration is
a = 4.0 m/s². At the initial time t = 0, the position is xo = 5.0 m and
the initial velocity is vox = 15 m/s.
SOLVE Part (a): We want to know the position and velocity (the val-
ues of x and Ux, respectively) at the later time t =.2.0 s. Equation 2.10
gives the position x as a function of time t:
x = xo + voxt + 1/a,t²
= 5.0 m+ (15 m/s) (2.0 s) + (4.0 m/s²) (2.0 s)² = 43 m.
Equation 2.6 gives the velocity Ux as a function of time t:
Ux = Vox + axt
= 15 m/s + (4.0 m/s²) (2.0 s) = 23 m/s.
Vox
xo = 5.0 m
t = 0
FIGURE 2.21
=
ax = 4.0 m/s²
gen
do the results change
15 m/s
X = ?
t = 2.0 s
TO
Ux = ?
→ x (east)
Video Tutor Solut
25 m/s. Note that
this occurs at a time later than 2.0 s and at a point farther than 43 m from
Part (b): We want to know the value of x when ux
the signpost. From Equation 2.11, we have
=
2
v² = voz² + 2ax(x − xo),
Ux
(25 m/s)² = (15 m/s)2 + 2(4.0 m/s²)(x - 5.0 m),
x = 55 m.
Alternatively, we may use Equation 2.6 to find first the time when
Ux = 25 m/s:
Ux = Vox + axt,
25 m/s = 15 m/s + (4.0 m/s²)(t),
t = 2.5 s.
Then, from Equation 2.10,
x = xo + voxt + 1⁄2 axt²
=
= 5.0 m+ (15 m/s) (2.5 s) + (4.0 m/s²) (2.5 s)²
= 55 m.
sors Rigodi pol
oniworla
A FIGUE
the polic
SOLUTION
SET UP B
the x axis
x compor
keep in m
nent of
The
so we c
objects
The or
both
be th
t. At
the
to E
ini
as
aF
REFLECT Do the results make sense? The cyclist accelerates from
15 m/s (about 34 mi/h) to 23 m/s (about 51 mi/h) in 2.0 s while travel-
ing a distance of 38 m (about 125 ft). This is fairly brisk acceleration,
but well within the realm of possibility for a high-performance bike.
Practice Problem: If the acceleration is 2.0 m/s² instead of 4.0 m/s.
where is the cyclist, and how fast is he moving, 5.0 s after he passes the
signpost if xo = 5.0 m and vox = 15 m/s? Answers: 105 m, 25 m/s.
Transcribed Image Text:OSAGE O EXAMPLE 2.8 Constant acceleration on a motorcycle Here we will look at a more complex problem for which we will use all three of our primary constant- acceleration equations. A motorcyclist heading east through a small Iowa town accelerates after he passes a signpost at x = 0 marking the city limits. His acceleration is constant: ax = 4.0 m/s². At time t = 0, he is 5.0 m east of the signpost and has a velocity of vox = 15 m/s. (a) Find his position and velocity at time 1 = 2.0 s. (b) Where is the motorcyclist when his velocity is 25 m/s? SOLUTION SET UP Figure 2.21 shows our diagram. The problem tells us that x = 0 at the signpost, so that is the origin of coordinates. We point the x axis east, in the direction of motion. The (constant) acceleration is a = 4.0 m/s². At the initial time t = 0, the position is xo = 5.0 m and the initial velocity is vox = 15 m/s. SOLVE Part (a): We want to know the position and velocity (the val- ues of x and Ux, respectively) at the later time t =.2.0 s. Equation 2.10 gives the position x as a function of time t: x = xo + voxt + 1/a,t² = 5.0 m+ (15 m/s) (2.0 s) + (4.0 m/s²) (2.0 s)² = 43 m. Equation 2.6 gives the velocity Ux as a function of time t: Ux = Vox + axt = 15 m/s + (4.0 m/s²) (2.0 s) = 23 m/s. Vox xo = 5.0 m t = 0 FIGURE 2.21 = ax = 4.0 m/s² gen do the results change 15 m/s X = ? t = 2.0 s TO Ux = ? → x (east) Video Tutor Solut 25 m/s. Note that this occurs at a time later than 2.0 s and at a point farther than 43 m from Part (b): We want to know the value of x when ux the signpost. From Equation 2.11, we have = 2 v² = voz² + 2ax(x − xo), Ux (25 m/s)² = (15 m/s)2 + 2(4.0 m/s²)(x - 5.0 m), x = 55 m. Alternatively, we may use Equation 2.6 to find first the time when Ux = 25 m/s: Ux = Vox + axt, 25 m/s = 15 m/s + (4.0 m/s²)(t), t = 2.5 s. Then, from Equation 2.10, x = xo + voxt + 1⁄2 axt² = = 5.0 m+ (15 m/s) (2.5 s) + (4.0 m/s²) (2.5 s)² = 55 m. sors Rigodi pol oniworla A FIGUE the polic SOLUTION SET UP B the x axis x compor keep in m nent of The so we c objects The or both be th t. At the to E ini as aF REFLECT Do the results make sense? The cyclist accelerates from 15 m/s (about 34 mi/h) to 23 m/s (about 51 mi/h) in 2.0 s while travel- ing a distance of 38 m (about 125 ft). This is fairly brisk acceleration, but well within the realm of possibility for a high-performance bike. Practice Problem: If the acceleration is 2.0 m/s² instead of 4.0 m/s. where is the cyclist, and how fast is he moving, 5.0 s after he passes the signpost if xo = 5.0 m and vox = 15 m/s? Answers: 105 m, 25 m/s.
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