Practice Problem: 11 = 2.00 s and 12 = What is the car's average acceleration 5.00 s? Answer: 3.5 m/s².

Physics for Scientists and Engineers, Technology Update (No access codes included)
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Author:Raymond A. Serway, John W. Jewett
Publisher:Raymond A. Serway, John W. Jewett
Chapter2: Motion In One Dimension
Section: Chapter Questions
Problem 2.37P: A speedboat travels in a straight line and increases in speed uniformly from i = 20.0 m/s to f =...
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38
CHAPTER 2 Motion Along a Straight Line
Part (b): The change in velocity divided by the time interval
gives the average acceleration in each interval. The time interval is
Ar = 3.00 s-1.00 s= 2.00 s, so
U2r- Ulx 4.00 m/s
12-11
2.00 s
Part (c): We use the same procedure to approximate the instantaneous
acceleration at x₁ by calculating the average acceleration over a very short
time period, 0.100 s. When At = 0.100 s, t₂ = 1.10 s, and it follows that
V2r = 60.0 m/s + (0.500 m/s³) (1.10 s)² = 60.605 m/s,
Au, = 0.105 m/s,
ax=
dav..x
Aux
At
=
=
0.105 m/s
0.100 s
=
1.05 m/s².
= 2.00 m/s².
REFLECT If we repeat the calculation in part (c) for At = 0.01 s and
At = 0.001 s, we get a, 1.005 m/s² and a, 1.0005 m/s², respec-
tively (although we aren't entitled to this many significant figures). As
At gets smaller and smaller, the average acceleration gets closer and
closer to 1.00 m/s². We conclude that the instantaneous acceleration at
t = 1.00 s is 1.00 m/s².
Practice Problem: What is the car's average acceleration between
t₁ = 2.00 s and t2 = 5.00 s? Answer: 3.5 m/s².
As with average and instantaneous velocity, we can gain added insight into the concepts
of average and instantaneous acceleration by plotting a graph with velocity U, on the vertical
axis and time t on the horizontal axis. Figure 2.15 shows such a graph for the race car from
Figure 2.13. Notice that now we are plotting velocity versus time, not position versus time as
before. If we draw a line between any two points on the graph, such as A and B (corresponding
to a displacement from x₁ to x₂ and to the time interval At = 1₂
equals the average acceleration over that interval. If we then take point B and move it closer
ti), the slope of that line
and closer to point A, the slope of the line AB approaches the slope of the line that is tan
the curve at point A. Accordingly
-
Transcribed Image Text:38 CHAPTER 2 Motion Along a Straight Line Part (b): The change in velocity divided by the time interval gives the average acceleration in each interval. The time interval is Ar = 3.00 s-1.00 s= 2.00 s, so U2r- Ulx 4.00 m/s 12-11 2.00 s Part (c): We use the same procedure to approximate the instantaneous acceleration at x₁ by calculating the average acceleration over a very short time period, 0.100 s. When At = 0.100 s, t₂ = 1.10 s, and it follows that V2r = 60.0 m/s + (0.500 m/s³) (1.10 s)² = 60.605 m/s, Au, = 0.105 m/s, ax= dav..x Aux At = = 0.105 m/s 0.100 s = 1.05 m/s². = 2.00 m/s². REFLECT If we repeat the calculation in part (c) for At = 0.01 s and At = 0.001 s, we get a, 1.005 m/s² and a, 1.0005 m/s², respec- tively (although we aren't entitled to this many significant figures). As At gets smaller and smaller, the average acceleration gets closer and closer to 1.00 m/s². We conclude that the instantaneous acceleration at t = 1.00 s is 1.00 m/s². Practice Problem: What is the car's average acceleration between t₁ = 2.00 s and t2 = 5.00 s? Answer: 3.5 m/s². As with average and instantaneous velocity, we can gain added insight into the concepts of average and instantaneous acceleration by plotting a graph with velocity U, on the vertical axis and time t on the horizontal axis. Figure 2.15 shows such a graph for the race car from Figure 2.13. Notice that now we are plotting velocity versus time, not position versus time as before. If we draw a line between any two points on the graph, such as A and B (corresponding to a displacement from x₁ to x₂ and to the time interval At = 1₂ equals the average acceleration over that interval. If we then take point B and move it closer ti), the slope of that line and closer to point A, the slope of the line AB approaches the slope of the line that is tan the curve at point A. Accordingly -
.
no necessarily true that the object is speeding up.
If a < 0, then it is not necessarily true that the object is slowing down.
If ax = 0, then it is not necessarily true that the velocity of the object is also zero.
The instantaneous acceleration at any point on a graph of velocity as a function of
time is the slope of the tangent to the curve at that point.
EXAMPLE 2.4 Average and instantaneous accelerations
Now let's examine the differences between average and instantaneous accelerations. Suppose that, at any
time t, the velocity v of the car in Figure 2.13 is given by the equation vx = 60.0 m/s + (0.500 m/s³)². In
this equation, v, has units of m/s and t has units of s. Note that the numbers 60.0 and 0.500 must have the
units shown in order for this equation to be dimensionally consistent. (a) Find the change in velocity of the
car in the time interval between t₁ = 1.00 s and t2 = 3.00 s. (b) Find the average acceleration in this time
interval. (c) Estimate the instantaneous acceleration at time t₁ = 1.00 s by taking At = 0.10 s.
SOLUTION
SET UP Figure 2.14 shows the diagram we use to establish a coordi-
nate system and organize our known and unknown information.
a₁₁x = ?
X₁
t₁ = 1.00 s
K
Avx = ?
=?
Gav, x
A FIGURE 2.14 Our sketch for this problem.
t₂ = 3.00 s
✈
→X
O'XO
Video Tutor Solution
SOLVE Part (a): We first find the velocity at each time by substituting
each value of t into the equation. From these values, we can find the
change in velocity in the interval. Thus, at time t₁ = 1.00 s,
Vlx = 60.0 m/s + (0.500 m/s) (1.00 s)² = 60.5 m/s.
At time t₂ = 3.00 s,
V2x = 60.0 m/s + (0.500 m/s³) (3.00 s)² = 64.5 m/s.
The change in velocity Au, is then
AUx = U2x
Vlx = 64.5 m/s
60.5 m/s = 4.00 m/s.
CONT
Transcribed Image Text:. no necessarily true that the object is speeding up. If a < 0, then it is not necessarily true that the object is slowing down. If ax = 0, then it is not necessarily true that the velocity of the object is also zero. The instantaneous acceleration at any point on a graph of velocity as a function of time is the slope of the tangent to the curve at that point. EXAMPLE 2.4 Average and instantaneous accelerations Now let's examine the differences between average and instantaneous accelerations. Suppose that, at any time t, the velocity v of the car in Figure 2.13 is given by the equation vx = 60.0 m/s + (0.500 m/s³)². In this equation, v, has units of m/s and t has units of s. Note that the numbers 60.0 and 0.500 must have the units shown in order for this equation to be dimensionally consistent. (a) Find the change in velocity of the car in the time interval between t₁ = 1.00 s and t2 = 3.00 s. (b) Find the average acceleration in this time interval. (c) Estimate the instantaneous acceleration at time t₁ = 1.00 s by taking At = 0.10 s. SOLUTION SET UP Figure 2.14 shows the diagram we use to establish a coordi- nate system and organize our known and unknown information. a₁₁x = ? X₁ t₁ = 1.00 s K Avx = ? =? Gav, x A FIGURE 2.14 Our sketch for this problem. t₂ = 3.00 s ✈ →X O'XO Video Tutor Solution SOLVE Part (a): We first find the velocity at each time by substituting each value of t into the equation. From these values, we can find the change in velocity in the interval. Thus, at time t₁ = 1.00 s, Vlx = 60.0 m/s + (0.500 m/s) (1.00 s)² = 60.5 m/s. At time t₂ = 3.00 s, V2x = 60.0 m/s + (0.500 m/s³) (3.00 s)² = 64.5 m/s. The change in velocity Au, is then AUx = U2x Vlx = 64.5 m/s 60.5 m/s = 4.00 m/s. CONT
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