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If you travel in a straight line at \(50 \mathrm{km} / \mathrm{h}\) for \(50 \mathrm{km}\) and then at \(100 \mathrm{km} / \mathrm{h}\) for another \(50 \mathrm{km},\) is your average velocity \(75 \mathrm{km} / \mathrm{h} ?\) If not, is it more or less?

Short Answer

Expert verified
The average velocity isn't 75 km/h, it is less, specifically 66.67 km/h.

Step by step solution

01

Calculate the time spent for each segment of the journey.

The time \(t\) for travelling a certain distance \(d\) at a certain velocity \(v\) is given by the formula \(t = d/v\). For the first segment of 50 km at 50 km/h, it will take \(t1 = 50/50 = 1\) hour. For the second segment of 50 km at 100 km/h, it will take \(t2 = 50/100 = 0.5\) hours.
02

Calculate the total time spent and the total distance covered

The total time spent is \(T = t1 + t2 = 1 + 0.5 = 1.5\) hours. The total distance covered is \(D = D1 + D2 = 50 + 50 = 100\) km.
03

Calculate the average velocity

The average velocity \(V\) is given by the formula \(V = D/T\). So, \(V = 100 / 1.5 = 66.67\) km/h.
04

Compare the calculated average velocity with 75 km/h

The calculated average velocity (66.67 km/h) is less than 75 km/h.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Velocity Calculation
When calculating velocity, it's important to understand what this term actually means. Velocity is the speed of something in a specified direction. In physics, it's a vector quantity, meaning it has both magnitude and direction. To calculate velocity, you need to know how far something has traveled (distance) and how long it took (time). The basic formula is:
  • Velocity (v) = Distance (d) / Time (t)
For instance, if you travel 50 km in 1 hour, your velocity is 50 km/h. When calculating, ensure that the units for distance and time match up, like using kilometers for distance and hours for time. This makes calculations straightforward and the resulting velocity clear. In scenarios where you change speeds, you add complexity. You need to calculate the time for each leg of the journey separately and then combine these to find the average velocity.
Distance and Time Relation
The relationship between distance and time plays a pivotal role in understanding motion. At its core, this relationship underpins velocity and speed calculations. When we talk about moving objects or people, distance tells us how much ground is covered, and time tells us how long it took. Using the formula mentioned above, you can calculate the velocity, or if you're looking for time, simply rearrange the formula. For instance:
  • To find time: Time (t) = Distance (d) / Velocity (v)
  • To find distance: Distance (d) = Velocity (v) × Time (t)
These simple equations illustrate how closely linked distance and time are. When evaluating real-world situations, knowing any two of these variables will allow you to solve for the third, enhancing your understanding of motion dynamics.
Average Speed
Average speed, unlike velocity, does not take direction into account. It's a scalar quantity and simply tells you how fast something is moving overall. To calculate average speed, you divide the total distance traveled by the total time taken:
  • Average Speed = Total Distance / Total Time
Using the original exercise scenario, you traveled two distances at different speeds. The average speed isn't a simple arithmetic mean of 50 km/h and 100 km/h. Instead, you need to calculate how long each distance takes. Then, sum these times and divide the total distance by this sum. In this example:
  • Total Distance = 100 km
  • Total Time = 1.5 hours
Thus, the average speed calculated is 66.67 km/h, showing how the mathematical approach reveals a clearer picture than simple assumptions might.

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Most popular questions from this chapter

A particle leaves its initial position \(x_{0}\) at time \(t=0,\) moving in the positive \(x\) -direction with speed \(v_{0}\) but undergoing acceleration of magnitude \(a\) in the negative \(x\) -direction. Find expressions for (a) the time when it returns to \(x_{0}\) and (b) its speed when it passes that point.

If you know the initial velocity \(v_{0}\) and the initial and final heights \(y_{0}\) and \(y,\) you can use Equation 2.10 to solve for the time \(t\) when the object will be at height \(y .\) But the equation is quadratic in \(t,\) so you'll get two answers. Physically, why is this?

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Amtrak's 20 th-Century limited is en route from Chicago to New York at \(110 \mathrm{km} / \mathrm{h}\) when the engineer spots a cow on the track. The train brakes to a halt in 1.2 min, stopping just in front of the cow. (a) What is the magnitude of the train's acceleration? (b) What's the direction of the acceleration? (c) How far was the train from the cow when the engineer applied the brakes?

A Frisbee is lodged in a tree 6.5 m above the ground. A rock thrown from below must be going at least \(3 \mathrm{m} / \mathrm{s}\) to dislodge the Frisbee. How fast must such a rock be thrown upward if it leaves the thrower's hand \(1.3 \mathrm{m}\) above the ground?

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