A gray kangaroo can bound across level ground with each jump carrying it 10 m from the takeoff point. Typically the kangaroo leaves the ground at a 20° angle. If this is so: What is its maximum height above the ground?
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5. Projectile Motion
Positive (Upward) Launch
Problem 57
Textbook Question
An Olympic long jumper is capable of jumping 8.0 m. Assuming his horizontal speed is 9.1 m/s as he leaves the ground, how long is he in the air and how high does he go? Assume that he lands standing upright—that is, the same way he left the ground.

1
Step 1: Recognize that the problem involves projectile motion. The horizontal motion and vertical motion are independent of each other. The horizontal distance (range) is given as 8.0 m, and the horizontal velocity is 9.1 m/s. Use this information to calculate the total time of flight.
Step 2: Use the formula for horizontal motion: , where is the horizontal distance, is the horizontal velocity, and is the time of flight. Rearrange to solve for : . Substitute the given values to find the total time of flight.
Step 3: To find the maximum height, focus on the vertical motion. The vertical velocity at the start () can be calculated using the relationship between the total time of flight and the symmetry of projectile motion. The total time of flight is twice the time to reach the maximum height, so . Use the formula , where is the acceleration due to gravity (9.8 m/s²).
Step 4: Use the vertical motion formula to calculate the maximum height: . Substitute the value of from Step 3 and the value of to find the maximum height.
Step 5: Summarize the results. The total time of flight is determined from Step 2, and the maximum height is calculated in Step 4. These are the two quantities requested in the problem.

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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Projectile Motion
Projectile motion refers to the motion of an object that is launched into the air and is subject to gravitational forces. It can be analyzed in two dimensions: horizontal and vertical. The horizontal motion is uniform, while the vertical motion is influenced by gravity, leading to a parabolic trajectory. Understanding this concept is crucial for determining the time of flight and maximum height of the jumper.
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Time of Flight
The time of flight in projectile motion is the total time an object remains in the air from launch to landing. It can be calculated using the horizontal distance traveled and the horizontal speed. For the long jumper, knowing the horizontal speed and the distance jumped allows us to find the time he is airborne, which is essential for further calculations regarding his vertical motion.
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Maximum Height
Maximum height in projectile motion is the peak vertical position reached by the object during its flight. It can be calculated using the initial vertical velocity and the effects of gravity. For the long jumper, this involves determining how high he rises based on the time spent in the air and the vertical component of his initial velocity, which is influenced by the angle of takeoff.
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