Lessons 7–8 · 2 lessons · N. Sh. Turdiyev, K. A. Tursunmetov, A. G. Ganiyev, K. T. Suyarov, J. E. Usarov, A. K. Avliyoqulov. Physics Grade 10, 1st edition. Niso Poligraf Publishing House, Tashkent, 2017
7
Motion of a body thrown at an angle to the horizontal
Textbook: pp. 18–21
GoalSplit the motion of a body thrown at an angle to the horizontal into components and calculate its flight time, height and range.
We split the initial velocity of a body thrown at angle α to the horizontal with speed υ₀ into two components: υ₀ₓ = υ₀cosα and υ₀ᵧ = υ₀sinα. Horizontally the body moves uniformly (υₓ = υ₀cosα, x = υ₀t·cosα); vertically it moves with uniform deceleration from the initial speed υ₀sinα (υᵧ = υ₀sinα – gt). At the highest point υᵧ = 0, so the speed there is the smallest, υ = υ₀cosα. The rise time is tₖ = υ₀sinα/g, the total flight time is t = 2υ₀sinα/g, the maximum height is h = υ₀²sin²α/(2g), and the range is s = υ₀²sin2α/g. Without air resistance the range is greatest at 45° and the same for angles α and 90° – α. The trajectory equation is y = x·tanα – gx²/(2υ₀²cos²α), a parabola through the origin opening downward. In real conditions air resistance shortens the range and the best angle is below 45° (roughly 30–40°).
Worked examples
A body is thrown at υ₀ = 25 m/s at 53° to the horizontal (sin53° ≈ 0.8; cos53° ≈ 0.6; g = 10 m/s²). υ₀ₓ = 15 m/s, υ₀ᵧ = 20 m/s; tₖ = 2 s; t = 4 s; h = 20²/(2·10) = 20 m; s = 15·4 = 60 m.
A ball is kicked at υ₀ = 20 m/s at 45° (g = 10 m/s²). s = υ₀²sin90°/g = 400/10 = 40 m; h = υ₀²sin²45°/(2g) = 400·0.5/20 = 10 m. At 30° and 60° the ranges are equal (sin60° = sin120°).
Class activity
In the school yard, only with the teacher: aim a stream of water from a hose at different angles (about 30°, 45°, 60°) and mark where the water lands. Find which angle sends it farthest. Keep the water away from electrical equipment and from people.
Practice
1
At which launch angle (no air resistance) is the range greatest?
45°
2
A body is thrown at 50 m/s at 37° to the horizontal (sin37° ≈ 0.6). What is the vertical component of the initial velocity (m/s)?
30
3
A football is kicked at 10 m/s at 45° (g = 10 m/s², sin90° = 1). What is the range (m)?
10
4
A body is thrown at 50 m/s at 53° (sin53° = 0.8; g = 10 m/s²). What is the maximum height (m)?
80
5
Why is the speed not zero at the highest point of the trajectory?
The vertical component is zero, but the horizontal component υ₀cosα stays unchanged.