Motion in Space. MATH 311, Calculus III. J. Robert Buchanan. Fall Department of Mathematics

Transcription

1 Motion in Space MATH 311, Calculus III J. Robert Buchanan Department of Mathematics Fall 2015

2 Background (1 of 2) Suppose the position vector of a moving object is given by r(t) = f (t), g(t), h(t),

3 Background (1 of 2) Suppose the position vector of a moving object is given by r(t) = f (t), g(t), h(t), then the tangent vector to the motion is r (t) = f (t), g (t), h (t),

4 Background (1 of 2) Suppose the position vector of a moving object is given by r(t) = f (t), g(t), h(t), then the tangent vector to the motion is r (t) = f (t), g (t), h (t), and the magnitude of the tangent vector is r (t) = [f (t)] 2 + [g (t)] 2 + [h (t)] 2.

5 Background (2 of 2) Recall the arc length of a parametric curve in three dimensions is t t s(t) = [f (u)] 2 + [g (u)] 2 + [h (u)] 2 du = r (u) du. t 0 t 0

6 Background (2 of 2) Recall the arc length of a parametric curve in three dimensions is t t s(t) = [f (u)] 2 + [g (u)] 2 + [h (u)] 2 du = r (u) du. t 0 t 0 The FTC, Part II implies s (t) = r (t) so r (t) is the speed of the moving object.

7 Background (2 of 2) Recall the arc length of a parametric curve in three dimensions is t t s(t) = [f (u)] 2 + [g (u)] 2 + [h (u)] 2 du = r (u) du. t 0 t 0 The FTC, Part II implies s (t) = r (t) so r (t) is the speed of the moving object. Consequently, r (t) is the velocity vector and r (t) is the acceleration vector.

8 Example (1 of 2) Find the velocity and acceleration vectors of an object whose position vector is 2 r(t) = 3 t3 + t + 1, t 3 t, e t.

9 Example (1 of 2) Find the velocity and acceleration vectors of an object whose position vector is 2 r(t) = 3 t3 + t + 1, t 3 t, e t. v(t) = r (t) = 2t 2 + 1, 3t 2 1, e t a(t) = r (t) = 4t, 6t, e t

10 Example (2 of 2) The acceleration vector of a moving object is t a(t) =, sin t, 0, 6 while its initial velocity is v(0) = 2, 0, 3 and its initial position is r(0) = 0, 0, 1. Find the velocity and position vectors as a function of t for this object.

11 Example (2 of 2) The acceleration vector of a moving object is t a(t) =, sin t, 0, 6 while its initial velocity is v(0) = 2, 0, 3 and its initial position is r(0) = 0, 0, 1. Find the velocity and position vectors as a function of t for this object. t s v(t) v(0) = 0 6, sin s, 0 ds t 2 v(t) = 2, 0, 3 + =, 1 cos t, t2, 1 cos t, 3 12

12 Finding the Position Vector r(t) r(0) = t s2 r(t) = 0, 0, 1 + =, 1 cos s, 3 12 ds 2t + t3, t sin t, 3t 36 2t + t3, t sin t, 1 + 3t 36

13 Newton s Second Law One of the most fundamental physical laws is Newton s second law of motion which states that the force vector acting on an object is the product of the object s mass (a scalar) and the object s acceleration vector. This is stated concisely as F = m a.

14 Example Find the force acting on an object moving along an elliptical path x 2 a 2 + y 2 b 2 = 1.

15 Example Find the force acting on an object moving along an elliptical path x 2 a 2 + y 2 b 2 = 1. Let the position vector of the object be then r(t) = a cos ωt, b sin ωt

16 Example Find the force acting on an object moving along an elliptical path x 2 a 2 + y 2 b 2 = 1. Let the position vector of the object be then r(t) = a cos ωt, b sin ωt F(t) = m a(t) = m r (t) = m ω 2 a cos t, b sin t = m ω 2 r(t).

17 Centripetal Motion r(t) = a cos ωt, b sin ωt y b 0 v(t) a(t) r(t) -b -a 0 a x

18 Equations of Motion Suppose an object is launched at an angle θ with respect to the horizontal from a height h. The object is given an initial speed of s 0. Find the position vector describing the path the object takes.

19 Equations of Motion Suppose an object is launched at an angle θ with respect to the horizontal from a height h. The object is given an initial speed of s 0. Find the position vector describing the path the object takes. Treating this as motion in the xy-plane, we have an acceleration vector a(t) = 0, g and initial condition vectors v 0 = s 0 cos θ, sin θ and r 0 = 0, h.

20 Equations of Motion Suppose an object is launched at an angle θ with respect to the horizontal from a height h. The object is given an initial speed of s 0. Find the position vector describing the path the object takes. Treating this as motion in the xy-plane, we have an acceleration vector a(t) = 0, g and initial condition vectors v 0 = s 0 cos θ, sin θ and r 0 = 0, h. Thus the velocity and position vectors are v(t) = s 0 cos θ, s 0 sin θ gt r(t) = (s 0 cos θ)t, h + (s 0 sin θ)t 12 gt2.

21 Rotational Motion We can adapt Newton s second law for spinning objects. τ torque (scalar, τ = τ ) I moment of inertia, measure of the force required to start an object rotating θ angle of displacement ω angular velocity α angular acceleration α(t) = ω (t) = θ (t)

22 Rotational Motion We can adapt Newton s second law for spinning objects. τ torque (scalar, τ = τ ) I moment of inertia, measure of the force required to start an object rotating θ angle of displacement ω angular velocity α angular acceleration α(t) = ω (t) = θ (t) τ = Iα

23 Example A merry-go-round of radius 6 feet and moment of inertia I = 12 rotates at 5 radians per second. Find the constant force applied tangent to the edge of the merry-go-round needed to stop the merry-go-round in 3 seconds.

24 Example A merry-go-round of radius 6 feet and moment of inertia I = 12 rotates at 5 radians per second. Find the constant force applied tangent to the edge of the merry-go-round needed to stop the merry-go-round in 3 seconds. implies α = 5/3. 5 = ω(3) ω(0) = 3 0 α dt = 3α τ = (12)(5/3) = 20 = r F = r F = (6) F which implies F = 10/3 pounds.

25 Momentum Suppose an object has mass m and velocity v. The object s linear momentum is p(t) = m v(t).

26 Momentum Suppose an object has mass m and velocity v. The object s linear momentum is p(t) = m v(t). The object s angular momentum is L(t) = r(t) m v(t). Example Show that torque is the derivative of angular momentum.

27 Momentum Suppose an object has mass m and velocity v. The object s linear momentum is p(t) = m v(t). The object s angular momentum is L(t) = r(t) m v(t). Example Show that torque is the derivative of angular momentum. L (t) = r (t) m v(t) + r(t) m v (t) = v(t) m v(t) + r(t) m a(t) = 0 + r(t) F(t) = τ (t)

28 Projectile Motion in Three Dimensions A projectile of mass 10 kg is launched to the east from a height of 1 m at a speed of 10 m/s. The launch angle is 45. The projectile spins as it flies and thus is subject to a Magnus force of magnitude 2 N in the southerly direction. Find the position of the projectile, its landing location, and its speed at impact.

29 Example (1 of 3) Assumptions: East is the positive x-direction and south is the negative y-direction. The only forces acting on the projectile are gravity and the Magnus force. The projectile is launched one meter above the origin on the positive z-axis.

30 Example (1 of 3) Assumptions: East is the positive x-direction and south is the negative y-direction. The only forces acting on the projectile are gravity and the Magnus force. The projectile is launched one meter above the origin on the positive z-axis. m a = F = 10 0, 0, , 2, 0 a = 0, 1 5, 9.8

31 Example (2 of 3) r (t) = 0, 1 5, 9.8 r (t) = 0, t, 9.8t + v(0) 5 where v(0) = 10 cos 45, 0, sin 45 = 5 2, 0, 5 2.

32 Example (2 of 3) r (t) = 0, 1 5, 9.8 r (t) = 0, t, 9.8t + v(0) 5 where v(0) = 10 cos 45, 0, sin 45 = 5 2, 0, 5 2. r (t) = 5 2, t 5, t r(t) = 5 2t, t2 10, 5 2t 4.9t 2 + r(0) where r(0) = 0, 0, 1.

33 Example (2 of 3) r (t) = 0, 1 5, 9.8 r (t) = 0, t, 9.8t + v(0) 5 where v(0) = 10 cos 45, 0, sin 45 = 5 2, 0, 5 2. r (t) = 5 2, t 5, t r(t) = 5 2t, t2 10, 5 2t 4.9t 2 + r(0) where r(0) = 0, 0, 1. r(t) = 5 2t, t2 10, t 4.9t 2

34 Illustration r(t) = 5 2t, t2 10, t 4.9t 2 r(t) No Magnus force

35 Example (3 of 3) Since r(t) = 5 2t, t2 10, t 4.9t 2 the projectile lands when which implies t 4.9t 2 = 0 = t , r( ) , , 0 r ( )

36 Homework Read Section Exercises: 1 55 odd.