Dynamics II Motion in a Plane. Review Problems

Transcription

1 Dynamics II Motion in a Plane Review Problems

2 Problem 1 A 500 g model rocket is on a cart that is rolling to the right at a speed of 3.0 m/s. The rocket engine, when it is fired, exerts an 8.0 N thrust on the rocket. Your goal is to have the rocket pass through a small horizontal loop that is 0 m above the launch point. At what horizontal point to the left of the hoop should you launch?

3 Problem 1 con t 3

4 Problem 1 con t Analysis: Essentially a projectile motion problem with positive vertical acceleration and constant horizontal velocity. Assume a particle model. Compute the time required to rise 0 m and use that to determine the horizontal distance. FT -mg = ma 1 0 = 6.t a = 8N-.5kg(9.8m / s ) t =.54sec d =.54s(3m / s) = 7.6m.5kg = 6.m / s s 4

5 Problem A highway curve of radius 500 m is designed for traffic moving at speeds of 90 kph. What is the correct banking angle of the road? Analysis: N r provides the centripetal acceleration and N z counters the gravitational force. Nsinθ= mv r v Ncosθ = mg tanθ= rg 5 = (9.8) θ θ 5

6 Problem 3 A 30 g ball rolls around a 40 cm diameter horizontal track at 60 rpm. What is the magnitude of the net force that the track exerts on the ball? Neglect rolling friction. Analysis: Force against the vertical element of the track provides centripetal acceleration and the horizontal supports the weight. 60 rev π rad 1min F r = mr = kg0.0 m = 0.4 N min 1 rev 60 s F r F z 6

7 Problem 4 A student has 65 cm arm length. What is the minimum angular velocity, in rpm, for swinging bucket of water in a vertical circle without spilling any? The distance from the handle to the bottom of the bucket is 35 cm. Analysis: The minimum angular velocity for swinging a bucket of water in a vertical circle without spilling any water corresponds to the case when the speed of the bucket is critical. In this case, n = 0 N when the bucket is in the top position of the circular motion. mvc F rg = 0 N+mg = = mrω r 9.8 m / s ω c = g / r = = 3.13 rad / s = 30 rpm 1.00 m c 7

8 Problem 5 A car is tested on a 00 m diameter track. If the car speeds up at a steady 1.5 m/s, how long after starting is the magnitude of its centripetal accelerations equal to the tangential acceleration? Analysis: NUCM; we know a t = 1.5 m/s from that we should be able to determine the angular velocity and acceleration, ω and α, and then the time. a =ω r =1.5 m/ s r 1.5 m / s 1.5 m / s ω = = = 0.1 rad / s r 100 m a r 1.5 m / s 100 m t - - α = = = s. ω = ω i +αδt ω - ω α 0.1 s - 0 s s -1-1 i Δt = = = 8. s - 8

9 Problem 6 A popular pastime is to see who ca push an object closest to the edge of a table without its going off. You push the 100 g object and release it.0 m from the table edge. Unfortunately you push a little too hard. The object slides across, sails off the edge, falls 1.0 m to the floor, and lands 30 cm from the edge of the table. If the coefficient of kinetic friction is 0.50, what was the object s speed as you released it? Analysis: After release velocity is affected by friction only. When it reaches the edge of the table it becomes a projectile motion problem with horizontal velocity constant. Best approaches appear to be to work backward from the projectile motion to the release velocity. 9

10 Problem 6 con t Δy = 1 m and v 1y = 0 F x = -f k =ma x = -μkmg x k 1 y = y + v t - t + -g t - t 1 1y 1 1 g 0 m =1.0 m+0 m- t - t1 Similarly in the x direction x = x + v t - t a = -μ g = m/s = -4.9 m/s t - t 1 1 1x 1.30 m =.0 m+ v1x s v 1x = s = m/s Given v 1x we can find v 0x given the acceleration. v = v +a x - x : 1x 0x x m/s = v m/s.0 m v = 4.5 m/ s 0x 0x 10

11 Problem 7 A motorcycle daredevil plans to ride up a.0 m high, 0 ramp, sail across a 10 m wide pool filled with crocodiles, and land at ground level on the other side. Unfortunately, the motorcycle engine dies just as he starts up the ramp. He is going 11 m/s at that instant, and the coefficient of rolling friction is 0.0. Does he make it? Analysis: Two part problem: First is a ramp problem to determine velocity at end of ramp. Second part is a projectile motion problem determining how far right he will travel while dropping.0 m. Different sets of axes will be required for each part. 11

12 Problem 7 con t First find the acceleration parallel to the ramp: (F net ) = -fr - mgsin0 =-μrn- mgsin0 =-μ mgcos0 - mgsin0 = ma 0 r r 0 Dividing by M and solving for a 0 gives: a = -g(μ cos0 + sin0 ) = -(9.8 m / s )((0.0)cos0 + sin0 ) = m / s The length of the ramp is m/ sin 0 = 5.85 m Using v = v + a (s - s ) v = (11.0 m / s) + ( m / s )(5.85 m) = 8.9 m / s 1

13 Problem 7 con t Now we have the velocity at the beginning of projectile motion. Axes are now horizontal and vertical. Velocity components are: v = v cos0 = x 1 v = v sin0 = y 1 Time in the air is given by: 1 y = 0 m = y + v t + a t For Δt = t - t m s m S 1 1y 1y 1 =.0 m + (3.05 m / s) t - (4.90 m / s ) t t = 1.01s X is given by: 8.38 m/s (Δt) = 8.56 m. Looks like crocodile food!! 13

14 Problem 8 A concrete highway curve of radius 70 m is banked at a 15 angle. What is the maximum speed with which a 1500 kg rubber tired car can take this curve without sliding? The summation of r and z forces gives: mv F r = fkcosθ +nsinθ = r F = ncosθ - f sinθ - F = 0 z Maximum speed is when the static friction force reaches its maximum value f k =μkn. max Substituting this into the above equations gives: mv nμscos15 + sin15 = r n cos15 - μ sin15 = mg s k Dividing the bottom into the top gives: G μ + tan15 v μ + tan15 = v = gr = 34 1-μ tan15 gr 1- μ tan15 s s m s s s 14

15 Problem 9 A conical pendulum is formed by attaching a 500g ball to a 1.0 m long string, then allowing the mass to move in a horizontal circle of radius 0 cm. a. Find the tension is the string and b. the angular speed of the ball in rpm. Analysis: The mass moves in a horizontal circle of radius The acceleration and the net force vector point to the center of the circle, not along the string. The only two forces are the string tension, which does point along the string, and the gravitational force. These are shown in the free-body diagram. Newton s second law for circular motion is z F = Tcosθ - F = Tcosθ - mg = 0 N G mv F r = Tsinθ = ma r = r mg kg 9.8 m / s From F Z : T = = = 5.00 N cosθ cos

16 Problem 10 A 500 g ball moves in a vertical circle on a 10 cm long string. If the speed at the top of the 4.0m/s, then the speed at the bottom will be 7.5 m/s. Find (a) the gravitational force acting on the ball, (b) The tension in the string at the top of the circle and (c) the tension in the string at the bottom of the circle. (a) The gravitational force is: G F = mg = kg 9.8 m / s = 4.9 N. (b) The tension in the string at the top of the circle is: v T 1 = m - g = kg m / s =.9N r 1.0 m 4.0 m / s 16

17 Problem 10 con t (c) Similarily at the bottom of the circle the tension is: mv F r = T - F G = r v 7.5 m / s T = mg + = kg 9.8 m / s + = 3 N r 1.0 m 17