Physics 1A: Homework #4 Assigned: July 14th, 2020 Due: Tuesday, July 21th, 2020, by 11:00 pm, on Gradescope 1. The following questions should be explained in 3-5 lines: (a) A plane and a fly have the same kinetic energy. Which one has the greater momentum? (b) Tug-of-war is played on a frictionless surface between two contestants. Contestant A is much stronger than contestant B. B weighs 80 kgs and A weighs 65 kgs. Determine who loses this contest. 2. Two masses m1 and m2 are at rest on a frictionless surface. They are connected by a stretched spring and released at time t = 0 as shown in Fig. 1. Calculate the ratio of their speeds, v1 and v2, and the ratio of their kinetic energies. Figure 1: Problem 2 3. Two particles collide elastically as one approaches the other directly from the rear. Particle A has a mass of 42 kg and particle B 49 kg. If A approaches at 7.50 m/s and car B is moving at 4.60 m/s. Calculate their velocities after the collision. 4. A mass m = 2 kg slides down a θ = 60◦ inclined plane from a height of 6 m. At the bottom of the incline, it collides with another 1 mass M = 5 kg, and the latter is initially at rest as shown in Fig. 2. The surface past the incline is horizontal and frictionless. Determine the speeds of the masses after collsion and how far back up the inclined plane m travels. Figure 2: Problem 4 5. Impact effects on different objects are tested by smashing heavy objects into fixed barriers at 18 m/s. An object of mass 3000 lbs takes 0.1 s from the time of impact until it is brought to rest. (a) Find the force (on average) exerted on the object by the barrier. (b) Determine the deceleration of the object. 6. You are given two forces F1 = 60 N and F2 = 4N. Which one exerts a greater torque. Explain your answer in under three lines. 7. An flywheel slows down from 3000 rpm to 1000 rpm in 2.75 s. Calculate the following quantities: (a) its angular acceleration. (b) the number of revolutions tmade in this time. 8. A boy of mass 100 lbs stands at the center of a rotating merrygo-round (MGR) in th shape of a disk of radius 4 m and mass 20 kg. The platform rotates at ω = 0.76 rad/s. At this moment the bout walks to the edge of the platform. (a) Determine the angular velocity of the system (boy + MGR) when the boy reaches the rim. (b) Additionally, find the rotational kinetic energy of the system. 9. Consider the diagram of a heart-shaped body as shown in Fig. 3. 2 Calculate the net torque on the body about the point O. Express your final answer in SI units. Figure 3: Problem 9 10. A wheel is accelerated from rest at a rate of 2.2m/s2 . (a) How fast will the point at the top of the wheel be moving after 3.1 s? (b) Now consider the same wheel as part of bicycle moving on the road. Given that the velocity of the lowest point of the wheel is zero at any given time, find the velocity of the top of the wheel. Figure 4: Problem 11 11. A mass m1 = 1kg is attached to the end of a string and revolves in a circle on a frictionless surface. The other end of the string 3 passes through a hole in the table and is attached to a mass m2 kg as shown in Fig. 4. m1 revolves with a constant speed v1i = 2 m/s in a circle of radius r1 = 0.5 m. If the mass m2 is slowly increased m2 = 1.6 kg, what is the final speed, v1f , of the mass m1 ? 12. An 220 lb football player is stopped in 1.5 s by a tackle. Find the average power is required to stop him if he was originally moving at 4.5m/s. 4
Purchase answer to see full attachment