Practice Problems for 1st Semester Exam

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6. Troy, a Pinellas county sheriff is trying to determine the speed of a car that went off the Howard Franklin bridge on a rainy night and landed in the water 5 m below. The car landed 10 m horizontally from the bridge in the water. How fast was the car going when it left the road? PS: the driver was not hurt , however Troy gave him a ticket for reckless driving! dy = 5m dx = 10m vx = ? vx = dx / t don’t know t; solve for t first t = 2dy / g t = 2(5m) /(10 m/s 2 ) t = 10m/ 10 m/s 2 t = 1s vx = dx / t vx = (10m) / 1s vx = 10 m/s 7. Mrs. Peck’s class is trying to calculate where a flour sack would land if dropped from a moving airplane. The plane would be moving horizontally at a constant speed of 50 m/ s at an altitude of 200m. Neglecting air resistance, how far horizontally from the dropping point would they predict the flour sack would land? vx =50 m/s dy = 200m dx = ? dx = vx t don’t know t; solve for t first t = 2dy / g t = 2(200m)/10 m/s 2 t = 400m /10 m/s 2 t = 40 s 2 t = 6.32 s dx = (50 m/s) (6.32 s) dx = 316 m 8. A pull of 6N causes a 0.5 kg object to move at a constant velocity. Complete the diagram. a. Fn = 5N b. Ff = c. 6N constant velocity Fapp = Ff & Fnet = 0N d. Fapp = 6N m = 5kg … Fg = mg … Fg = (.5kg)(10m/s Fg e. = f. m = _______ g. h. 2 ) = 5N 5N .5 kg Fg = Fn … Fn = 5N 0N a = Fnet/ m a = 0N/ .5kg = 0 m/s 2 Ff = c. a = _______ a. Fn = b. Fg e. = d. Fapp = f. m = _______ 8 kg Fnet = Fapp – Ff = 30N – 10N = 20N g. h. 0 m/s 2 9. An applied force of 30N is used to accelerate an object with a weight of 80N to the right across a frictional (with friction) surface. The object encounters 10N of friction. Complete the diagram. 10N a = _______ 80N Fg = Fn Fn = 80N 80N 30N Fg = mg m = Fg/ g = 80N/ 10 m/s 2 = 8kg 20N a = Fnet / m a = 20N/ 8kg a = 2.5 m/s 2 2.5 m/s 2
10. A rightward force of 25N is applied to a 5kg object to move it across a rough surface with a rightward acceleration of 2 m/s a. Fn = b. Ff = c. 2 . Complete the diagram 50N Fg = mg Fg = (5kg)(10 m/s 2 ) 15N 25N Fg = Fn … Fn = 50N Fg e. = d. Fapp = f. m = _______ 5 kg Fnet = Ff + Fapp Ff = Fapp – Fn = 25N – 10N = 15N g. h. 10N 2 m/s 2 a = _______ 50N Fnet = ma Fnet = (5kg)(2 m/s 2 ) = 10N 11. a. What is the momentum of a 55 kg carton that slides at 4 m/ s across an icy surface? m = 55kg v = 4 m/s p = ? p = mv p = (55kg) (4m/s) p = 220 kg m/s b. The carton skids onto a rough surface and stops in 4 s. What is the change in momentum of the carton? t = 4s vf = 0m/s vo = 4m/s ∆p = m∆v ∆p = (55kg) (0m/s – 4m/s) ∆p = - 220kg m/s c. What was the impulse that changed the momentum of the carton, thus bringing it to a stop ∆p = J ∆p = - 220 kg m/s J = - 220 Ns d. Calculate the force of friction the carton encountered in order to stop it in 4 s. J = Ft F = J/t F = -220 Ns/ 4s F = - 55N 12. a. What impulse occurs when an average force of 20 N is exerted on a cart for 5s? F = 20N t = 5s J = ? J = Ft J = (20N) (5s) J = 100Ns b. What change in momentum does the cart undergo? ∆p = J ∆p = 100 kg m/s c. If the mass of the cart is 5 kg and the cart is initially at rest, calculate its final velocity? ∆p = m∆v ∆p = m (vf - vo) 100kg m/s = (5kg) ( vf – 0m/s) 100kg m/s = 5kg (vf) 100kg m/s / 5 kg = 5kg (vf) / 5 kg 20 m/s = vf 13. Hamlet, a hamster, runs on his exercise wheel, which turns around once every 3.5 s. What is the frequency and period of the wheel? T = 3.5s f = 1/ T f = 1/ 3.5s f = .29 Hz
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Practice Problems for 1st Semester Exam

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