Presentation on theme: "Estimating Speed According to a rule-of-thumb, every five seconds between a lightning flash and the following thunder gives the distance of the storm in."— Presentation transcript:
1 Estimating SpeedAccording to a rule-of-thumb, every five seconds betweena lightning flash and the following thunder gives the distanceof the storm in miles. Assuming that the flash of light arrivesin essentially no time at all, estimate the speed of sound in m/sfrom this rule.
2 Kinematics in One Dimension MECHANICS comes in two parts:kinematics: motion (displacement, time, velocity)x, t, v, adynamics: motion and forcesx, t, v, a, p, F
8 d = 2100 km + 1800 km = 3900 km v = d/t = 881 km/hr An airplane travels 2100 km at a speed of 800 km/h, and thenencounters a tailwind that boosts its speed to 1000 km/h for thenext 1800 km.What was the total time for the trip?What was the average speed of the plane for this trip?d = 2100 km km = 3900 kmv = d/t = 881 km/hr
9 Acceleration How to express a change in velocity? Again, two kinds of acceleration:
10 Kinematics defined by - x, t, v, a x displacementt timev velocitya acceleration
11 An automobile is moving along a straight highway, and the driver puts on the brakes. If the initial velocity isv1 = 15.0 m/s and it takes 5.0 s to slow to v2 = 5.0 m/s,what is the car’s average acceleration?
18 Try It! Consider an airport runway. A light aircraft must reach a speed of 100 km/hr (27.8 m/s) to lift off. It canaccelerate at 2.00 m/s2.A) If the runway is 150 m long, can the airplane takeoff?B) If it cannot take off, how long of a runway wouldbe required?
19 Try It! Doing part B) first: For part A), runway length is not sufficient.
20 Problem Solving Read the problem Draw a diagram List what is known and what is wantedWhat physics principles are appropriateList relevant equations and their applicability(may have to derive the best equation)Calculate the requested quantityMake an estimate - are the results reasonableBalancing units can serve as another check
21 A car speeding at 80 mi/hr passes a stationary police car. The police car immediately gives pursuit. If the speedingcar remains at a constant velocity, and the police car canmaintain a constant acceleration of 4.5 m/s2, how long isrequired to catch the speeder and how fast is the policecar traveling?vs = 80 mi/hr = 35.8 m/sap = 4.5 m/s2 = 10.0 mi/hr-s
24 If the one-dimensional motion is vertically oriented… Try a = g (9.807 m/s2 or ft/s2 , down)Galileo derived kinematics based on experiments.Concerning the motion of falling objects,all objects fall with the same constant accelerationIn the absence of air resistance, regardless of the sizeor mass, all objects fall with the same acceleration g.
25 A ball is dropped from a tower that is 70.0 m in height. How far will it have fallen in 1.00 s, s, and 3.00 s?How long will it take to reach the ground?
26 A person throws a ball upward with an initial velocity of 15.0 m/s. How long will the ball take to be caught?