Concepts The 70.0-kg swimmer in Figure 7.44 starts a race with an initial velocity of 1.25 m/s and exerts an average force of 80.0 N backward with his arms during each 1.80 m long stroke. (a) What is his initial acceleration if water resistance is 45.0 N? (b) What is the subsequent average resistance force from the water during the 5.00 s it takes him to reach his top velocity of 2.50 m/s? (c) Discuss whether water resistance seems to increase linearly with velocity.
• SpeedingUpIndex o Somethingpersomethingelse=index o something/somethingelse=speed/time=acceleration • Tofindacceleration…(twoways) 1. DataTableWay Example: t v Δt(t 2t 1) Δv(v 2v )1 a(Δv/Δt) 0s 5mph -- -- -- 2s 25mph 2 20 10 4s 45mph 2 20 10 2. AccelerationEquation:(a)=Δv/Δt (0s,5mph) (4s,45mph) a=Δv/Δt=(45mph-5mph)/(4s-0s)=40mph/4s=10mph/s v=v 0+at v 0=v–at a=v–v /0 t=v–v /0 Example#1:Ifat0seconds,atruckismovingat10mphandisat30mph10secondslater, whatistheaccelerationofthecar a=v 1–v /0 v 0=10mph v 1=30mph a=30mph–10mph/10s-0s t0=0s t1=10s a=20mph/10s a=2mph/s Example#2:Acarstartsat5mphandacceleratesatarateof3mph/s.Howlongdoesittake togetat30mph t=v–v 0aàt=(30mph-5mph)/(3mph/s)=25/3=8.33seconds • “Falling” o velocity
