Movie Review Pat One due Tuesday (in class) please pint Test in class on Fiday. You may stat at 8:30 if you want. (The topic of powe is not on test.) Chaptes 4-6
Main Ideas in Class Today Afte class, you should be able to: Undestand and calculate linea momentum and impulse Detemine the aveage/maximum foces and pessue acting on an object Exta Pactice Poblems: 5.1, 5.7, 5.9, 5.11, 5.13, 5.15, 5.17, 9.3, Conceptual 9.1
Chapte 6: Linea Momentum How we deal with collisions Ball and bat, cas, etc. p = mv Linea momentum (p) of an object is mass times velocity Linea momentum is a vecto Diection of linea momentum is the same as the velocity of the object Units: kg m/s
Example Review A pitche claims he can thow a 0.145 kg baseball with as much momentum as a 3.00 g bullet moving with a speed of 1500 m/s! What must the baseball s speed be if the pitche s claim is valid? Which has geate kinetic enegy? A. the ball B. the bullet C. they ae the same Q63
Newton s Second Law Goal: Wite Newton s Second Law in tems of momentum. F net = ma = m v t F net = = m( v v t p t o ) = F net = mv mv t p = o mv ma This is the geneal fom of Newton s 2nd Law and applies even if mass changes, like in a ocket buning fuel (end of this chapte). Rockets not on test.
Impulse If a constant (o aveage) foce F acts on an object, the impulse I deliveed to the object ove a time t is: I p = F t = ( ) t avg t = p = mv f mv i units = kgm / Notes: 1. A foce must act on an object fo impulse to occu. 2. The time is the collision time (just befoe touching to afte) 3. In a collision, an impulse occus in the diection of the foce acting on the object [when I walk into the wall?] s
Spideman s Gwen Stacy (Example movie eview) Gwen who weighs 50 kg falls 300 feet and is then bought to est by Spideman s webbing in 0.5 s. What is the impulse and foce on Gwen? Ignoe ai esistance. What does Spideman do to stop this in the futue?
A 0.40 kg ball is initially moving to the left at 30 m/s. Afte hitting the wall, the ball is moving to the ight at 20 m/s. What is the impulse of the net foce on the ball duing its collision with the wall? A. 20 kg m/s to the ight B. 20 kg m/s to the left C. 4.0 kg m/s to the ight D. 4.0 kg m/s to the left E. none of the above Q64
A 100-g lump of clay hits a wall at 70 cm/s and sticks. A 100-g ubbe ball hits the same wall at 60 cm/s and ebounds with a speed of 30 cm/s. Which object expeiences the lage impulse deliveed by the wall duing the collision? A. The clay B. The ball C. Both impulses ae the same. D. Cannot be detemined. Q65
Main Ideas in Class Today You should be able to: Undestand and calculate linea momentum and impulse Detemine the aveage/maximum foces and pessue acting on an object
Impulse = Foce * contact time What if that foce isn t constant? Can you think of an example?
Hitting a Baseball A 150 g baseball is thown at a speed of 20 m/s. It is hit staight back to the pitche at a speed of 40 m/s. (Contact time is 0.006s) What is the maximum foce F max and the aveage foce F av that the bat exets on the ball?
Hitting a Baseball: Conside just befoe and just afte ball touches bat We don t have to conside gavity because it acts in the y diection. p = I x = (aea unde foce cuve) = F (6 ms) = F (.003 s) 1 2 x max max p = mv mv = m( v v ) x fx ix fx ix = = (0.15 kg)(40 m/s + 20 m/s) 9.0 kg m/s Aea of tiangle = ½ base * height p (9.0 kg m/s) F av = x = = 1,500 N t (.006 s) Theefoe, F = (9.0 kg m/s) /(.003 s) = 3,000 N max
If you eceive a big impulse, will you necessaily beak a bone? (Section 9.2)
Pessue (mainly fo liquids and gases) o Stess (fo solids) tells you ove what aea the foce is spead. Pessue o Stess = Foce / Aea Damage depends moe on pessue than foce! (may be useful fo movie assignments, moe Ch. 9)
Man on a Chai A man sits on a fou-legged chai with his feet off the floo. The combined mass of the man and chai is 95 kg. If the chai legs ae cicula and have a adius of 0.50 cm at the bottom, what pessue does each leg exet on the floo? What could you do to educe pessue and chance of scatching?
Ca Ties The fou ties of an automobile ae inflated to a gauge pessue of 2.0 x 10 5 Pa. Each tie has an aea of 0.024 m 2 in contact with the gound. Detemine the weight of the automobile. 0.024 m 2
Pick a patne Explain to each othe you movie poject. Minute Pape: Explain the appoach of you patne.
Main Ideas in Class Today You should be able to: Undestand and calculate linea momentum and impulse Detemine the aveage/maximum foces and pessue acting on an object Exta: 5.1, 5.7, 5.9, 5.11, 5.13, 5.15, 5.17
Anothe baseball example A baseball playe of mass 84.0 kg unning at 6.70 m/s slides into home plate and comes to a stop. What magnitude of impulse is deliveed to the playe by fiction? If the slide lasts 0.750 s, what is the aveage fiction foce exeted on the playe?
A 75.0 kg ice skate moving at 10.0 m/s cashes into a stationay skate of equal mass. Afte the collision, the two skates move as a unit at 5.00 m/s. Suppose fo this poblem the aveage foce a skate can expeience without beaking a bone is 4500 N. If the impact time is 0.100 s, does a bone beak fo eithe skate? Fo each skate: Fav = p / t Mass of each skate does not change Fav = p / t = m v / t
p = mv A 0.15 kg ball is thown staight up with an initial speed of 20 m/s. Find the momentum (p) of the ball halfway to its maximum height. What do you know: (V fy?) V oy, a y Find y max and divide by 2 My goal fo poblem: Show not vey diffeent fom what we ve been doing
Clicke Answes 1=D, 2=E, 3=D, 4=B, 5=C, 6=A, 7=C, 8=E, 9=A, 10=C, 11=B, 12=C, 13=E, 14=A, 15=B, 16=C, 17=B, 18=D, 19=A, 20=C, 21=B, 22=B (had), 23=A Ch.4A: 25=A, 26=B, 27=E, 28=C, 29=C, 30=D, 31=C, 32=D, 33=C, 34=B, 35=D Ch.4B: 36=A, 37=C, 38=B, 39=E, 40=B, 41=B, 42=C, 43=A Ch.4C: 44=B, 45=A, 46=A, 47=C, 48=C, 49=A Ch.5A: 50=C, 51=E, 52=D, 53=E, 54=D Ch.5B: 55=A, 56=D, 57=A, 58=A, 59=B, 60=B, 61=C, 62=D Ch.6A: 63=B, 64=A, 65=B