# Unit 08 “Impulse and Momentum” “Problem Solving”.

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Unit 08 “Impulse and Momentum” “Problem Solving”

A 0.65kg soccer ball is moving downfield with a velocity of 10m/s. A player kicks the ball so that it has a final velocity of 15m/s downfield. a. What is the change in the ball’s momentum? b. Find the constant force exerted by the player’s foot on the ball if the two are in contact for 0.030s.

m = 0.65kg V i = -10m/s V f = -15m/s a)Δp = ? Δp = mv f - mv i Δp = -9.75kgm/s – (-6.5kgm/s) Δp = -9.75kgm/s + 6.5kgm/s Δp = -3.25kgm/s b)F =? t = 0.030s Δp = FΔt -3.25kgm/s= F(0.030s) -108N = F Δp = (0.65kg)(-15m/s) – (0.65kg)(-10m/s)

A man’s 1.5kg hand hits a punching bag with a velocity of 3m/s. If the bag takes 0.3 seconds to stop the man’s hand, how much force was applied to stop it?

m = 1.5kg V i = 3m/s V f = 0m/s Δt = 0.3s F =? FΔt = mv f - mv i F(0.3s)= (1.5kg)(0m/s) – (1.5kg)(3m/s) F(0.3s)= 0kgm/s – 4.5kgm/s F(0.3s)= – 4.5kgm/s F = -15 N

Problem Solving Review 1.An Egg Drop Apparatus is dropped from the roof of Hope High School. The apparatus hits the cement with a velocity of 21m/s and comes to a complete stop. The apparatus was designed with a suspension system that allowed 2.5 seconds to stop the 0.050kg Egg, what is the force on the Egg? 2.How long does it take a trampoline to stop a 45kg boy if it puts a 200N force on the boy when lands on it with velocity of 10m/s. 3.(a) How long would the a 4kg tennis ball take to come to a stop from its initial velocity of 8.0m/s if a constant force of 150N is applied by the tennis racket? (b) How far would the tennis racket strings stretch out before stopping the ball? 4.An 2kg egg drops from rest from the roof of Hope High School 20m above the concrete ground and comes to rest 0.05s hitting the ground. What force does the ground exert of him? Work in groups using the Whiteboards to solve the following problem Answer: a) 0.21s b) 0.853m Answer: -792N Answer: -0.42 N Answer: 2.25s

1. An Egg Drop Apparatus is dropped from the roof of Hope High School. The apparatus hits the cement with a velocity of 21m/s and comes to a complete stop. The apparatus was designed with a suspension system that allowed 2.5 seconds to stop the 0.050kg Egg, what is the force on the Egg? V i = 21m/s V f = 0m/s Δt = 2.5s m = 0.05kg F =? FΔt = mv f - mv i F(2.5s)= (0.05kg)(0m/s) – (0.05kg)(21m/s) F(2.5s)= 0kgm/s – 1.05kgm/s F(2.5s)= – 1.05kgm/s F = -0.42 N

2. How long does it take a trampoline to stop a 45kg boy if it puts a 200N force on the boy when lands on it with velocity of 10m/s. FΔt = mv f - mv i -200N(Δ t)= (45kg)(0m/s) – (45kg)(10m/s) -200N(Δ t)= 0kgm/s - 450kgm/s -200N(Δt)= - 450kgm/s Δt = 2.25s m =45kg V i = 10m/s V f = 0m/s Δt = ? F = -200N

3. How long would the a 4kg tennis ball take to come to a stop from its initial velocity of 8.0m/s if a constant force of 150N is applied by the tennis racket? How far would the tennis racket strings stretch out before stopping the ball? FΔt = mv f - mv i -150N(Δ t)= (4kg)(0m/s) – (4kg)(8.8m/s) -150N(Δ t)= 0kgm/s - 32kgm/s -150N(Δt)= - 32kgm/s Δt = 0.213s m =4kg V i = 8.0m/s V f = 0m/s Δt = ? F = -150N Δx = ? V i = 8.0m/s V f = 0m/s Δt = 0.213s Δx = ½(v f + v i )Δt Δx = ½(0 +8m/s)(0.213s) Δx = 0.853m

4. An 2kg egg drops from rest from the roof of Hope High School 20m above the concrete ground and comes to rest 0.05s hitting the ground. What force does the ground exert of him? FΔt = mv f - mv i F(0.05s)= (2kg)(0m/s) – (2kg)(19.8m/s) F(0.05s)= 0kgm/s – 39.6kgm/s F(0.05s)= - 39.6kgm/s F = 792N m =2kg V i = V f = 0m/s Δt = 0.05s F = ? V i = 0m/s V f = ? a = -9.8m/s 2 Δy = -20m v f 2 = v i 2 + 2aΔy v f 2 = 0 2 + 2(-9.8m/s 2 )(-20m) v f = 19.8m/s 19.8m/s

Hard Difficult How long would the a 4kg tennis ball take to come to a stop from its initial velocity of 8.0m/s if a constant force of 150N is applied by the tennis racket? How far would the tennis racket strings stretch out before stopping the ball? An 2kg egg drops from rest from the roof of Hope High School 20m above the concrete ground and comes to rest 0.05s hitting the ground. What force does the ground exert of him? Answer: Δt = 0.21s Δx = 0.853m Answer: (you need the velocity it hit the ground with first!) V hit = 19.8m/s F = -792N