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Unformatted text preview: Forces and Motion Forces and Motion Newton Newton ’ ’ s Laws Review s Laws Review Newton Newton ’ ’ s Third Law s Third Law Hook Hook ’ ’ s Law s Law – – Springs Springs Lecture 8 Example: Predicting Motion Using Newton Example: Predicting Motion Using Newton ’ ’ s 2 s 2 nd nd A force acts on a 1.2 kg object which is initially at rest at the origin. (a) What is the object’s acceleration? (b) What is its location after 3.5s ? (c) How fast and in what direction is it moving after 3.5s ? (a) From Newton’s 2 nd the acceleration is: F 2.4 i 1.7 j a F / m 2.4 i 1.7 j /1.2 2.0 i 1.4 j m / s 2 (b) From the kinematics for an object with uniform acceleration the location is: r t v t 1 2 at 2 1 2 2.0 i 1.4 j t 2 r 3.5 1 2 2.0 i 1.4 j 3.5 2 12.25 i 8.68 j m Example: Predicting Motion Using Newton Example: Predicting Motion Using Newton ’ ’ s 2 s 2 nd nd A force acts on a 1.2 kg object which is initially at rest at the origin. (a) What is the object’s acceleration? (b) What is its location after 3.5s ? (c) How fast and in what direction is it moving after 3.5s ? The object’s speed after 3.5s is: F 2.4 i 1.7 j (c) From the kinematics for an object with uniform acceleration the velocity is: v t v at 2.0 i 1.4 j t v 3.5 2.0 i 1.4 j 3.5 7 i 5 j m / s v v x 2 v y 2 7 2 5 2 8.6 m / s It’s direction (as measured from the x axis) is: tan − 1 5 7 35.5 ∘ Example: Hockey Player and that Puck Example: Hockey Player and that Puck A hockey player strikes a 170g puck accelerating it from rest to 50 m/s . If the hockey stick is in contact with the puck for 2.5 ms , what is the average force on the puck? To find the average force we need the average acceleration: 〈 a Δ v Δ t 50 m / s 2.5 10 − 3 s 20 10 3 m / s 2 From Newton’s 2 nd the average force is: 〈 F m 〈 a .17 kg 20 10 3 m / s 2 3.4 kN Example: A Truck and That Pole Example: A Truck and That Pole A truck moving at 70km/h collides with a pole. The front of the truck is compressed by .94m . What average force must a seatbelt exert in order to restrain a 75 kg passenger in this accident?...
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 Spring '08
 Hicks
 Acceleration, Force, Mass

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