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Unit 7 & Essential Questions Essential Questions: In a head-on collision between a Mack Truck and a Volkswagon Bug, why does the Mack Truck always win? How does an air bag help to reduce the injury to a person in a car crash? Momentum Any mass in motion – – – – – Inertia depends on mass (Newton’s 1st law) Momentum = mass x velocity p mv Unit is kg x m/s Momentum is directly proportional to both the mass and the velocity. Momentum is a vector quantity. p m v Example 1 Impulse p A change in momentum Requires a force F ma v F m( ) t F t m (v) Impulse = force x time Impulse is how long it takes a force to change the momentum of an object ∆p = F x t = m x ∆v Impulse is directly proportional to both force and time. Example 2 Example 3 Example 4 Example 5 A 1400 kg car moving westward with a velocity of 15 m/s collides with a utility pole and is brought to rest in 0.30 s. Find the magnitude of the force exerted on the car during the collision. Example 6 A 2250 kg car traveling to the west slows down uniformly from 20.0 m/s to 5.00 m/s. How long does it take the car to decelerate if the force on the car is 8450 N to the east? How far does the car travel during the deceleration? When the egg hits the plate, the time decreases & force increases F t mv When the egg hits the pillow, time increases & force decreases F t mv Essential Questions In a head-on collision between a Mack Truck and a Volkswagon Bug, why does the Mack Truck always win? The Mack Truck has more Momentum How does an air bag help to reduce the injury to a person in a car crash? The air bag decreases the force by increasing the time over which it is applied. Conservation of Momentum Newton’s Third Law of Motion For every action (force) there is an equal and opposite reaction (force) If the net force is zero then the total change in momentum is zero ... in every interaction, there is a pair of forces acting on the two interacting objects. – – – The size of the force on the first object equals the size of the force on the second object. The direction of the force on the first object is opposite to the direction of the force on the second object. Forces always come in pairs - equal and opposite actionreaction force pairs. Conservation of Momentum Momentum is never lost , but is simply transferred from one object to another. Elastic vs. Inelastic Collisions Elastic Collisions – – – – – two objects collide and then bounce off each other. Both objects return to their original shape. Objects move separately after the collision. Total kinetic energy is conserved ½mv2 1i + ½mv2 2i = ½mv2 1f + ½mv2 2f Total momentum is conserved m1v1i m 2v 2i m1v1 f m 2v 2 f Elastic collision where a larger mass hits a ball that is initially at rest Elastic collision where a smaller mass hits a larger mass Collisions That are not Head On Inelastic Collisions – – – Two objects collide and stick together and move as one mass. The velocity of the two objects after the collision is the same (obviously since they are stuck together). KE is NOT conserved Turned into other forms of energy: – – – Sound energy Heat/Thermal energy Friction Inelastic Collision m1v1i m2 v2i (m1 m2 )v f Example 7 Example 8 Example 9