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A change in momentum for an object is equal to impulse. Momentum changes when (a) A net force of 117 newtons is applied to a 20 kilogram cart that is already moving at velocity changes. 3 meter per second. The final speed of the cart was 11 meters per second. For how long was the force applied? IMPULSE = CHANGE IN MOMENTUM IMPULSE = IMPACT FORCE X TIME Force is what changes velocity. Therefore, when momentum changes a force must be involved for a period of time. The following equation relates impulse to change in momentum. If you are looking for… Use this equation (b) A 3.8 kilogram ball is accelerated from rest to a speed of 12.7 m/sec. What is the ball’s impulse? What is the ball’s change in momentum? Impulse/Change in Momentum Units: _________/____________ Force Units: _________ If a constant force of 41 newtons is applied to change the momentum in this situation, for how long does the force act? Time Units: _________ (c) A 2200 kilogram car uses a braking force of 12,000 newtons to stop in 5.5 seconds. (f) In certain martial arts, people practice breaking a piece of wood with the side of What impulse acts on the car? What is the change in momentum of the car? their bare hand. Use your understanding of impulse to explain how this can be done without injury to the hand. 1 The artist hits the wood with the fleshy side of the hand. The artist rapidly pulls backwards after the wood breaks before damage to the hand can be done. The artist pushes through the wood, continuing forward after the wood breaks. The artist hits the wood with the hard knuckles with the hand in the shape of a fist. (g) If identical bullets are shot from a pistol and a rifle, a bullet shot from the rifle will travel at a higher speed than a bullet from the pistol. Why? (Hints: Assume shooting (d) A baseball player hits a 0.155 kilogram fastball traveling at 42.7 m/sec into center force is the same in each case. The barrel of the rifle is longer than the barrel of the field at a speed of 51.6 m/sec. If the impact lasts for 0.00450 second, with what force pistol.) does he hit the baseball? 1 The rifle exerts the same amount of force, but over the longest time. The rifle has the most recoil. The pistol has the least recoil. The pistol exerts the same amount of force, but over the shortest period of time. (h) Boxers attempt to move with an opponent's punch when it is thrown. In other words, a boxer moves in the same direction as their opponent's punch. This movement may prevent a knockout blow being delivered by their opponent. Explain how. Moving with the punch reduces the force delivered to the head. For questions (e)-(i), select all statements that are correct answers to each question. Moving with the punch decreases the amount of time the glove is in contact with the body. (e) In terms of impulse, why is the ride much more comfortable when an airplane is Moving with the punch increases the force delivered, but decreases the amount of flying at constant speed versus when it is taking off or landing? time of contact, therefore a smaller impulse results. Moving with the punch increases the force delivered, but decreases the amount of An airplane moving at a constant speed has zero impulse, and an airplane time in contact, therefore a larger impulse results. accelerating has impulse. The faster airplane has more impulse, therefore more force on the passenger at (i) Mats in a gym, airbags, and padding in sports uniforms are used to protect people higher speeds. from being injured. Explain why these soft objects are used instead of rigid objects The slower airplane has less impulse, therefore less force on the passenger at lower using your understanding of impulse and change of momentum. (Select all statements speeds. that are correct explanations.) An airplane moving at constant speed has impulse, and an airplane accelerating has Soft mats extend the time over which an upward force brings your momentum to no impulse. zero. Soft mats decrease the time over which a force brings your momentum to zero. Soft mats decrease your change in momentum. Soft mats increase your change in momentum.
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