Learning Goal: To practice Tactics Box 9.1 Drawing a before-and-after visual ove

Question

Learning Goal: To practice Tactics Box 9.1 Drawing a before-and-after visual overview. During a collision, two objects exert forces on each other that vary in a complex way during the time of the collision. Using Newton's second law to predict the outcome of a collision would thus be a daunting challenge. Nevertheless, some collisions have very simple outcomes that can be predicted by analyzing the motion of the objects before and after the collision. A before-and-after visual overview is therefore an important problem-solving tool in such problems. The following Tactics Box explains how to draw a visual overview focusing on the connection between before and after. Which of the following sketches best represents the situation before and after the collision? Vectors vi and vf represent, respectively, the velocities of the puck before and after the collision with the stick.

Explanation / Answer

we know that impulse is equal to change in momentum

=> impulse = change in momentum

let v be the puck initial velocity

given speed is same after impulse => -v is the puck velocity after impulse

=> change in momentum = m(v-(-v))

=> m(2v) = 4

=> m=2/v = 2/12 = 1/6 kg = 0.1667 kg

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