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Physics 101: Lecture 14 Impulse and Momentum

Physics 101: Lecture 14 Impulse and Momentum. Today’s lecture will be on Chapter 7.1 - 7.2. Impulse and Momentum. Consider a ball hit by a baseball bat: At t=t 0 : ball approaches bat with initial velocity v 0 At t=t f : ball leaves the bat with final velocity v f

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Physics 101: Lecture 14 Impulse and Momentum

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  1. Physics 101: Lecture 14Impulse and Momentum • Today’s lecture will be on Chapter 7.1 - 7.2

  2. Impulse and Momentum • Consider a ball hit by a baseball bat: At t=t0: ball approaches bat with initial velocity v0 At t=tf: ball leaves the bat with final velocity vf During the time interval Dt=tf-t0 the ball is hit by the bat, i.e. the bat exerts an average force Fave on the ball. To describe the effect of such a time-varying force on the motion of an object we introduce two new concepts: Impulse and Momentum

  3. Impulse and Momentum • The effect on the motion of an object when a time- varying force is applied will be the larger the longer the force is acting on the object and the larger the bigger the force. This observation is described by the concept of impulse: J = FaveDt SI unit: N s To describe the response of an object to a given impulse we need the concept of linear momentum: p = m v SI unit: kg m/s

  4. Impulse and Newton’s second law • If there is a average net-force acting on an object during a time interval Dt, the object experiences an acceleration, thus a change in velocity, thus a change in momentum: J = SFave Dt = m aave Dt = m (vf-v0)/Dt Dt = = m vf – m v0 = pf – p0 In our example (neglect weight of ball => SFave =Fave): The bat exerts force Fave on ball in time Dt • impulse given to the ball • ball’s velocity changes • ball’s momentum changes From the change of momentum, SFave can be determined.

  5. correct Conceptual Question Two identical balls are dropped from the same height onto the floor. In case 1 the ball bounces back up, and in case 2 the ball sticks to the floor without bouncing. In which case is the impulse given to the ball by the floor the biggest? 1. Case 1 2. Case 2 3. The same The impulse-momentum theory says that the impulse that acts on an object is given by the change in the momentum of the object, and this change is proportional to the change in velocity. The ball that sticks has a velocity of downward to zero, but the velocity of the ball that bounces goes downward then upward. This change in momentum is greater and therefore has a greater impulse on it.

  6. correct time Conceptual Question In both cases of the above question, the direction of the impulse given to the ball by the floor is the same. What is this direction? 1. Upward 2. Downward

  7. Conservation of Linear Momentum • Consider a system of two colliding objects with masses m1 and m2 and initial velocities v01 and v02 and final velocitiesvf1 and vf2 : If the sum of the average external forces acting on the two objects is zero ( = isolated system), the total momentum of the system is conserved: SFave,ext Dt = Pf - P0 => Pf = P0 if SFave,ext = 0 Pf andP0arethetotal momenta of the system: Pf = pf1 + pf2 and P0 = p01 + p02 This is true for any number of colliding objects.

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