P11 Dynamics 3 Momentum and Impulse Bundle.notebook November 05, 2013

Transcription

1 Momentum 1

2 Momentum Momentum is a physics term that refers to the quantity of motion that an object has. Sometimes it is defined as "inertia in motion" or "mass in motion." The amount of momentum which an object has is dependent upon two quantities: mass (how much stuff is moving) and velocity (how fast and in which direction the stuff is moving). The equation for momentum is: p = mv p momentum m mass kg v velocity Momentum is a vector quantity! direction of motion direction of momentum direction of motion direction of momentum 2

3 Sample Problem p = mv A taxi possesses a certain amount of momentum, p. What would be the taxis's new momentum if... a) its velocity were doubled? b) its velocity were halved? c) its mass were doubled (by adding passengers) d) both its velocity and its mass were doubled? 3

4 Example Calculate the momentum of a 1200 kg car travelling at 90.0 km/h north. 4

5 Check Your Understanding Which has more momentum, a falling raindrop or a transport truck parked along the side of the road? 5

6 Check Your Understanding Calculate the momentum of a kg cricket ball that has a velocity of 110 km/h. 6

7 Change in Momentum and Impulse A force acting for a given amount of time will change the velocity of the object. If the object's velocity changes, then the object's momentum will also change. Δp = mδv Δp = m(v f v i ) Example In football, the defensive player applies a force for a given amount of time to stop the momentum of the offensive player with the ball. Impulse, J, is equal to an object's change in momentum. J = Δp It is also equal to the product of force and time. J = Force x Time J = FΔt Impulse (When Push Comes to Shove) 7

8 Change in Momentum Change in velocity causes change in momentum This product is referred to as the Impulse 8

9 Impulse Change in Momentum and Impulse are equivalent: an impulse of 1Ns will provide a change in momentum of 1 kgm/s 9

10 A large impulse results if: a) a large force acts over a short time interval. b) a small force acts over a long time interval. J = FΔt J = FΔt FΔt = mδv FΔt = m(v f v i ) (This is equation represents Newton's second law of motion.) Understanding Car Crashes (When Physics meets Biology) 10

11 Application 11

12 Collisions Collisions usually occur in a short time; force changes quickly The area of the graph represents the impulse 12

13 Example 1 An egg when thrown against a brick wall will break; however, an egg thrown at the same speed into a sagging sheet will likely survive. Explain why using the concepts of impulse and momentum. 13

14 Example 2 A tennis player hits a ball with a racket, giving it a speed of 43 m/s. If the ball has a mass of kg and is in contact with the racket for 4.5 x 10 3 s, what is the average force exerted on the ball by the racket? 14

15 Newton's Cradle Netwon's cradle is a device that demonstrates conservation of momentum and energy. It is constructed from a series of pendulums (usually 5) abutting one another. Each pendulum is attached to a frame by two strings of equal length angles awayed from each other. This string arrangement restricts the pendulums' movement to the same plane. It has been a popular desk toy since being invented, named and produce in 1967 by Simon Pebble. At first a wood cradle version was sold by Harrods of London, and later a chrome design was created by sculptor Richard Loncraine. 15

16 Video Clip 16

17 Momentum and Impulse Sample Problems 1. Two cars of equal mass are traveling down King Avenue with equal velocities. They both come to a stop over different intervals of time. The ticker tape digrams for each car are shown below. a) At which point does each car begin to experience the impulse? b) Which car (A or B) experiences the greatest acceleration? Justify your answer. c) Which car experiences the greatest change in momentum? Justify your answer. d) Which car experiences the greatest impulse? Justify your answer. 17

18 Check Your Understanding A 0.15 kg cricket ball is approaching a bat with a speed of 95 km/h. After it is hit by the bat, it has a speed in the opposite direction of 115 km/h. The interaction between the bat and the ball lasted s. A) what was the change in momentum of the ball? B) What impulse was applied to the ball? C) What was the average force exerted on the ball? 18

19 Module Summary 19