Today’s lesson Impulse

 Measures the effect of the force.  Measured in Newton seconds Ns.  Equivalent to kgm/s  1Ns ≡ 1kg m/s

Impulse  a= ∆v/∆t  F=ma  F=m x ∆v/∆t  F∆t = m x ∆v  Remember: ∆v = (v 2 -v 1 )  F∆t = m x (v 2 -v 1 ) = ∆ p

Impulse Ft = mv – mu The quantity Ft is called the impulse, and of course mv – mu is the change in momentum (v = final velocity and u = initial velocity) Impulse = Change in momentum

Units Impulse is measured in Ns or kgm/s

Impulse Note; For a ball bouncing off a wall, don’t forget the initial and final velocity are in different directions, so you will have to make one of them negative. In this case mv – mu = 5m - -3m = 8m 5 m/s -3 m/s

Example  Jack punches Chris in the face. If Chris’s head (mass 10 kg) was initially at rest and moves away from Jack’s fist at 3 m/s, and the fist was in contact with the face for 0.2 seconds, what was the force of the punch?

Example  Jack punches Chris in the face. If Chris’s head (mass 10 kg) was initially at rest and moves away from Jack’s fist at 3 m/s, and the fist was in contact with the face for 0.2 seconds, what was the force of the punch?  m = 10kg, t = 0.2, u = 0, v = 3

Example  Jack punches Chris in the face. If Chris’s head (mass 10 kg) was initially at rest and moves away from Jack’s fist at 3 m/s, and the fist was in contact with the face for 0.2 seconds, what was the force of the punch?  m = 10kg, t = 0.2, u = 0, v = 3  Ft = mv - mu

Example  Jack punches Chris in the face. If Chris’s head (mass 10 kg) was initially at rest and moves away from Jack’s fist at 3 m/s, and the fist was in contact with the face for 0.2 seconds, what was the force of the punch?  m = 10kg, t = 0.2, u = 0, v = 3  Ft = mv – mu  0.2F = 10x3 – 10x0

Example  Jack punches Chris in the face. If Chris’s head (mass 10 kg) was initially at rest and moves away from Jack’s fist at 3 m/s, and the fist was in contact with the face for 0.2 seconds, what was the force of the punch?  m = 10kg, t = 0.2, u = 0, v = 3  Ft = mv – mu  0.2F = 10x3 – 10x0  0.2F = 30

Example  Jack punches Chris in the face. If Chris’s head (mass 10 kg) was initially at rest and moves away from Jack’s fist at 3 m/s, and the fist was in contact with the face for 0.2 seconds, what was the force of the punch?  m = 10kg, t = 0.2, u = 0, v = 3  Ft = mv – mu  0.2F = 10x3 – 10x0  0.2F = 30  F = 30/0.2 = 150N

Force time graphs The graph shows the force acting on a tennis ball during a return shot. What does the area under the curve tell us? AREA= F X ∆t = impulse produced during the small time interval ∆t Area under F vs t graph = total impulse acting = total change in momentum produced.

Now let’s try some fun questions!