Do now! Can you write in your planners that today ’ s homework is to read pages 54 to 61 of your text book! Due Friday 22 nd October

Last lesson  Momentum YouTube - Spectacular 100mph Train Crash Test

Momentum  Momentum is a useful quantity to consider when thinking about "unstoppability". It is also useful when considering collisions and explosions. It is defined as Momentum (kgm/s) = Mass (kg) x Velocity (m/s) p = mv

An easy example  A lorry has a mass of kg and a velocity of 3 m/s. What is its momentum? Momentum = Mass x velocity = x 3 = kg.m/s.

Conservation of momentum  In a collision between two objects, momentum is conserved (total momentum stays the same). i.e. Total momentum before the collision = Total momentum after Momentum is not energy!

A harder example!  A car of mass 1000 kg travelling at 5 m/s hits a stationary truck of mass 2000 kg. After the collision they stick together. What is their joint velocity after the collision?

A harder example! 5 m/s 1000kg 2000kg Before After V m/s Combined mass = 3000 kg Momentum before = 1000x x0 = 5000 kg.m/s Momentum after = 3000v

A harder example The law of conservation of momentum tells us that momentum before equals momentum after, so Momentum before = momentum after 5000 = 3000v V = 5000/3000 = 1.67 m/s

Momentum is a vector  Momentum is a vector, so if velocities are in opposite directions we must take this into account in our calculations

An even harder example! Snoopy (mass 10kg) running at 4.5 m/s jumps onto a skateboard of mass 4 kg travelling in the opposite direction at 7 m/s. What is the velocity of Snoopy and skateboard after Snoopy has jumped on? I love physics

An even harder example! 10kg 4kg-4.5 m/s 7 m/s Because they are in opposite directions, we make one velocity negative 14kg v m/s Momentum before = 10 x x 7 = = -17 Momentum after = 14v

An even harder example! Momentum before = Momentum after -17 = 14v V = -17/14 = m/s The negative sign tells us that the velocity is from left to right (we choose this as our “negative direction”)

Today’s lesson Impulse

Let’s follow Mr Porter

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

Now let’s try some fun questions!