1. Do now: Rātapu, 3 Haratua 2015 A tennis ball of mass 50g is moving at a speed of 20ms -1. It is then hit directly back with a speed of 15ms -1. a) Calculate the change in momentum of the ball b)If the ball is in contact with the racquet for 0.10s, calculate the size of the average force on the ball

2. This week’s learning: Translational Motion (Chapter 7) -last term you did the Year 12 review questions on pages 82-86 – if you need extra practice: do Activity 7A Homework: Set 1 Today’s kaupapa (learning): Impulse (in 2D) – Activity 7B

4. Example: To test cycle helmets, a model head containing a digital force recorder is dropped onto concrete. Use the graphs below to explain why it is safer for cyclists to wear a helmet.  The two graphs have the same area, since the change in momentum (impulse) is the same in both cases (both heads come to rest on the concrete)  With the helmet on, the change in momentum occurs over a longer time, Δt, and with a smaller maximum force F max  Therefore the risk of head injury is reduced if the cyclist is wearing a helmet

5. Impulse in 2D: