1.23 starter 14 March 2012 16:23
· Why does kicking a brick hurt your foot?
· Why does kicking a football not hurt your foot?
> 1.23 Tuesday, July 06, 2010 3:04 PM
· 1.23 use the relationship between force, change in momentum and time taken: force = change in momentum time taken F = Δp / t From N2L F = m x a but a = v - u / t so F = m (v - u) / t F = mv - mu / t but p = mv so F = pf - pi / t F = Δp / t can be written as Δp = F x t = Impulse We can express the green equation in words as... "Force is the rate of change of momentum" How to catch a high velocity egg! Example 1 Why would you prefer to fall onto a bed than onto concrete? (3 marks) Answer
· Concrete or soft bed, your change in momentum is equal
· With a soft bed, this change in momentum takes place over a longer time
· This means that the force needed to change your momentum is less or use the equation
· Impulse = Δp = F x t where Δp = constant
· soft bed => increases t
· and therefore decreases F Example 2 - Pearson, p.41 [cid:image001.jpg@01CD0CE6.B2259030] Answer F = Δp / t 10,000 = Δp / 60 Δp = 600,000kgm/s Δp = mv - mu Δp = m(v - u) 600,000 = 1,200(v - 2000) v - 2000 = 500 v = 2500m/s 1.23 Plenary questions 13 March 2012 15:20 Physics for You p.147 Q1d-f, Q2b 1 [cid:image002.png@01CD0CE6.B2259030] 2b. When you jump down from a table, why do you bend your legs rather than keep them rigid? (3 marks) 
· Why does kicking a brick hurt your foot?
· Why does kicking a football not hurt your foot?
> 1.23 Tuesday, July 06, 2010 3:04 PM
· 1.23 use the relationship between force, change in momentum and time taken: force = change in momentum time taken F = Δp / t From N2L F = m x a but a = v - u / t so F = m (v - u) / t F = mv - mu / t but p = mv so F = pf - pi / t F = Δp / t can be written as Δp = F x t = Impulse We can express the green equation in words as... "Force is the rate of change of momentum" How to catch a high velocity egg! Example 1 Why would you prefer to fall onto a bed than onto concrete? (3 marks) Answer
· Concrete or soft bed, your change in momentum is equal
· With a soft bed, this change in momentum takes place over a longer time
· This means that the force needed to change your momentum is less or use the equation
· Impulse = Δp = F x t where Δp = constant
· soft bed => increases t
· and therefore decreases F Example 2 - Pearson, p.41 [cid:image001.jpg@01CD0CE6.B2259030] Answer F = Δp / t 10,000 = Δp / 60 Δp = 600,000kgm/s Δp = mv - mu Δp = m(v - u) 600,000 = 1,200(v - 2000) v - 2000 = 500 v = 2500m/s 1.23 Plenary questions 13 March 2012 15:20 Physics for You p.147 Q1d-f, Q2b 1 [cid:image002.png@01CD0CE6.B2259030] 2b. When you jump down from a table, why do you bend your legs rather than keep them rigid? (3 marks)
