1.23

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?
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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

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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

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2b. When you jump down from a table, why do you bend your legs rather than keep them rigid? (3 marks)

Impulse - kicking a ball and kicking a brick.swf Download this file

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