Physics Answers

QUESTION SEVEN: PRESSURE
Hayley and Sarah were running across the football field. Hayley has a
mass of 54 kg and Sarah has a mass of 60 kg.
(a) Sarah was running with a forward force of 65 N and a friction force
of 50N. Calculate the acceleration of Sarah.
Hayley was wearing football boots and Sarah was wearing
running shoes. Hayley noticed that when they both stood still,
she left marks in the field but Sarah did not.
QUESTION SEVEN ANSWERS: PRESSURE
a) Calculation of acceleration:
F net = m x a so a = F net / m = 15 / 60 = 0.25 ms -2
b) Calculation of pressure:
Hayley
P = F/A Force is F w = mg = 54 x 10 = 540N
Area is that of studs A = 0.0001 x 12 = 0.0012 m 2
P = 540 / 0.0012 = 450 000 Pa
Sarah
P = F/A Force is F w = mg = 60 x 10 = 600N
Area of both shoes A = (0.10 x 0.27)x2 = 0.054 m 2
P = 600 / 0.054 = 11 111 Pa
The bottoms of their shoes are shown below:
Each stud
has a surface
area of
Hayley’s
football boots
Sarah’s running
shoes
Length
= 0.27 m
Discussion of physics principles:
1. Sarah’s running shoes have a much larger area than the studs on
Hayley’s boots.
2. P = F/A so as A increases the pressure must get smaller.
3. Even though Sarah is heavier, her weight is distributed over a
larger area while Hayley’s is applied over a small area.
Hayley puts more pressure on the ground causing the studs to ‘sink in’
and leave marks.
For Achievement your answer should have two of:
in (a), net force is calculated OR formula correctly arranged but
incorrect force value substituted
in (b), calculates the area or weight force for one person correctly
in (b), states that Hayley leaves marks as her boots exert more
pressure
0.0001 m 2 Average width = 0.10 m
(b) Discuss the physics principles that caused Hayley to
leave marks while Sarah, who was heavier, did not.
Include calculations in your answer.
For Merit your answer should have two of:
in (a), net force is calculated AND correct acceleration
calculated including unit
in (b), shows an understanding of how the two formula should
be applied to each person but fails to give correct pressures or
units
in (b), shows an understanding of the physics principles
involved by stating that smaller area of studs would exert more
pressure than area of running shoes
For Excellence your answer should have everything