Physics Answers

QUESTION NINE: MOTION-TIME GRAPHS
A passenger training for a harbour swim boards a ferry that travels
from wharf A across the harbour to wharf B.
The ferry leaves wharf A and slowly accelerates. It takes 150 s to
travel the first 500 m. The ferry then travels the next 2000 m at a
constant speed. This section of the journey takes 300 s. The ferry
decelerates over the final 500 m, taking a further 150 s.
(a)
Sketch a distance-time graph for the ferry’s journey.
(b) (i) Calculate the average speed of the ferry during the
journey.
The same passenger then dives off wharf B and swims across the
harbour to the far shore as shown in the speed-time graph below.
Speed
(ms -1 )
0.8
0
Section
A
Speed-time graph for swim across the
harbour
Section
B
400 4000
Section
C
(b) (ii) Calculate the distance the person swam while her
speed was changing.
Time (s)
(c) Discuss whether the forces on the swimmer are balanced or
unbalanced during each section of the swim and how this results in
the types of motion produced.
4500
QUESTION NINE ANSWER: MOTION-TIME GRAPHS
a) Plotting graph correctly and drawing of correct graph shapes:
3000
2500
tra
vell
ed
(m)
500
b) (i) Calculation
0
of average
1 2
speed:
3 4
v = d/t
5 6
= 3000/600 = 5 m s -1
(ii) Calculation of distance:
0
Only
0
applies
0
to
0
section e 0
A and
0
C
Area of section A: 0.5 x
0
400
0
x 0.8
0
= 160
0
m (sec 0 0
Area of section C: 0.5 x 500 x 0.8 = 200 m onds
Total distance = total area 160 + 200 ) = 360 m
c) Explanation of motion:
Section A: Forces are unbalanced; there is a net force in the forward direction causing the
swimmer to accelerate.
Section B: Forces are balanced; there is zero net force, therefore swimmer moving at
constant speed.
Section C: Forces are unbalanced; there is a net force in the backward direction causing
the swimmer to decelerate.
For Achievement your answer should have two of:
in (a), completes the graph accurately showing correct graph shapes for at least two
sections (with the y axis labelled correctly).
in (b), attempts to calculate average speed (E.g. writes the formula and substitutes
values OR finds average speed but does not give the unit). AND recognises the distance
as the area under the speed-time graph (E.g. attempt made at finding an area of any of
the three sections).
in (c), shows understanding that section B represents constant speed and A and C
represent changing speed.
In (c) correctly states whether forces are balanced or unbalanced for each section.
For Merit your answer should have two of:
in (a), draws accurate graph with appropriate shapes for all three sections
in (b), calculates average speed correctly (with unit) AND calculates the area under at
least one section of the graph and states the distance.
in (c), states whether the forces are balanced or unbalanced for two sections of the
graph AND links them to the motion. (balanced = constant speed, unbalanced =
acceleration)
For Excellence your answer should have everything