Session E: Water Resistance
Whole class teaching: In a large space ask the children to imagine that they are standing in
water up to their necks, how does it feel as they start to move around? It’s hard work - there is a lot of resistance
(drag force) - discuss why it’s so much easier to move around on dry land –
less resistance/lower drag force.
Discuss how when you swim (actually
kicking your legs and moving your arms) you can move through the water, but if
you stop applying the forces you slow down (similar to when you are moving
through air - compare to cycling, when you stop pedalling you slow down). Tell
the children that you are now going to fill the pool with different liquids. What about oil? Syrup? How does it feel now? Why is it different? Back in the
classroom take a look at three jars containing water, oil and syrup. Place the
same small object (e.g. a marble, penny) into each jar in turn. Use the Discussion
Drawing to stimulate discussion (session
resource). How does it behave differently? What do the children notice?
Will it be placed flat or edge on? Remind children of the opposite force theory
as studied in the Session B.
Remember the book on the table? The push force
from the table was enough to stop the book from passing through it, in water
the pull of gravity is greater than the pushing upthrust and so many objects
sink. As a liquid becomes thicker (more viscous) its upthrust force
increases. Finally place a plasticine ball into a bowl of water – it sinks as
the forces are not balanced. Retrieve the plasticine and make a large flat
shape (&/or a boat shape) - it floats – the increased surface area makes
the most of the upthrust.
Group activities:
Adult-led
activity:
Set up a demonstration for the children.
Weigh two pieces of plasticine so that they have the same mass and roll each
into a ball. Fill a tall clear cylinder with water and place it so that the
children can see it. Tell the children that you are going to drop both balls
from the same height at the same time. One into the water and one onto the
table. Get the children to countdown… 3, 2, 1, Drop! What do they notice? The ball in water falls slower than that
falling through the air. What is the explanation for this? Water resistance slows the ball travelling
through the water in the same way that air resistance can slow a parachute.
Adult-led
activity:
Place a number of different objects
(that don’t float) one at a time into the fruit net and measure their weight
using a force meter. Repeat the measurements this time while the net is
suspended in water. Record each measurement in a table repeating if necessary
to find an average/mean result. What do the results show? – Gravity still works under water, even though
the weight seems different – water resistance is greater than air
resistance. Remember the mass stays the same!
Adult-led activity:
Challenge children to predict which boat
design will move most quickly through water (session resources).
Lesson Materials
Boat Shape
Enquiry
You will need:
Length
of household guttering (2m is sufficient) with a cap fitted on each end
Water
Three
balsa wood boats prepared beforehand with three different prows: square,
rounded and pointed
3
small hooks
Embroidery
thread or thin cord
50g
mass
Stopwatch
Method:
1.
Before placing the first boat in the water ask children to predict what will
happen. It will float! Give children
pieces of balsa wood to feel – it is very light and they know wood floats from
previous experience.
2.
Show children the three boats and ask which shape they predict will move
through the water most easily/with the least resistance acting on it. Children
record their individual predictions.
3.
Attach a piece of thread at least 2m long to the hook on each boat.
4. Attach an
identical 50g mass to the other end of each thread.
5. Place the boat at
one end of the trough and hold the weight above the end at the other end.
6. Drop the mass
allowing it to fall to the floor pulling the thread and the boat through the
water. Discuss what is driving the boat through the water – force of gravity
pulling down on the mass.
7. Use a stopwatch to
time how long it takes each boat to travel the 2m distance. Discuss what is
slowing the boats – water resistance (drag force)
8. Repeat twice with
each boat and record all results in a table.
9.
After each run ask how each boat travelled through the water – smoothly? Did it
create waves or ripples on the surface?
10.
Work out a mean average for each set of boat results.
11.
Together draw a graph of the results.
12.
Ask children to each write a conclusion along the lines:
The boat with the
________ prow recorded the fastest time because …
The boat with the
________ prow was slowest because …
The Dead Sea
It is normally as calm as glass, with barely a ripple disturbing its surface. During most days the water shimmers under a beating sun. Where rocks meet its lapping edges, they become snow-like, covered with a thick, gleaming white deposit that gives the area a strange moon like appearance.
The Dead Sea has no life due to an extremely high content of salts and minerals which is how it got its name! Its rumoured powers of curing many illnesses and its buoyancy have been recognized since the days of Herod the Great, more than 2000 years ago.
The salt content is four times that of most of the World’s oceans; you can float in the Dead Sea without even trying, which makes swimming interesting! It is the only place in the world where you can sit back on the water to read a newspaper.
References
Forces (Year 5) | Hamilton Trust. 2015. Forces (Year 5) | Hamilton Trust. [ONLINE] Available at:https://www.hamilton-trust.org.uk/browse/science/y5/forces-year-5/86859. [Accessed 16 November 2015].
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