# Talk

## Help - any science graduates out there?

(11 Posts)
KingscoteStaff Thu 07-Mar-13 22:50:41

I am teaching Forces to Year 6.

We have taught them that mass is not the same as weight, and they have weighed objects in air and water to see that the weight changes while the mass stays the same.

We have also demonstrated that a medicine ball and a basket ball hit the floor at the same time when dropped from the fire escape. I told the children that this proves that objects that weigh more do not fall faster.

Today, we experimented with parachutes. Would the size of the parachute affect the speed at which it fell? 1 group said that they would like to change the size of the weight at the bottom of the parachute. They made 3 parachutes the same size (40x40cm) out of recycling bags, and tied unifix blocks to the strings. The parachute with 32 unifix blocks dropped MUCH faster than the parachute with 16 blocks, which dropped faster than the one with 4 blocks.

Why??

Any explanations appreciated, as they are writing conclusions tomorrow!

Knowsabitabouteducation Thu 07-Mar-13 22:58:08

Weight is always the same (on Earth). It doesn't change whether measured in air or water. When something is "weighed" in water, there is up thrust acting in a direction opposite to weight (weight is a vector, which means it has both size and direction - towards the centre of the Earth). Weight doesn't change, but upthrust becomes relevant, and this is subtracted from weight to give a much smaller resultant force.

Knowsabitabouteducation Thu 07-Mar-13 23:09:17

Regarding the parachute experiment...

Galilieo's experiment only holds true for a vacuum. You are working in air, so have to deal with air resistance (an upwards force).

When an object falls, it accelerates because of gravitational acceleration. When it is moving at a slow velocity, air resistance is small, but as it picks up speed, the air resistance increases. At some point, air resistance matches with weight of the object (but in an opposite direction), so the resultant force is zero and the object no longer accelerates - it has reached terminal velocity.

A heavier object accelerates for longer before the air resistance matches it weight, so has a higher terminal velocity. This means the overall time for the drop is less.

If you want to do a Galileo experiment, you need to set up the experimental conditions to ensure that you do not reach terminal velocity.

KingscoteStaff Fri 08-Mar-13 06:47:33

So WHY did the medicine ball (Very heavy) and the basket ball reach the floor at the same time?

And WHY does the parachute with the greater weight reach the ground first?

This is bringing back the full horrors of GCSE Physics...

Knowsabitabouteducation Fri 08-Mar-13 17:03:19

Presumably the balls did not reach terminal velocity, being relatively heavy objects.

The parachutes, with their light loads and large surface area reached terminal velocity early on. The heavier of the two would have taken longer to reach terminal velocity, so accelerated for longer. The average velocity of the heavier parachute would have been larger, and therefore the time to fall to the ground shorter.

Sat 09-Mar-13 07:38:34

I don't think year six need to know terminal velocity.

Can you do a new experiment with the weights from the parachute dropped? They should drop at the same time. Then attach the parachutes.

The parachutes will all slow down the acceleration.

You can then use v=u+at to find out the different acceleration for an object with a parachute attached.

Knowsabitabouteducation Sat 09-Mar-13 08:16:14

Your independent variable is still the mass/weight of the object, and the heavier object is falling faster when air resistance is significant.

The problem is saying that weight can change (introduced by weighing objects in air and water). It doesn't. Never ever. It introduces or reinforces a misconception that needs to be unpicked by their secondary school teacher.

Year 6 students should know about upthrust and air resistance, so it is not that difficult to subtract this from the weight. Those who have a good grasp of the particle theory can make the link to air resistance and velocity.

KingscoteStaff Sun 10-Mar-13 19:12:50

I think the problem is that we teach that the newton meter measures weight in N, while the balance scales measure mass in g and kg.

Therefore, when they use a newton meter meter to weigh something in air and then in water, it appears to 'weigh' less.

As a secondary teacher (I'm guessing!) do you think that we are teaching something too complicated to Year 6 - is this a concept that they really need to know at 10? I haven't checked the new primary curriculum, so I don't know whether weight and mass is still in.

Knowsabitabouteducation Sun 10-Mar-13 21:39:03

I think you have to teach them the truth.

Weight is weight - it doesn't change (unless on the Moon or Jupiter). You can't just change this fact for convenience.

Y6 students are well able to discern different forces (I imagine you do a forces 'circus' early on in the topic, where the students identify the various forces). They can easily understand that a force in one direction can be offset by another in the opposite direction (think tug-of-war).

I teach my Y5s and Y6s a prep school curriculum, designed to be delivered by specialist teachers in labs. I don't dumb down for their age. They cope well. They appreciate upthrust when trying to push a balloon under water, and air resistance when making helicopters. These forces can be discussed in the open!

I am very carefull to always refer to mass as mass and weight as weight. I never say weight when referring to mass (and I extend this to PSHE lessons or when cooking at home). I do use the opportunity to talk about everyday language and scientific language, but am rigid about adhering to scientific language.

Mon 18-Mar-13 23:08:40

But miss! Weight does change! F = m x g!

Mass is constant. Amount of stuff. Force acting on (or weight ) changes due to the gravitational constant.

E.g mass of 100g on earth has a weight of 0.1 kg x 10 ms^-2 = 1 Newton
Mass of same 100g on moon has a weight of 0.1 kg x 1.6 ms ^-2 = 0.16 Newtons

And SUVAT really wouldn't be suitable for year 6.... Well only the v small minority. If they could grasp that they could certainly get weight = drag for term velocity....

Mon 18-Mar-13 23:12:25

Ahhh I misread your qualifier knows I.e unless you change little 'g'.

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