Saturday, 30 September 2017

Inertia experiments 3 and 4

In these experiments, a tub full of sand and an empty tub are taped to the top of two trolleys. In one experiment you push the trolleys away from you, one with your left hand and one with your right hand. You catch them with elastic bands. The heavier one stretches the elastic band more, showing that a bigger external force is needed to effect the change in motion. You then use the elastic bands to pull them towards you instead and the same thing happens. The bigger the mass, the bigger the external force needed to change the motion. This is INERTIA.

Friday, 29 September 2017

Inertia experiment 2

The two margarine tubs are supposed to look identical but one is full of sand and is much heavier. Pull the paper gently - one with each hand - and you can feel the difference but both move. Then give the papers a sudden tug.
The heavy one stays where it is but the light one is more likely to fall over. The heavy object has inertia. If an external force acts it is more likely to carry on doing what it was doing beforehand - in this case, staying still. Newton's First Law again.

Wednesday, 27 September 2017

Inertia experiment 1

In this experiment a trolley is allowed to roll down a ramp so that it hits a solid barrier at the bottom. The marble is not pressed in to the plasticine; it rests loose on top. The trolley carries on down the ramp in a straight line. When it hits the barrier, the barrier exerts a force on the trolley to stop the trolley. We would call this an external force, a force external to the system of the trolley and marble. Since the force acts on the trolley and the marble is not physically attached to the marble, the marble is unaffected by the external force and carries on as before in a straight line. The marble is not thrown forward by the collision; it merely continues with what it was already doing, completely unaware that a force has acted on the trolley. This is why you should wear a seat belt. It demonstrates the property of mass called inertia, as expressed in Newton's First Law. Objects with mass carry on in a straight line with a steady speed unless an external force acts on them.

Monday, 25 September 2017

Cadiar Berwyn and the Roswelsh incident

We climbed Cadair Berwyn which turned out to be a hidden gem. A shapely peak from which we could see most of Wales. Our guidebook mentioned the Roswelsh incident (see https://www.higgypop.com/news/berwyn-mountains-ufo-crash/) Sadly we were not visited by aliens but did love this guide to lights in the sky that appeared on my favourite NASA website yesterday https://apod.nasa.gov/apod/ap170924.html

Saturday, 23 September 2017

Free fall at Pistyll Rhaeadr

We went to see the tallest waterfall south of Scotland. It is supposed to be taller than Niagara. Objects feel weight because of the normal contact force (formerly known as the normal reaction force) pushing back up on them. This is when the weight of an object pushing down on a solid compresses the springy electostatic bonds between atoms. When objects fall through the air, this can't happen. There is no normal contact force so there is no reason for an object to "feel" the gravitational pull, even though it is falling. I watched the water falling and made it seemingly freeze by fixing my gaze on a particular piece of falling water. For this piece of water, it would have seemed to exist in a world devoid of gravity until it hit the bottom.

Friday, 22 September 2017

How far away are we from Blackpool?

I was impressed that you could see the Lakeland fells so clearly from Blackpool. I used the usual trick. My little finger is worth 1 degree. The fells subtend half a degree. Half a degree is 8.7 x 10^3 radians. Arc length = radius x angle in radians. The height of the fells is the arc length. The height of the fells is roughly 800m. So distance to the fells is 800/8.7 x 10^3 = 92km. That's far too big!

Tuesday, 19 September 2017

Centripetal acceleration on the big wheel at Blackpool


I timed the big wheel as it went round. 42 seconds for a complete spin. 2 pi radians in 42 seconds is 0.15 radians per second - the angular velocity. Linear velocity v = radius x angular velocity. I estimate the radius to be 15 metres. So the people are moving at a speed v of 2.2 metres per second along a tangent to the circle. They have an angular acceleration towards the centre which is v squared/r = 0.33 m/s/s.