Stages 3 · 4
Physics
Forces change motion, energy is kept, and it all depends on where you stand.
Seeds
PHY-1
A force changes motion, it does not keep it going
PHY-2
Energy and momentum are conserved
PHY-3
Fields push and pull across empty space
PHY-4
Waves carry energy, not stuff
PHY-5
Motion depends on who is watching
Stages
3
Structure and Evidence
How are force and motion related?
- Do heavy things fall faster? Without air, everything falls with the same acceleration g; differences come from air resistance, which matters more for light, wide objects. Try it
- How far would a cart roll with no friction? The smoother the floor, the farther the cart rolls; with no friction at all it would never stop. Try it
- Is current used up in a bulb? The current is the same all the way round a series loop: a bulb turns energy into light and heat, it does not use up charge. In parallel each branch gets the full battery voltage and the branch currents add up. Try it
- Where does a lens put the picture? Rays from one point of the object meet again at one point of the image; move the object closer (but still beyond f) and the image moves away and grows; inside f there is only an upright, magnified virtual image, as in a magnifier. Try it
- Why doesn't a bed of nails hurt? What presses in is pressure, force divided by area: the same weight spread over a thousand nails or a wide snowshoe presses a thousand times more gently than on one nail. Try it
- How hard does water push up? The upward push equals the weight of the water the object shoves aside; once it is fully under, going deeper does not change it. Try it
- Where is the person in the mirror? A plane mirror's image sits behind the glass, as far behind as you are in front, so to see your whole body you need a mirror half your height, wherever you stand. Try it
4
Systems and Argument
Why must things be conserved?
- A ball dropped on a moving train Sideways and downward motion are independent: the passenger sees the ball fall straight down, someone on the platform sees a parabola. Try it
- What sets a pendulum's beat? For small swings the period depends only on the length (T = 2π√(L/g)); height energy and speed energy keep trading places. Try it
- Who pushes harder in a crash? Both carts push each other equally hard for the same time, so the total momentum is the same before and after, even when energy turns into heat. Try it
- Drawing a force you cannot see Field lines leave positive charges and end on negative ones; where they crowd together the force on a test charge is strongest. Try it
- Does the water travel with the wave? A wave moves a pattern and its energy along while each bit of the medium only moves back and forth or round a small loop in place; its speed is frequency times wavelength, v = fλ. Try it
- Which way does the ball fly when the string breaks? Going round a circle needs a force pointing to the centre; take it away and the ball flies off in a straight line along the tangent, not outward and not in a curve. Try it
Ideas we expect to meet
- Heavy things fall faster.
- Motion needs a force to keep it going.
- Current is used up by a bulb.
- In a crash the bigger object pushes harder.