Distance is how much ground you actually cover. Walk to a shop that is 300 m away in a straight line, but the road bends around a park, and you might walk 500 m. Your legs feel the 500.
The straight line from start to finish has its own name: displacement. It only cares where you started and where you ended up, not how you got there.
Distance is the whole winding path. Displacement is the straight arrow from start to finish.
The running track puzzle
Run one lap of a 400 m track. Your distance is 400 m. Your displacement is zero, because you finished exactly where you started. Your phone's step counter measures distance; a map pin dropped at start and finish measures displacement.
In one line
Distance is the path you travel; displacement is the shortcut from start to finish.
Concept 2
Speed: distance spent each second
Speed tells you how much distance you cover in each second (or hour). Think of it as a budget: a fast runner "spends" lots of metres every second, a slow walker only a few.
speed = distance ÷ time
A sprinter who runs 100 m in 10 seconds has a speed of 10 metres per second.
Same time, different distance: the faster runner covers twice the ground.
Average speed vs speed right now
Drive 120 km in 2 hours and your average speed is 60 km/h, even if you stopped for tea and later drove at 90. The speedometer shows your speed right now. Add a direction ("60 km/h heading north") and speed becomes velocity.
Handy trick: to turn metres per second into kilometres per hour, multiply by 3.6. So the sprinter's 10 m/s is 36 km/h.
In one line
Speed is distance divided by time: how much ground you cover each second.
Concept 3
Acceleration: how fast speed changes
Acceleration is not speed. It is how quickly your speed changes. A car that goes from 0 to 100 km/h in 5 seconds has a big acceleration. A car cruising at a steady 100 has none, however fast it is going.
Imagine taking a photo of a moving ball once every second. At a steady speed the gaps between photos are equal. When it accelerates, every gap is bigger than the last.
Faint dots are one-second snapshots. Both balls finish together, but only one of them is accelerating.
Three ways to accelerate
Speed up: you feel pushed back into your seat.
Slow down: braking is acceleration too, just backwards. You lurch forward.
Turn: change direction, even at the same speed, and you are accelerating. You feel pushed to the side on a bend.
That last one is the secret of orbits: the Moon's speed hardly changes, but its direction changes all the time. Which brings us to gravity.
In one line
Acceleration is how quickly your speed or direction changes.
Concept 4
Gravity: the Moon is falling
Gravity keeps your feet on the ground and the Moon in the sky. Spoiler: they're the same thing. Near Earth's surface, anything that falls accelerates, getting about 10 m/s faster every second.
Everything pulls on everything
You pull on your phone, your phone pulls on you. The pull is far too weak to notice, because it grows with mass. Earth has a huge amount of mass, so its pull wins and everything falls towards its centre.
Distance matters too: twice as far away, the pull is only a quarter as strong.
An orbit is falling and missing
Isaac Newton imagined a cannon on a very tall mountain. Fire a ball slowly and it curves down to the ground. Fire faster and it lands further away. Fire fast enough (about 8 km per second) and the ground curves away beneath the ball as fast as it falls. It never lands. That is an orbit.
Slow: lands close. Faster: lands further. Fast enough: keeps falling around the Earth.
The Moon does exactly this. It is falling towards Earth all the time, but it moves sideways so fast that it keeps missing. Astronauts on the space station float for the same reason: they are falling with their station, not free of gravity.
Space bends like a trampoline
Albert Einstein gave a deeper picture. Imagine space as a stretched trampoline. A heavy bowling ball makes a dip, and a marble rolled nearby curves towards it. Nothing "pulls" the marble; it just follows the shape of the surface. Mass bends space, and things move along the bends.
Heavy things make deeper dips. Earth's dip holds the Moon, and the Sun's dip holds the Earth.
Try it yourself
Hold a book and a sheet of paper at the same height and drop them. The book wins. Now lay the paper flat on top of the book and drop them together. They land at the same time. Air slowed the paper down, not gravity. In 1971 an astronaut on the Moon, where there is no air, dropped a hammer and a feather. They hit the ground together.
In one line
Mass bends space, everything falls along the bends, and an orbit is just falling sideways fast enough to keep missing the ground.