Idea 5of 6Builds onHow fast, and which wayA minus sign is a direction
Whose velocity is it
A child walks up a moving train. How fast?
Today you'll discover
- why one person can have two speeds at once
- who the speed is measured against
- how to add the train's speed to the walk
Think firstA child walks forward on a fast train
How fast is the child moving?
Decide first, then press Next. Watch which velocity is measured against the ground and which against the train.
A train drawn from the side on a track, north to the right, with a child inside. After a prediction, an arrow on the train shows its 250 km/h against the track; an arrow on the child shows her 10 km/h against the floor; then one arrow shows the two added, 260 km/h against the ground. The last step turns the child round to walk toward the rear.
See the whole ideathe big idea and the key word, in one place
The big idea
A velocity is relative to something. Add along one line.
For exampleA boat makes 5 km/h north against the shore while the river runs 7 km/h south. Against the water it is doing 5 north take away 7 south: 12 km/h north. The water is going backward under it.
Key word
- relative velocity
- the velocity of one thing as measured against another; A relative to C is A relative to B plus B relative to C
About this modelThe arrows are velocities, not paths; the dotted line is where the swimmer actually goes.
The arrows are velocities, not paths; the dotted line is where the swimmer actually goes.
Ready to use it?
- Find a velocity relative to the ground from a velocity relative to a moving carrier along one line.