Idea 3of 6Builds onChange needs a net forceHow fast the speed changes
Which way it changes
A tram slows while rolling forward. Which way is the force?
Today you'll discover
- where the net force points on something that is slowing
- why moving one way does not mean pushed that way
- a ball thrown upward, and the arrow on it
Think firstA tram rolls forward and is slowing down
Which way it changes: as the clock runs, the dot traces the velocity–time line and the area under it is shaded; the shaded area is the ground covered so far, printed beside it.
The tram moves forward and slows. Which way does its net force point?
Decide first, then press play. Watch whether the line rises or falls.
A tram on the ground beside a velocity graph, at rest in time before play. After a prediction the play button runs the tram forward while its velocity line falls. The crate's drawing then shows the one sideways arrow pointing against the slide and the net arrow beside it. The last step shows a scooter speeding up and asks which way its net force points.
See the whole ideathe big idea and the key word, in one place
The big idea
The net force points where the velocity changes.
Easy to get wrongAn arrow for the net force is not an arrow for the motion. Ask which way the velocity is CHANGING. Slowing down means the net force points against the motion.
About this modelArrows are forces, not motion: a box can move with no arrow pointing its way.
Arrows are forces, not motion: a box can move with no arrow pointing its way.
Ready to use it?
- Give the direction of the net force on a described object from how its velocity is changing.