Esther steps off a low wall twice. Once she lands on concrete, and once on a thick foam mat. The mat landing takes longer to bring her to rest.
Which landing presses on her legs with the smaller average force?
Both landings remove the same momentum: all of it.
During a landing, the ground's force on her legs changes from instant to instant, so we talk about its average over the stop.
You have seen the trade already: a force acting for a longer time changes more momentum.
Run that trade backward. If the momentum change is fixed, a force given more time needs less strength.
The foam stop takes longer, so a smaller average force removes the same momentum.
The concrete stop is quick, so a larger average force must do the removing.
A glass jar drops onto carpet, and an identical jar drops the same way onto tile. The carpet brings its jar to rest over a longer time. Which jar feels the smaller average force?
Wilma's bike helmet has a crushable liner. In a fall, the liner squashes, and her head takes longer to stop than it would on the bare shell. The average force on her head is smaller. Why?
Note what stays fixed: the momentum change.
Only the time and the average force trade against each other.
Gunnar catches a baseball. He lets his glove ride backward as the ball stops. Compared with holding the glove rigid, is the average force on the ball larger or smaller?
The ball's momentum change is the same either way: from full speed to rest.
The riding glove makes the stop take longer.
A longer stop with the same momentum change means a smaller average force.
The average force on the ball is smaller when the glove rides backward.
A gym removes the thick crash mat under a high bar and lays down a thin rubber sheet. Landings now stop in less time. What happens to the average force on a landing gymnast?
The gymnast's momentum change is the same: from falling speed to rest.
The thin sheet makes the stop take less time.
A shorter stop with the same momentum change means a larger average force.
The average force on the gymnast is larger on the thin sheet.
Haruki says: 'Two identical eggs roll off a counter. The egg that stops over a longer time feels a smaller average force.' Is Haruki correct?
A truck's cargo crate can be brought to rest by a stretchy strap or by a rigid chain. The chain stops it in less time. Which statement is correct?
An egg lands on foam instead of a hard counter. Which quantity is the SAME for the two landings?
Esther lands a jump twice: once with stiff legs, stopping in 0.1 s, and once bending her knees, stopping in 0.8 s. Which statement is correct?