The other day my daughter was standing on my back. She weighs, let's say, 35 kg. I assume that the force involved is product of gravity (as in $F=ma$)?
Then, she jumped off my back, and suddenly the force increased significantly (and painfully). Am I correct in assuming that the mass involved didn't change, in which case, it had to be the acceleration? If so, what happened? How did she change the acceleration caused by gravity, and by how much?
Best Answer
You are correct, her mass did not change. And while gravity is still acting on her, what she did was use her legs muscles to store energy (like springs) such that the resulting spring force was greater than the force caused by her mass (weight).
So if you look at the force balance you get:
$$\text{Leg Force} - \text{Weight} = \text{mass} \cdot \text{acceleration}$$
Where $\text{Weight} = \text{mass} \cdot a_{\text{gravity}} $
So when $\text{Leg Force} > \text{Weight}$ you get a net positive acceleration in the direction opposite gravity (up) which causes her to "jump" off your back.