NapkinCalc

Physics 1 — Motion & Energy

Newton's second law & friction

continues from lesson 3 — values defined earlier in the course stay live here

ELI5: F = m·a, but that F is the net force — everything pushing and pulling, added up. Shove a box and friction shoves back with f = μ·N, where N is how hard the surfaces press together (on flat ground, just the weight). Draw all the forces first, then add them.

free-body diagram of the box — the four forces computed below
free-body diagram of the box — the four forces computed below
mbox:=10kgm_{box} := 10 kg = 10 kg
Fpush:=50NF_{push} := 50 N = 50 N applied force
μk=0.4\mu_{k} = 0.4 kinetic friction coefficient
Nf:=mbox∗gN_{f} := m_{box} * g = 98.1 N normal force
fk=μk⋅Nff_{k} = \mu_{k} \cdot N_{f} = 39.24 N friction force
anet=Fpush−fkmboxa_{net} = \frac{F_{push} - f_{k}}{m_{box}} = 1.076 N / kg net acceleration
✓ pass Fpush>fkF_{push} > f_{k} does it move at all? push must beat friction

Real-world hook: friction is why tyres grip (and why anti-lock brakes matter), why a heavy box won't budge until you push hard enough, and the whole reason walking works.

Try it yourself: with the box above (push 50 N, friction f_k), what is the net acceleration? (a = (F − f)/m.)

ayou=Fpush−fkmboxa_{you} = \frac{F_{push} - f_{k}}{m_{box}} = 1.076 N / kg ✏️ Your turn: net force is F_push − f_k; divide by m_box for the acceleration.
✓ pass abs(ayou−(Fpush−fk)/mbox)<0.01m/s2abs(a_{you} - (F_{push} - f_{k}) / m_{box}) < 0.01 m/s^2 green when your acceleration is correct