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Forces · 12 min

Friction and resultant force

Friction allows walking and braking, but it also resists motion and transfers mechanical energy into thermal energy.

01Why this matters

Friction allows walking and braking, but it also resists motion and transfers mechanical energy into thermal energy.

02See it

20 N8 N
The opposing arrows represent 20 N right and 8 N left. The resultant is 12 N right.
push 20 N →← friction 8 Nnet 12 N →a = 2.4 m/s²
Read left to right: push 20 N → → ← friction 8 N → net 12 N → → a = 2.4 m/s². Each panel connects one part of the example.

03Understand it

Friction is a contact force that opposes relative sliding, or the tendency to slide, between surfaces.

For a simple horizontal model, subtract opposing friction from the applied force to find the resultant. Sliding friction is often approximated by Ff = μN, where μ is a coefficient and N is normal force. Static friction adjusts up to a maximum; it is not always equal to that maximum.

04Guided example

A 5 kg crate is pushed right with 20 N while friction acts left with 8 N. Find acceleration.

  1. Choose right as positive: the forces are +20 N and −8 N.
  2. The resultant is 20 − 8 = 12 N to the right.
  3. Apply a = Fnet/m: 12/5 = 2.4 m/s² to the right.

05Try it

A 25 N push opposes 10 N friction. Resultant?
A little hint

Subtract the opposing force.

06Practise it

Resultant 15 N acts on 3 kg. Acceleration?
A little hint

Use Fnet/m.

Sliding coefficient 0.2 and normal force 50 N. Friction?
A little hint

Use μN.

07Use it in the real world

A box sliding at constant velocity under a 10 N horizontal pull has 10 N opposing friction, assuming no other horizontal forces.

Go a little deeper

The coefficient model is approximate. Surface materials, lubrication and conditions matter. Rolling resistance and fluid drag need different models.

08Quick check

A constant-velocity box is pulled with 7 N. Opposing friction?
A little hint

Constant velocity means zero resultant.

09Summary

Friction is a contact force that opposes relative sliding, or the tendency to slide, between surfaces. Apply a = Fnet/m: 12/5 = 2.4 m/s² to the right.

Before moving on, explain the worked example in your own words. Use the hint on any question you want to revisit.

10Key idea

Fnet = Fapplied − Ffriction

All forces are in N; this scalar form assumes opposite forces along one line.

sliding model: Ffriction ≈ μN, with μ unitless

Find this in the formula library →