Contact Forces

Thursday, 8 March 2018

11:53 AM

The normal component of a contact force is called the normal force N. The component in the plane of contact is called the friction force . This division is arbitrary, but useful.

Normal force: at right angles to the surface

Friction force: in the plane of the surface

If there is relative motion, we call it kinetic friction, whose direction opposes the relative motion

If there is no relative motion, it is static friction, whose direction opposes the applied force

 

Kinetic friction (k)

 

Static friction (s)
 

 

 is often less than  (it takes less force to keep sliding than to start sliding)

 

Normal force

 

Machine generated alternative text: Newton 2 in normal direction: N — mg cos 0 Never write N = mg without thinking Newton 2 in direction down plane: 8 W = mg Classic example. 0 is gradually increased to Oc when sliding begins. What is oc? What is a at oc? mg sin 0 —F = ma. No sliding: a = 0 (ii) mg sin 0 = F (i) gives N = mg cos0 definition of us is F S us N (ii) Substitute in (ii) (ii*) Note S substitute both of these in (ii*) gives (for no sliding mg sin 0 S us mg cos0 tan 0 S us, Oc = tan— us useful technique for finding

Machine generated alternative text: Also use to get (Us — '1k) 8 W = mg What is a at Oc? we had mg sin 0 —F = ma. and N - mg cos 0 = 0 (ii) (i) Sliding at 0 = we had so a = g sin Oc = g sin Oc = g sin Oc m m — 'lk,g cosOc a — tan— = g cos0c (PIs —Llk)

 

Created with Microsoft OneNote 2016.