More misconceptions

Friction and electrical resistance

Catherine LowRangi Ruru Girls’ School

Four forces act on Toni and the bike while she rides: weight, support, friction and push.

On the diagram below, draw in arrows to show the directions of ALL FOUR forces acting on Toni and the bike. Label the forces.

FRICTION PUSH

SUPPORT

WEIGHT /GRAVITY

FRICTIONPUSH

SUPPORT

WEIGHT /GRAVITY

FRICTION PUSH

SUPPORT

WEIGHT /GRAVITY

FRICTIONPUSH

SUPPORT

WEIGHT /GRAVITY

FRICTION

Static

• Surface friction

Kinetic

• Fluid friction/Drag

Common phrase:

FRICTION OPPOSES MOTION

More correct:

(surface) FRICTION OPPOSES SLIDING

Drag opposes the relative motion of the object through the air

KINETIC FRICTION

STATIC FRICTION

sliding

Kinetic friction

Direction object WOULD slide if there was no grip

Static friction

DRAGSTATIC FRICTION

SUPPORT

WEIGHT /GRAVITY

FRICTION PUSH

Accelerating

braking

Constant speed

Rolling resistanceDirection of motionSupport force

Rolling resistanceDirection of motion

Overall support force

SUPPORT

WEIGHT /GRAVITY

DRAGSTATIC FRICTION

• DRAG

• RESISTANCE

• STATIC FRICTION• GRIP•TRACTION

SUPPORT

WEIGHT /GRAVITY

DRAGSTATIC FRICTION

SUPPORT

WEIGHT /GRAVITY

Explain how the effect of the spikes on the sport shoes changes the effect of forces acting on the athlete and how this leads to an improvement in performance.

Answer: Spikes on shoes increase the effect of friction. Resulting in a greater reaction/push/thrust force.

ELECTRICAL RESISTANCE

R = Slope of the V vs I graph

OR

R = dVdI

R = VI

Ohmic conductors – not an issue

V

I

The diagram below is a graph showing the voltage – current characteristics of a particular diode.

State what physical quantity is represented by the gradient of the graph line.

Answer: Resistance

Resistance = how hard it is to push charges through the device

R = VI

V

I

V

I

V

I

P = I2R

P =V

R2

Halliday, Resnick, Walker

Serway

Direction you want to go

Direction of sliding

STATIC FRICTION