## What is Friction? To spin or not to Spin

Friction is the Force between two surfaces rubbing together. Here are some very simple facts:

1. When two smooth surfaces rub together there is very little friction.

2. When two rough surfaces rub together there is more friction.

3. There is less friction when there is a liquid (e.g. oil) between the two surfaces.

4. There is more friction if the two surfaces are forced against each other.

Just think of walking on a wet marble floor or an icy pavement with new leather soled shoes: you might slip over! Worse still, if the tyres on your car are worn out and the road is wet and slippery, you will probably skid the car and have an accident. Both your shoes and the car's tyres need good grip so their surfaces are rough. This increases the friction between them and the ground.

Friction is also very important for your car's brakes to work properly. When you put you foot on the brake pedal, some rough pads are squeezed tight against the brake discs. This friction slows the car down. If oil gets on the discs, the brakes will not work so well.

No matter which direction something moves in, Friction pulls it the other way. Move something left, Friction pulls right. Move something up, Friction pulls down.

Friction is actually a force that appears whenever two things rub against each other. Although two objects might look smooth, microscopically, they're very rough and jagged, as this picture shows:

Certain materials exert more friction than others. For example, a rough surface (such as a carpet) exerts more friction than a smooth one (such as an ice skating rink). The state of the material an object is in contact with also affects friction.

There is more friction between two solid surfaces, for example, than between a solid object and a gas.

### Golf Ball Spin

If the ball is to spin when it leaves the clubface then a spin force must be present on the ball while the ball is in contact with the clubface.

A spin force is any force on the ball that does not pass through the centre of the ball.

The compression force, due to the elasticity of the ball during its deformation by the clubhead collision, will pass through, or close to, the centre of the ball and so is not a great source of spin for the separating ball.

However if the ball does not slip up the clubface during contact then a “Friction Force” must be pulling on the ball in a Tangential direction (down the clubface) at the surface of the ball to keep it from slipping up the clubface.

This “Friction Force’’ will spin the ball (backspin) on separation.

If the ball slips up the clubface during contact then this Friction Force is much reduced and hence the ball will not spin (backspin) nearly as much on separation.

The grooves in the clubface will help the ball grip (Larger Friction Force) the clubface better and prevent the ball from slipping up the clubface during contact.

The nature of the surface of the ball (e.g. if it is soft and “sticky”) will also influence how effectively the ball grips the clubface during contact and so influences the amount of backspin on the ball at separation.

### Drill

You can get an insight into this Friction Force idea if you gently throw a ball (a table tennis ball with a line around it is good – or a golf ball) onto a gripping surface. After the bounce the ball will be spinning. Try not to spin the ball as you throw it so that before contact with the surface the ball is not spinning.

Now find a slippery surface (ice is good or a hard smooth surface with oil on it to make it slippery) and throw the ball onto it.

Now the ball will slip on the surface during contact and the Friction Force is quite small and so the ball will have little or no spin after the bounce. The angle of the bounce is also affected by the Friction Force.

The presence of any lubricant on the clubface (e.g. moist grass or from the rough) will reduce the Friction Force and hence reduce the backspin.
Try putting some oil on your wedge and see if you can get backspin. – I doubt it. Put something sticky on the clubface and see how you can spin it like Adam Scott.

### Summary

In summary it is the Friction Force, acting on the ball, from the clubface that produces the backspin. The presence of the Friction Force also reduces the elevation angle of the trajectory, all other factors being equal.

The Friction Force pulls down the clubface on the surface of the ball (tangential to the ball) reducing the separation angle of the ball.

Once you get into the 60 degree wedge type of club, the Friction Force cannot stop the ball slipping up the clubface during contact. Once slipping occurs the Friction Force reduces significantly.

Two things happen compared to a ball that does not spin.

1. There will be much less backspin

2. The separation elevation angle will be greater.

Think of it this way. Whenever the ball tries to slip it is the Frictional Force that stops it from slipping.

The Frictional Force will be in a direction that stops the slipping.

With the clubface square to the direction of travel of the club there will be no sideways tendency for the ball to slip sideways. Because of the slope (loft) of the clubface the ball will only tend to slip up the clubface in this case.

If the clubface is open at contact with the ball the ball will slice to the right. With the clubface open the ball will try to slip outwards (as well as upwards) and the Frictional Force stopping the ball slipping outwards will put side spin on the ball causing the slice.

Similarly with the clubface “closed “at contact with the ball will try to slip inwards (as well as upwards) and the Friction Force stopping the ball from slipping inwards will put sise spin on the ball causing the ball to draw.

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