Dynamic and static friction

Autor:

Magdalena Blicharska

Słowa kluczowe:

siła tarcia,tarcie

Cel doświadczenia:

Why is it hard to push a wardrobe that is standing still? Why can you push it with less force when it has started to move? The purpose of the experiment is to familiarise club members with the difference between static and dynamic friction.

Spis materiałów:

1. a long wooden stick, e.g. a broomstick
2. a ribbon
3. several heavy metal nuts

Etapy realizacji:

1. Hang some heavy metal nuts or other objects as a load on the ribbon.
2. Keep the wooden stick at an angle and hang the ribbon with the nuts on it. The inclination angle of the stick should not be too large; otherwise the ribbon might slide down at once.
3. Gently turn the stick around its axis. Observe what happens when you rotate the stick and when you stop.

Pytania do doświadczenia:

1. What is the difference between static and dynamic friction?
2. When does the ribbon start to move? Why?

Opis zjawiska:

Interpretacje wyników:

We are dealing with static friction when despite a force acting on a body this body remains at rest relative to the ground. The resting surfaces have some irregularities that ‘hold’ both bodies together, and it is hard to set such a body in motion. You can set the body in motion by acting with a force that exceeds the maximum static friction. After setting the body in motion, the irregularities do not ‘obstruct; one another so much and thus hold the body with less force than at rest.

When making an attempt at moving a heavy object we start pushing but it will not move because the force of friction counteracts our force. This is static friction. As we increase our force, we come to a point at which the object starts moving. The force with which we were acting on the object at the point when it started to move is the maximum value of static friction. When the object is already in motion, then we are dealing with dynamic friction the value of which is lower than the maximum value of static friction. Therefore, now you can push the object using less force and it will keep moving anyway. A similar situation is observed in the test with the broomstick and ribbon; when the stick remains at rest, the ribbon will not move but when we slightly rotate it, the ribbon starts moving downwards.

Ciekawostki:

1. The phenomenon of friction was used for making fire. When you rub a hard piece of wood against a softer piece this leads to obtaining high temperature. If you apply a flammable material to the wood heated this way we may start a fire.
2. Changing winter or summer tyres depending on the season of the year is related, among other things, to the properties of the material from which these tyres are made. Summer tyres are composed of a mixture of synthetic rubber and soot, and so they are elastic and ensure better grip in positive temperatures. At lower temperatures they get harder and do not ensure appropriate friction against the surface. Winter tyres contain a silica additive and therefore are more flexible in negative temperatures than summer tyres and ensure better grip.