How does the weight of an object affect the friction it has on the surface.

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                        Physics coursework

How does the weight of an object affect the friction it has on the surface.

Part 1: Planning the experiment

1. Friction

When two surfaces slide overreach other, there will be a force acting against the motion, called friction.

The force needed to push an object along a surface is the force necessary to overcome the friction between the object and the surface. This frictional force depends on two things. Firstly, the nature of the object and also the friction of the surface itself. Secondly, the force between the object and the surface, so it will be harder to push the object over the surface if the downward force of the object, the weight, increases.

In my experiment, I will be investigating the later statement. As all the testing will be done on horizontal planes, the force between the body and the surface will be the weight of the body.

Friction occurs in almost all situations involving physical objects. We usually try to minimise friction, as it is a nuisance to us. However, friction is also useful and necessary in many ways: the friction between the soles of our shoes and the pavement allows us to walk without slipping and sliding. Friction is also used to light matches. The friction between the screws and the wooden beams prevent them from sliding out and keeps buildings from collapsing.

2. Dynamic and static friction

 

The different types of friction are due to various kinds of motion. Static friction occurs between objects that are not moving. Dynamic friction occurs between objects that slide across each other. I will be investigating these two types of friction. Other types are fluid friction and rolling friction.

The difference between static friction and dynamic friction can be shown by performing a simple experiment, shown in the diagram bellow.

A rectangular block of wood placed on a flat surface has a string and a force meter attached to it so that horizontal force can be applied.

If a gradually increasing force is applied to the block it will, at first, continue to remain at rest. This is because an equally increasing but oppositely directed force of friction comes into action at the under surface of the block.

If we continue to increasing the pull, a point will eventually be reached when the block just starts to move. At that point, the friction between the two surfaces has reached its maximum value. This is called static friction.

If the block continues to be pulled along so that it slips at a steady pace, it has been noticed that the force meter gives a reading that is lower than the static friction.

Static Friction

Static friction prevents an object from moving against a surface. In order to move something, you must overcome the force of static friction between the object and the surface on which it is lying. If a horizontal force P is applied to an object lying on a horizontal surface, the object does not necessarily move. There is a frictional force F opposing the force P. If P and F are the only horizontal forces acting on the object, then so long as the object is stationary, P=F.

The magnitude of the static frictional force is just sufficient to prevent motion.

The frictional force F for a particular surface is not constant. It increases as the applied force P increases until the force F reaches a value F maximum beyond which it cannot increase. The object is then just about to move and is said to be in a state of limiting equilibrium. At this point, friction is said to be limiting.

Dynamic friction

Dynamic friction, also called sliding or kinetic friction acts opposing the direction of motion. It prevents an object from moving as fast as it could if friction was not present. When dynamic friction is acting, another force must be present to keep the object moving. In the case of a falling object, the force is gravity.

How Newton’s Second law of Motion is related

Newton’s 2nd law states simply that:

Force= mass acceleration

Or

F=ma

Where friction is present, Newton’s second law is expanded to state that:

Motion force – Frictional force+ mass acceleration = mass acceleration

Or

F motion – F frictional = ma

As the object is being pulled at a constant velocity, a=0

The equation then becomes:

F motion - F frictional= m x 0

So, F motion= F frictional

4. Factors and variables

 In my experiment I am going to look at the effects of weight on friction. In my preliminary results I will look at both static and dynamic friction, and the various ways of measuring them.

Mass will be an independent variable

Throughout the experiment only one variable, mass or surface, will be changed at one time, the remaining variables, surface area of the object used, direction of motion, will remain constant at all times. This is to keep the results as precise and acurate as possible.

5. Preliminary work

This experiment is investigating how the weight of an object affect the friction it causes on a surface. To do this, I am going to use two methods and compare the results, to find out which one is the more precise and reliable to use in next parts of the experiment.

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(i) Method using pulley  

The apparatus will be set up as shown on the diagram above. A block of wood will be attached to some weights with some string, via a pulley. Weights shall be added or removed until the block of wood just begins to move, for static friction, at which point the sum of the added weights shall be recorded (force needed to overcome the static friction). This will be repeated 2 times. Then, the surface the block of wood is being pulled over will be changed, and the mass of the block of wood ...

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