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### Forces - friction

posted Nov 13, 2015, 6:07 PM by Upali Salpadoru   [ updated Nov 25, 2015, 10:35 PM ] John is pulling a load of 500 N weight.  What is the force necessary to pull the load? Is it 500N ?

Fig.1 John dragging a heavy load

The weight of an object is always a vertical force. It cannot oppose a horizontal movement. If you lift a load you would be doing work against gravitational force. So the body gains potential energy.   P.e. = mgh.

Such a thing does not happen for a horizontal movement. Yet John seem to be using a powerful force to drag the object.  What is the force opposing him?

This is a force that will mainly depend on the nature of the surfaces. In other words rubbing one surface on another. If you can prevent the rubbing action objects can be moved with a reduced force. Making the surfaces is a way to do this. On a very slippery surface things can be pushed or pulled with almost negligible force.

We call this strange force that makes it difficult for us to pull and push, FRICTION. Friction is a contact, hidden force unlike the attractions like weight etc.  If a body is stationary,and the forces are balanced, there will be no friction. The moment you pull a block on another surface, friction will oppose you, even if the body is not moving.

Fig.2 Rollers reduce friction.

We know how difficult it is to pull a heavy log along the ground but if you place some rollers underneath the job can be done with a little effort. Rollers can reduce friction very effectively.

Magnitude of friction

As friction is a force, it must have a magnitude and a direction. If you have a force measurer ( Spring balance) the value of friction can be measured. The direction of force will be always opposite to motion. Here is a way to get the value of friction. Static friction and Kinetic friction.

The first part of the diagram shows the block being pulled using a force measurer. (Spring balance) When the pull is 2N, the block does not move. Is there a friction force acting now? “ Yes ! of course.”  For the body not to move the forces must be balanced. So the friction must be equal to the pull. Yet this will not be the maximum friction the surfaces can offer.

By increasing the pull gradually, you can find out the minimum pull necessary to make it move. This will be equal to the maximum friction between the two surfaces. This value may be taken as the limiting static friction.

If you can pull the block at a uniform speed the friction will be a little less. This will be the kinetic friction or dynamic friction.

Factors affecting sliding friction

The value of friction will depend mainly on the following factors.

 Increases friction Reduces Friction Rough surface. Smooth surface. Soft elastic materials like rubber. Hard rigid materials like steel. High pressure between the sliding plates. Low pressure between the surfaces.

Area does not seem to affect friction.

Although movement reduces friction slightly (kinetic friction)  speed does not alter sliding friction.

Finding the coefficient of limiting static friction.

Here is an easy way to determine how the friction varies with the pressure between two surfaces. Fig. 3. Friction on a slope.

An object is kept on a plank and one side is raised slowly. The block on the slope exerts a component of the weight as shown. This will tend the object to slide down, a friction force will oppose it. As the angle a. becomes bigger component force will increase. Ultimately this will overcome friction.

When the object is about to slide measure the angle a.  ( If the friction is high this angle will be high)

Using trigonometry we can get the Normal reaction force and the friction.

Fr. =  W sin.θ.  ….. and    R = W cos..θ

Coefficient of friction =  W cos θ / W sine θ.

Coefficient of friction  = F/R   =  λ

Here is an example Fig 4. Measuring the coefficient of friction.

Friction /Normal Reaction = 0.577

Tan 30˚ = 0.577.

This method can be used to find the coefficient friction between any two surfaces

Coefficient of friction for some common materials.

 Surface 1 Surface 2 Coefficient of limiting static friction Ice Wood 0.05 Graphite Steel 0.1 Glass Glass 0.11 - 0.14 Cast Iron Cast Iron 1.1, to 0.15 Carbon Steel 0.14 Brass Steel 0.35 Wood Concrete 0.62 Car tire Asphalt 0.72

Indebted to Engineering Tool box.

Rolling Friction Wheel or a ball placed on a slope will roll down even with a slight incline. This is due to the torque (moment) caused by the weight acting slightly ahead of the base. But still there will be some friction due to roughness of the surface and the invisible deformities caused due to elasticity of the materials. You can find the rolling friction by the tangent of the angle θ.

Fig. 5  A tyre rolling on a ramp.

Compare these values with sliding friction.

 Surface 1 Surface 2 Coefficient of rolling friction. bicycle tire asphalt road 0.004 car tires asphalt road 0.03 car tire sand 0.2 - 0.4 steel wheels steel rails 0.001 - 0.002

Indebted to Engineering Tool box.

Jerry wants to catch Sam that has overtaken him; so he is accelerating. As he increases the speed he feels a force opposing him is also increasing. This is neither sliding friction nor rolling friction but a force due to the molecules in the air. We call this AIR RESISTANCE or FLUID FRICTION.  Unlike sliding friction this force increases with speed.
Fluid friction is the friction due to the matter in the medium in which the object is moving.

Fig. 6. Jerry has to create a path for him by pushing the air molecules. 1.0 Fill the three columns as directed. 0.0 is answered as a sample.
 A Frictional force objet is getting. B Wanted or unwanted, C  Adaptation to change friction. 1.0 Example. Water friction,  Drag,( Wind gives the force to move) Unwanted Long and linear shape.(Streamlined hull) 1.1 1.2 1.3 1.4 1.5 (2x3 x 5 =30)

2.0 Here are some materials or  methods used to change friction. Give an example of application, and state.

 Material or Method. How or where used? Effect. 2.0  Ex. Graphite   Example. Lubricate at high temperatures. Reduces sliding friction. 2.1. Using rubber 2. 2 Using ball bearings. 2.3  Treads. 2.4  Magnetic force 2.5   Ice

( 3 x 2x 5 = 30 )

3.0 The blue box is about to slide on a ramp inclined at 35˚. The weight of the box is 250 N. Find the following.

3.1  The normal reaction.      3.2   Magnitude of the force that tends to drag it.

3.3  The friction force.   3.4  The coefficient of friction.

4.0  John is pulling a crate weighing 240 N. The rope makes an angle of 10 with the horizontal. Draw free body force diagram and find the following. 4.1   What is the normal reaction force on the crate?

4.2   If the man is moving at a constant speed using a force of !20 to East what is the dynamic or kinetic  friction.

4.3   What is the coefficient of friction?

4.4   If the crate is now loaded with vegetables weighing  200 N What will be  the friction?

( 5 x 4 = 20)