Forces Chapter 34 Exploring Science 3 rd Edition

Forces Chapter 34 Exploring Science 3 rd Edition Ms. O’Rourke

Aim and objective l Investigate examples of friction and the effects of lubrication l Investigate the relationship between the extension of a spring and the applied force.

Friction as a force l Friction is a force that opposes the sliding motion of an object when in contact with another object. l Friction causes an object to slow down.

Friction as a force Friction strips on a box of matches feels rough. All surfaces are rough even though they don’t seem to be. (close up photo of a surface appears rough and jagged)

Friction as a force When a block of wood is placed on a table the rough surface of the block is caught in the rough surface of the table. When pushing the block some of the pushing force moves the block. Other force is needed overcome the friction between the table and the block. The amount of pushing force needed to overcome the friction depends on how rough the surface is.

Advantages of friction l Friction has many advantages and is very useful in everyday life. l When driving a car around a corner friction is needed. (With worn tyres there is a lack of friction and could cause the car to skid) l Friction is needed to stop a bicycle. (Friction between the brake pads and the wheel)

Advantages of friction l Walking (friction between shoes and the ground is needed) l If walking on an oily surface one is likely to slip because the oily patch reduces the friction between shoes and the ground. l If there is not enough friction, you wouldn’t be able to walk.

Disadvantages of friction l Getting a blister (new shoes on a long walk continued rubbing of skin off shoe) l Friction causes the wearing of moving surfaces in contact. (soles of shoes, tyres, bade of a carving knife)

Disadvantages of friction l When a car is moving there is friction between the air and the surface of the car. Friction is overcome by the engine l However this means that some fuel is needed to over come the friction. Therefore friction wastes fuel.

Reducing friction l To reduce friction between two surfaces one could try and make it smoother. l There is no perfect smooth surface so we use special materials to reduce friction. These are called lubricants. Common examples are oil, liquid polish, soap

Lubrication Used to reduce the friction between two surfaces in contact.

Reducing the force of friction Moving parts in a car engine. Friction leads to wear and heat damage (on the engine) Oil and grease are used as lubricants l l Bones at joints are surrounded by a fluid that acts as a lubricant that greatly reduces friction. Fluid is synovial fluid

Reducing the force of friction Squeaky doors Noise cased by friction in the hinge. Liquid polish on the hinge reduces the friction and noise. l

Activity 34. 1 P 286 l To investigate friction in the laboratory

The effects of a force on a spring l Robert Hooke discovered the relationship between the force applied to an elastic body and the extension of the elastic body caused by the force. This is Hooke’s Law The extension of an elastic body (a spring) is directly proportional to the force causing the extension. l

The effects of a force on a spring l When a spring is being extended as in Hooke’s law experiment a problem may arise. l If the force used to extend the spring is too large, the spring is over stretched and loses it’s elasticity. l Hooke’s law will no longer be valid so the spring has exceeded its elastic limit.

Mandatory experiment 22 P 288 l To investigate the relationship between the extension of a spring and the applied force.

Homework l Finish mandatory write up.