7.3 Application of Frictional force

Frictional force

• Frictional force is defined as force that opposes the motion of an object.
• Frictional force occurs whenever two surfaces are in contact with each other.
• Frictional force is in opposite direction to the moving object and it is parallel to the surface.
• Frictional force has direction and magnitude.

Direction and Magnitude of Frictional Force

• Frictional force has direction and magnitude (value) that can be measured.
• The direction of friction is opposite to the motion of the object.
• Thus, friction can  :                                                                                                                                       

           a) Prevent a stationary object from moving    

                                                                                                              

           b) Slow down and stop a moving object

Diagram 1 : Frictional force always opposes the motion of an object

• The factors that affect the magnitude of frictional force are :

           a) Surface condition. The rougher the surface, the larger the frictional force.

           b) Weight of object. The heavier the object, the larger its frictional force.

Examples showing the existence of force

a) Friction enables a ladder to lean against the wall without slipping.

b) Heat produced by friction lights up the match

c) Friction between soles of shoes and the road surface enables us to walk without slipping

d) Friction between the tyres and the road surface prevents the car form skidding on the road

Advantages of friction

• Enables animals to climb trees.
• Allows brakes to slow down or stop a moving vehicle.
• Enables us to hold objects.
• Produces heat to light up the match.
• Enables us to write on paper.

Disadvantages of friction

• Wastes energy.
• Slows down moving objects.
• Wears out surfaces (og. soles of shoes, machines parts, tyres).
• Produces unnecessary heat that can damage machines.
• Produces unnecessary sound.

Ways to increase friction

• Soles of shoes have rough patterns to prevent slipping while walking.
• Tyres have tread patterns or grooves to prevent vehicles from skidding.
• The rough surface of the steering wheel cover helps the driver in turning the steering wheel.
• Handles of axes, knives and scissors have rough surfaces to ensure a firmer grip on the fools.

Ways of reducing friction

• Using ball bearings

Ball bearing are placed around the axles of wheels of vehicles to reduce friction

• Using wheels

Vehicles and heavy objects such as piano, sewing machine and sofa set are fitted with wheels so that they can moved easily

• Using rollers

Conveyor belts fitted with metal rollers are used to transfer luggage in airports and heavy objects in factories

• Using lubricants such as grease and oil

Oil or grease is applied to the hinges of doors and windows to reduce friction. Engines and machines are lubricated with oil to reduce friction.

• Using a cushion of air

Friction between two surfaces can be reduced by a layer of air between them. The air puck glides easily when it is pushed. Air that comes out of the balloon forms a cushion of air puck and table surface, whereby reducing the friction between them.

The cushion of air between the hovercraft and the water surface enables hovercraft to move faster.

7.2 Measurement of force

Measuring force

• The SI unit for measuring force is NEWTON (N).
• A force can be measured using Spring balance or Newton balance.
• The reading of the spring balance is the measurement of the magnitude of the force exerted.
• The spring balance works on the principle that the extension of the spring is directly proportional to the force acting on it.
• The greater the force acting on the spring, the larger the extension of the spring.
• Extension of the spring depends on the force of gravity acting on it.

Diagram 1 : Spring balance

Weight VS Gravity

• The weight of an object depends on pull of gravity.
• The force of gravity on the moon is only one-six of the force of gravity on earth.
• Thus, different object will have different weight on the earth and the moon.

Diagram 2 : Different size of earth and moon

Calculation 

 

Note : 1 kilogram (kg) = 10 newton

Diagram 3 : Weight and Mass

The answer depends on question. If the question wants you to give the answer in g, so, 

convert kg to g. So, after converting, 0.5 kg = 500 g.

7.1 Force

Definition of Force 

• Push or pull that is exerted on an object
• Cannot be seen
• Its effect can be seen or felt
• Has magnitude and direction
• Is involved in many of our daily activities

Examples of activities that involve force 

• Pushing a trolley
• Opening a door
• Opening the lid of a can
• Pulling a box
• Cycling
• Lifting a chair

Effects of force 

• Can change the shape of object

        

Diagram 1 : A platicine is flattened

• Can change the size of an object

Diagram 2 : A can is crushed

• Can move a stationary object

Diagram 3 : A stationary ball moves when it is kicked

• A goalkeeper stops the ball

Diagram 4 : A goalkeeper stop the ball

• Can change the speed of a moving object

Diagram 5 : A ping-pong ball moves faster 

• Can change the direction of moving object

Diagram 6 : A ping-pong ball changes its direction

Note : A force cannot change the mass of an object

Types of forces

1. Frictional force 

• Occurs when two surfaces are in contact
• Always opposes the motion of the object
• Acts in the opposite direction to movement
• The direction of the force is parallel to the surface
• Can slow down or stop a moving object

2. Gravitational force

• Force that pulls all objects to the earth
• Causes all objects to fall towards the Earth
• Causes all objects to have weight
• Weight of an object is the pull of the force of gravity on it

3. Electrostatic force

• Force of attraction or repulsion between charged substances
• Enables charged substance to attract neutral substances such as small pieces of paper
• Substances with like charges repel each other
• Substances with unlike charges attract each other

4. Magnetic force

• Force of attraction exists between two unlike poles of magnets
• Force of repulsion exists between two like poles of magnets
• Force of attraction between the magnetic substances and the magnet
• Enables a magnet to attract magnetic substances like iron, nickel and cobalt