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Chapter 14: Interactions Through the Application of Forces - Coggle Diagram
Chapter 14: Interactions Through the Application of Forces
Types of forces
Contact forces
Whenever an object is touched, contact forces act on it.
The contact force that acts in two materials rubbing against each other is called friction.
examples of friction:
Walking (the rough soles of our shoes/feet rubbed against the ground)
Writing on paper (the tip of a pencil rubbed against the rough surface if the paper)
Lighting a fire (the head of a matchstick rubbed against the rough sides of the matchstick box)
Non-contact forces :
A force that acts on an object without touching it is called a non-contact force. Gravitational force, electrostatic force and magnetic force are three examples of non-contact forces.
Gravitational force
Gravitational force is the force that attracts objects to planets. This force affects the earth, the moon, and the planets in the solar system.
example of gravitational force:
Ocean tides - the effect of gravitational force in the movement of high and low tides in harbours, ports, and coastal areas
Weight - the force of gravity pulls paperweights down on loose sheets of paper, preventing paper from being blown away by wind. It also keeps us in our seated in our chairs.
Magnetic force
Magnetic force or magnetism, is the force exerted by a magnet on magnetic materials such as iron and steel.
Every magnet has a North pole and a South pole.
Unlike poles attract each other. The force acting on the poles is called the magnetic force of attraction
Like poles repel each other. The force acting on the poles is called the magnetic force of repulsion.
Electrostatic force
We can feel electrostatic forces when we use a transparent sheet of plastic to wrap a book. The hair on our arms and the dust on the table are attracted to the plastic wrap because of electrostatic forces.
An electric charge can be positive or negative
Unlike charges attract each other. The force acting on the positive and negative charges is called the electrostatic force of attraction
Like charges repel each other. The force acting on the negative charges is called the electrostatic force of repulsion.
Measuring Force
Instruments for measuring force:
Extension spring balance (stretches when a pulling force acts on it)
Compression spring balance (compressed when a pushing force acts on it)
Mass and Weight
The force of gravity (weight) is different on different massive objects, e.g. planets
The weight of a person on earth is greater than his weight on the moon. This is due to the stronger gravity on earth.
example:
mass on earth = 60kg
weight on earth = 600N
mass on moon = 60kg
weight on moon = 100N
mass is the measure of the amount of matter in a body.
SI unit: kilogram (kg)
mass is measured using a balance such as the 1.beam balance and 2.electronic balance
weight is a measure of the gravitational force acting on an object
SI unit: Newton (N)
dependent on the location's gravitational field strength as given by the formula
W = m x g
weight is measured using a balance such as the 1.extension spring balance and 2.compression spring balance
Unit of W = N
Unit of m = kg
g = N/kg
gravitational force is not the same as gravitational field strength.
Effects of forces
Effects of forces on the size or shape of an object
when we apply a force to knead a lump of plasticine, the shape of the object changes.
when we mould a lump of wet clay, the size and shape of the object changes.
Speed
speed is the distance travelled per unit time
speed can be calculated using this formula
speed = distance travelled/time taken
OR v = d/t
Unit of v = m/s
Unit of d = m
Unit of t = s
Effect of forces on the motion of an object
forces can move a stationary object
forces can change the direction in which an object is moving
forces can change the speed of a moving object
forces can stop a moving object
Turning effect of forces on an object (not a force)
examples of turning effect of forces - levers
using a crowbar: force is applied on the handle of a crowbar. Crowbar turns and pulls out a nail from a wooden plank.
using a fishing rod: a large force is applied on the fishing rod handle to lift the fishing rod. The fish that is caught at the other end of the rod moves over at a large distance.
using a wheelbarrow: an upward force is applied on the handles of a wheelbarrow. The heavy load becomes easier to move.
when we open the door, we apply a force on the door knob or handle.
a force acting at the door knob makes the door rotate at the hinges
the force produces a turning effect on the door, causing it to open
examples of how forces help us with daily things
opening a window or a door (our hands touch the handle of a window or door to push or pull it open
pressing the button in a lift (we use our fingers to press the buttons)
squeezing a tube of toothpaste (our fingers press on a tube to squeeze out toothpaste.
Pressure
Pressure is the amount of force acting perpendicularly on a unit area
formula:
pressure = force/area
F = force , (N)
A = area , (m square)
SI unit of pressure = N/m square or Pa
pressure in everyday life
The sharp tip of a pin exerts a large pressure on notice boards
The sharp edge of a knife blade exerts a large pressure on food
The spikes on football shoes exert a large pressure on the ground
The large force of the hammer exerts a large pressure on the finger, causing a bruise
The sharp tip of the nail exerts a large pressure on the tyre
The small areas of stiletto heels produce large pressures on the floor.
when the force acting on a surface decreases, the pressure decreases
when the area of the surface increases, the pressure decreases
To increase pressure:
Increase applied force
Decrease contact area
atmospheric pressure
The air around us exerts pressure on all objects that are exposed to air. The pressure exerted by the air in the earth's atmosphere is called atmospheric pressure. We do many things that rely on atmospheric pressure.
Drinking straws: when we drink liquid using a drinking straw, we make use of atmospheric pressure to push water up the tube.
By sucking through the straw, we lower the air pressure in the straw.
The higher atmospheric pressure outside the straw pushes the liquid up the drinking straw.
Pressure in liquids
Submarines: submarines dive to great depths underwater. The rigid metal body of a submarine is built to withstand the very high pressure deep underwater
The deeper the submarine dives, the greater the underwater pressure becomes.
If the submarine dives beyond a certain depth, the pressure my become too high and damage the submarine.
Water supply: In most high-rise buildings, water is delivered to and stored in a tank at the top of the building. This causes water in the pipes below the tank to be at a higher pressure. The high water pressure ensures that water flows to the units below.
The units on the higher floors receive water at a high pressure.
The units on lower floors receive water at a pressure higher than the upper floors.