Chapter 14: Interactions Through the Application of Forces

Types of forces

Contact forces

Non-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.

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.

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)
  1. Gravitational force
  1. Magnetic force
  1. Electrostatic 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.

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.

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.

Measuring Force

Effects of forces

Pressure

  • 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.

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

Effects of forces on the size or shape of an object

Effect of forces on the motion of an object

  1. when we apply a force to knead a lump of plasticine, the shape of the object changes.
  2. when we mould a lump of wet clay, the size and shape of the object changes.
  1. forces can move a stationary object
  2. forces can change the direction in which an object is moving
  3. forces can change the speed of a moving object
  4. forces can stop a moving object

Turning effect of forces on an object (not a force)

examples of turning effect of forces - levers

  1. using a crowbar: force is applied on the handle of a crowbar. Crowbar turns and pulls out a nail from a wooden plank.
  2. 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.
  3. 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.

  1. a force acting at the door knob makes the door rotate at the hinges
  2. 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.

Speed

speed is the distance travelled per unit time

speed can be calculated using this formula

Pressure is the amount of force acting perpendicularly on a unit area

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.

speed = distance travelled/time taken

OR v = d/t

Unit of v = m/s
Unit of d = m
Unit of t = s

formula:
pressure = force/area

F = force , (N)
A = area , (m square)
SI unit of pressure = N/m square or Pa

pressure in everyday life

  • when the force acting on a surface decreases, the pressure decreases
  • when the area of the surface increases, the pressure decreases

atmospheric pressure

To increase pressure:

  1. Increase applied force
  2. Decrease contact area
  1. The sharp tip of a pin exerts a large pressure on notice boards
  2. The sharp edge of a knife blade exerts a large pressure on food
  3. The spikes on football shoes exert a large pressure on the ground
  4. The large force of the hammer exerts a large pressure on the finger, causing a bruise
  5. The sharp tip of the nail exerts a large pressure on the tyre
  6. The small areas of stiletto heels produce large pressures on the floor.

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.

  1. By sucking through the straw, we lower the air pressure in the straw.
  2. 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.

  1. The units on the higher floors receive water at a high pressure.
  2. The units on lower floors receive water at a pressure higher than the upper floors.