Forces in Physics Basic Concepts

Symbols

F = Force

W = Weight

M = Mass

G = Gravity / Gravitational force

N = Normal force

Gravitational Force

• The gravitational force is a force that attracts objects with mass towards each other

• Any object with mass exerts a gravitational force on any other object with mass

• The Earth exerts a gravitational pull on you, the desks in your house, and the chair in your room, holding you on the surface of the Earth and stopping you from drifting off into space

• All the planets in our solar system are held in orbit around the Sun by the gravitational force of attraction between the Sun and planets, and between planets and their moons

• The bigger the mass of the objects, the greater the gravitational force of attraction between them

  • This means that two small objects would have a very weak gravitational attraction and so it has no noticeable effect

  • However, bigger objects such as the Moon and the Earth have a much greater gravitational force

• The further objects are away from each other, the smaller the gravitational force

• Gravity is measured in meters per second squared (m/s²)

  • The standard acceleration due to gravity on Earth is approximately 9.8 m/s²

Magnetism

• Magnets are objects that show permanent magnetism

  • Magnets are often made up of iron or cobalt

• Magnets have two different poles:

  • A north pole

  • A south pole

• A magnetic field exists around a magnet. This is a region of space around a magnet, where a magnetic force can be detected

  • This field is invisible but can be visualized using lines to show its strength and direction

  • Magnetism is an example of a non-contact force

• Magnetism is a physical phenomenon where materials exhibit attractive or repulsive forces on other objects

  • It arises from the alignment and movement of charged particles, particularly electrons, within a material

Drawing magnetic field lines of magnets

Rules:

1. Magnetic Field lines never cross each other

2. Magnetic field lines drawn from the north to the south pole. Show direction of field lines by adding arrows, pointing “North to South”.

3. Field lines are continuous, this means that the field lines that are drawn must not have “any gaps” from where they exit the north pole, to where they enter the south pole

Mass

• Mass is the amount of matter in an object

  • It tells you how many particles that object has

• The SI unit of mass is the kilogram (kg)

• Eg.

  • The mass of flour in the bag on the scale, block tells us how many atoms there are in the bag

• The mass of an object stays the same, no matter where it is

  • Eg. a wooden block with a mass of 10 kg on Earth also has a mass of 10 kg on the Moon

Weight

• Weight is the force of gravity pulling an object towards the centre of the Earth (or Moon or any other planet)

• An object's weight differs on other planetary bodies

  • This is because the weight is dependent on the object's mass and the acceleration due to gravity/the force of gravity pulling down on an object

• Weight is a force and therefor it has magnitude and direction

• Unit for weight is newtons (N)

Calculating the force on an object

Formula - Force = Mass × Gravitational acceleration (9.8 m/s²)

Formula - F = m x g

F = force in newtons (N)

m = mass in kilograms (kg)

g = gravitational acceleration in m/s²

Steps:

1. Write down the formula

2. Substitute values

3. Give the answer AND unit

Normal Force

• In physics, normal force is the contact force exerted by a surface on an object acting perpendicular to the surface

• Eg. 1

  • Normal force on a box, resting on a table

  • If the box is not in contact with the table = no Normal force on a box

• Eg. 2

  • Normal force on a box, resting on an inclined surface

  • The Normal force prevents objects from passing through each other, acting as a support force that resists gravity

  • The Normal force has magnitude and direction

  • The magnitude of the normal force depends on the situation; for an object on a flat, horizontal surface, it is equal to the object's weight

Net Force

• Net force is the overall force acting on an object

• We can calculate the Net force by adding all individual forces

• It is important to take into account the direction of the forces when adding them together

How to draw Free Body Diagrams

• A free body diagram (FBD) is a graphical representation of all external forces acting on a single object

• How to draw FBD:

  • Show the object as a point or shape

  • Use arrows to represent forces

  • The length of the arrow indicates the magnitude

    • 1 cm = 1 N

  • The direction of the arrow indicates the force's direction

Contact force examples

• Friction

  • A force that opposes motion, or attempted motion, between surfaces that touch. It acts parallel to the surface.

• Normal force

  • A support force exerted by a surface. It acts perpendicular, at 90°, to the surface.

• Applied force

  • A push or pull exerted directly by a person or another object.

• Tension

  • A pulling force transmitted through a tight rope, string or cable.

• Drag

  • A resistive force from a fluid, such as air resistance or water resistance, acting opposite to motion.

Non-contact force examples

• Gravitational force

  • An attractive force between objects with mass. Near Earth, weight is the gravitational force pulling an object towards Earth’s centre.

• Magnetic force

  • An attractive or repulsive force between magnets, or between a magnet and certain magnetic materials.

• Electrostatic force

  • An attractive or repulsive force between electrically charged objects