Forces - An Introduction

• Scalar and Vector Quantities

  . A scalar quantity has magnitude (size) only, e.g. number of apples

  . A vector quantity has magnitude and direction, e.g. velocity, which shows the speed and the direction of travel

  . Arrows can be used to represent vector quantities:

  - the length of the arrow shows the magnitude

  - the arrow points in the direction that the vector quantity is acting

  . Forces are vector quantities

  . The diagram shows the forces acting on a boat. The arrows indicate the direction they are acting
• Contact and Non-Contact Forces

  . A force occurs when two or more objects interact

  . Forces are either:

  - contact forces - the objects are actually touching, e.g. the tension as two people pull against one another

  - non-contact forces - the objects are not touching, e.g. the force of gravity acts even when the objects are not touching

  Contact Forces: friction, air-resistance/drag, tension, normal contact force, upthrust

  Non-Contact Forces: gravitational force, electrostatic force, magnetic force
• What is a key point to remember about force?

  A force is a vector quantity. It occurs when objects interact
• Gravity


  A force of attraction between all masses
• Gravitational field around the Earth

  The force of gravity close to Earth is due to this
• Mass


  The amount of matter an object contains, and is constant
• Weight


  The force acting on an object due to gravity
• Weight of an object

  Depends on the gravitational field strength where the object is and is directly proportional to its mass
• Weight calculation


  1. weight = mass x gravitational field strength

  2. W=mg
• Weight (W)


  Measured in newtons (N)
• Mass (m)


  Measured in kilograms (kg)
• Gravitational field strength (g)


  Measured in newtons per kilogram (N/kg)
• Resultant Forces


  . When more than one force acts on an object, these forces can be seen as a single force that has the same effect as all the forces acting together

  . This is called the resultant force
• What is a key point to remember about gravitational field strength?

  The gravitational field strength (g) on Earth is 10N/kg, so a student with a mass of 50kg has a weight of (50x10=) 500N
• What is a key point to remember about gravitational field strength?

  The gravitational field strength (g) on Earth is 10N/kg, so a student with a mass of 50kg has a weight of (50x10=) 500N
• Free body diagram


  Used to show different forces acting on an object
• Scale vector diagram


  Used to illustrate the overall effect when more than one force acts on an object
• Using scale vector diagrams


  1. Forces are added together to find a single resultant force, including both magnitude and direction

  2. Vectors are added head to tail and a resultant force arrow is drawn
• Using scale vector diagrams for diagonal forces

  1. Expressing the diagonal force as two forces at right-angles to each other can help to work out what effect the force will have

  2. The force F(R) can be broken down into F(1) and F(2)

  3. F(1) is the same length as the length of F(R) in the horizontal direction

  4. F(2) is the same length as the length of F(R) in the vertical direction

  5. F(R) is also the vector found by adding F(1) and F(2) head to tail
• What is a key point to remember about weight?

  Weight is a force that can be measured using a newtonmeter (a calibrated spring-balance). The unit of measurement is newtons (N). Weight is directly proportional to mass