5 - Forces

• What is the difference between scalar and vector quantities?

  - scalar quantities have a magnitude (size) only

  - vector quantities have both magnitude and direction
• Give 6 examples of scalar quantities?

  - mass

  - temperature

  - speed

  - energy

  - distance

  - time
• What is the difference between distance and displacement?

  - distance is a scalar quanitity so it does not give us the direction

  - displacement is a vector quantity so it tells us both the magnitude and direction (displacement is distance with a given direction, e.g. 500m due West)
• Give 6 examples of vector quantities?

  - displacement

  - weight

  - force

  - velocity

  - acceleration

  - momentum
• How can we represent vector quantities?

  - with an arrow (length of the arrow tells us the magnitude and the direction in which the arrow is pointing tells us the direction)
• What is a force?

  - a push or pull that acts on an object due to the interaction with another object

  - a vector quantity

  - measured in newtons, N
• What is the difference between contact and non-contact forces?

  - contact forces involve objects that are physically touching

  - non-contact forces involve objects that are physically separate
• Give 4 examples of a contact force.

  - friction

  - air resistance

  - tension

  - normal contact force
• Give 3 examples of a non-contact force.

  - gravitational force

  - electrostatic force (e.g. repulsion or attraction)

  - magnetic force
• What is gravity?

  - a non-contact force that pulls objects toward each other

  - a vector quantity
• What is the difference between mass and weight?

  - mass is the amount of matter in an object; a scalar quantity measured in kilograms (kg)

  - does not depend on where the object is (e.g. Eshan will still be in the 98th percentile on Mars)

  
  

  - weight is the measure of the force acting on an object due to gravity; a vector quanity with both magnitude and direction, measured in newtons (N)
• What does the weight of an object depend on?

  
  

  What is the Earth's gravitational field strength?

  - the gravitational field strength at the point where the object is (g is a measure of the force of gravity in a particular location)

  
  

  - Earth has a g of 9.8N/kg
• What is the equation for the weight of an object?

  weight (N) = mass (kg) x g (N/kg)

  
• What is the relationship between the weight and the mass of an object?

  - the weight is directly proportional to the mass of an object

  
  

  weight ∝ mass
• What device can we use to determine an object's weight?

  a calibrated spring-balance (aka newtonmeter)

  
• Where does the weight of an object act at?

  - weight acts at a single point (centre of mass)
• What is a resultant force?

  - a single, overall force that has the same effect as all of the original forces acting together
• How do we calculate the resultant force?

  larger force - smaller force
• How can we display the forces acting on an object?

  - by drawing a free-body diagram (object is a point as arrows show the forces)
• What are the 4 forces that act on an aeroplane flying at a constant velocity?

  lift ↑

  weight ↓

  
  

  ← air resistance (drag)

  → thrust
• What is the equation for work done, in joules (J)?

  
  

  What is a key thing about the distance?

  
  

  What else can work done be measured in?


  work done = force x distance

  W = Fs

  
  

  - distance must be in the line of action of the force

  
  

  - work done can also be measured in newton-meters (Nm)
• What does work done against the frictional forces acting on an object cause?

  - a rise in temeprature of the object
• What are elastic materials? Why are they different to inelastic materials?

  - materials that will always return to their original length or shape if we remove the forces acting on them

  - inelastic materials do not return to their original length when the forces are removed
• Give 6 examples of elastic materials.

  - slinky

  - rubber bands

  - rubber gloves

  - eraser

  - playground surface

  - tennis ball
• Give an example of an inelastic material.

  - certian polymers
• What is elastic deformation?

  - a temporary change in shape

  that is self-reversing after the force is removed, so that the object returns to its original shape

  
  

  - e.g. stretching, compressing, bending
• What is inelastic deformation?

  - inelastic deformation occurs when an object is stretched beyond its elastic limit

  - when the applied force is removed the object does not return to its original size/shape
• What is the equation for the force needed to stretch an elastic object?


  Force = spring constant x extension

  F (N) = k (N/m) x e (m)

  F = ke

  
• What can 'e' in the equation 'F = ke' also represent?

  - compression instead of extension
• Describe a method to investigate the relationship between force and extension for a spring.

  1) Place a heavy weight on the clamp stand to stop it from falling over.

  2) Attach a metre ruler and a spring to the clamp stand, using two clamps and two bosses.

  3) Make sure the top of the spring is at the zero point on the metre ruler.

  4) Also make sure the meter ruler is vertical.

  5) Attach a wooden splint to the bottom of the spring as a pointer, making sure this pointer is horizontal or else the readings will be inaccurate.

  6) Read this initial point on the meter ruler (unstretched length of the spring).

  7) Hang a 1N weight on the spring using a hook.

  8) Read the new position of the pointer on the meter ruler.

  9) Continue adding 1N weights to the spring and reading the position of the pointer.

  10) Calculate the extension produced by adding each weight (subtract the length of the initial unstretched spring from each reading on the meter ruler.

  11) Plot the extension (m) to the weight (N) on a graph => linear, directly proportional relationship
• What happens if we add too much weight to the spring?

  - we have exceeded the limit of proportionality
• How can we calculate the force required to extend the spring from the graph?

  F = ke

  1. Workout the spring constant of the spring by dividing the weight by the extension.

  2. Multiply the spring constant by any extension (m) to calculate the force required to extend the spring by that same number of meters.
• What is a moment?


  - a force may cause an object to rotate =>

  - a moment is the turning effect of a force
• Give 5 examples of moments.

  - wheelbarrow

  - crowbar

  - seesaw

  - crane

  - hammer being used to remove a nail
• What is the equation to calculate the moment (Nm) of a force?

  moment (Nm) = force x distance

  M = Fd

  
• What is important about the distance when calculating a moment?

  - distance has to be perpendicular from the pivot to the line of action of the force
• What is involved in a balanced object?

  - if an object is balanced, the total clockwise moment about a pivot equals the total anticlockwise moment

  
• Why do some objects topple over?

  - normally, the weight of the object acts directly down from the centre of mass

  - if we tilt the object, there comes a point where the line of action of the weight lies outside the base of the object => resultant unbalanced moment
• What are levers?


  - levers are force multipliers that allow us to lift a heavy object by applying a relatively low force
• What can a fluid be?

  - gases or liquids