Topic 15 - Forces and Matter

• How many forces are required to compress, bend or stretch an object?

  More than one; it takes multiple forces (in different directions) to deform an object.
• Explain the relationship between the force applied and the extension of an elastic object.

  The extension is directly proportional to the force applied, provided that the limit of proportionality is not exceeded.
• What is meant by an inelastic (plastic) deformation?

  ● A deformation which results in the object being permanently stretched

  ● The object doesn't return to its original shape when the force is removed
• State the equation relating force, spring constant and extension. Give appropriate units.

  Force = Spring Constant x Extension

  Force (N), Spring Constant (N/m) Extension (m)
• What type of energy is stored in a spring when it is stretched?

  elastic potential energy
• What can be replaced with in the equation for spring force?

  Compression
• Differentiate between elastic and inelastic distortion

  An elastic distortion is a temporary change. The object reverts to its original shape once the forces are removed. An inelastic distortion leads to a permanent change in shape. It is irreversible.
• How can you calculate the work done/stored in stretching a spring?

  - Energy (J) = ½ x Spring constant (N/m) x Extension² (m²)

  - E = ½kx²
• When is there a linear relationship between force and extension?

  When an object is deforming elastically; when it has not yet reached the limit of proportionality.
• When is there a nonlinear relationship between force and extension?

  When the limit of proportionality has been exceeded, and the object is undergoing plastic deformation.
• What is meant by the term 'fluid'?

  A liquid or gas
• In any fluid, at what angle do the forces due to pressure act on a given surface?

  At right angles (normal to) the surface.
• State the equation used to calculate pressure. Give appropriate units.

  Pressure (Pa) = Force (N) / Area (m²)

  P = F / A
• Why does atmospheric pressure vary with the height above the earth's surface?

  The number of air molecules decreases the higher you go. This means that the weight of air above a point decreases, meaning the pressure also decreases.
• True of false? The pressure in a fluid is determined by the fluid and atmospheric pressure.

  True
• What direction does pressure act in?

  all directions
• How does pressure in fluids increase with depth?

  As the depth increases, the mass of liquid above that depth also increases. This means that the force due to the mass increases. Since the force has increased whilst the area has remained constant, the pressure will increase.
• Why does pressure in fluids increase with density?

  As the density of a fluid increases, the number of particles in a given volume increases. Consequently the weight of the fluid is greater. This means that the force from the fluid above a certain point is larger. Since the force has increased, the pressure also increases.
• What equation shows the magnitude of pressure in liquids at different depths?

  - Pressure (Pa) = Column height (m) x density (kg/m³) x gravitational field strength (N/kg)

  - P = hρg
• Why are objects in a fluid subject to upthrust?

  - when an object is submerged in fluid, it experiences a higher pressure below than it does on top of it

  - this leads to an upwards force called upthrust
• What is the magnitude of upthrust felt by an object?

  Upthrust is equal to the weight of fluid displaced by the object.
• How can you determine if an object will sink or float?

  If the upthrust is greater than the weight of the object, it will float. If the weight is the greater force, it will sink.
• Explain why an object with a density more than that of water would never float

  Upthrust is equal to the weight of the displaced fluid. If the density of the object is high, there would not be enough volume displaced to produce an upthrust larger than the object's weight. This means that it will sink.