P5: Forces

• Upthrust

  The force acting on an object in a fluid, equal to the weight of the fluid displaced by the object
• Atmospheric pressure


  The pressure exerted by the weight of the air molecules in the atmosphere on a surface
• As altitude increases

  Atmospheric pressure decreases
• Submarines make use of upthrust to sink and rise
• Distance

  A scalar quantity that measures the length of the path an object has travelled
• Displacement

  A vector quantity that measures the straight-line distance between the start and end points of an object's motion
• Speed

  A scalar quantity that measures how fast an object is moving, without regard to direction
• Velocity

  A vector quantity that measures how fast an object is moving in a given direction
• Typical everyday speeds

  • Person walking - 1.5 m/s
  • Person running - 3 m/s
  • Person sprinting - 5 m/s
  • Sound - 330 m/s
  • Train - 30 m/s
  • Plane - 250 m/s
• Calculating average speed


  Distance travelled (m) / Time taken (s)
• Acceleration

  The rate of change of velocity with respect to time
• Calculating average acceleration

  Change in velocity (m/s) / Time taken (s)
• Acceleration due to gravity (g) is approximately 9.8 m/s^2 near the Earth's surface
• Uniform acceleration equation

  v^2 = u^2 + 2as
• Uniform acceleration

  • Acceleration is constant
  • Acceleration due to gravity is an example of uniform acceleration
• Interpreting distance-time graphs

  1. Gradient of line = speed

  2. Horizontal sections = stationary

  3. Curved sections = changing speed
• Interpreting velocity-time graphs

  1. Gradient of line = acceleration

  2. Area under graph = distance travelled
• Friction always acts in the opposite direction to the motion of an object
• Drag

  The force that opposes the motion of an object through a fluid (air or water)
• Objects falling through fluids reach a terminal velocity
• Terminal velocity

  • The maximum constant speed reached by an object falling through a fluid
  • Depends on the object's shape and area
• On the Moon, where there is no air, objects fall at the same rate due to gravity alone
• Terminal velocity

  The maximum speed an object reaches when falling through a fluid
• Reaching terminal velocity

  1. Object accelerates at first due to gravity

  2. Friction builds up as speed increases

  3. Frictional force eventually equals accelerating force

  4. Object reaches maximum/terminal velocity and falls at steady speed
• Terminal velocity

  • Depends on shape and area of object
  • Depends on drag compared to weight of object
• On the Moon, where there's no air, objects fall at the same rate
• On Earth, air resistance causes objects to fall at different speeds
• Newton's First Law

  An object at rest stays at rest, and an object in motion stays in motion, unless acted upon by an unbalanced force
• Newton's Second Law

  The acceleration of an object is proportional to the net force acting on it, and inversely proportional to its mass
• Inertia

  The tendency of an object to resist changes in its state of motion
• Newton's Third Law

  For every action, there is an equal and opposite reaction
• Newton's Third Law

  • When you push on an object, it pushes back on you with an equal force
  • A book resting on the ground has its weight balanced by the normal contact force from the ground
• Investigating motion

  1. Set up a trolley with masses attached

  2. Measure the acceleration of the trolley as you change the mass or force

  3. Use Newton's Second Law to explain the results
• Stopping distance

  • Thinking distance + Braking distance
  • Affected by speed, reaction time, weather/road surface, tyre condition, brake condition
• Braking relies on friction between the brakes and wheels
• Faster vehicle speed

  More kinetic energy, so more work needed to stop, so greater braking force required
• Very large decelerations can be dangerous, causing brakes to overheat or the vehicle to skid
• Braking


  Relies on friction between the brakes and wheels
• Braking process

  1. Brake pedal pushed

  2. Brake pads pressed onto wheels

  3. Friction causes work to be done

  4. Energy transferred from kinetic energy of wheels to thermal energy of brakes

  5. Brakes increase in temperature
• Faster vehicle speed

  More kinetic energy, more work needed to stop, greater braking force needed