Forces

• What is a scalar quantity?

  A quantity that only has magnitude

  Is not direction dependant
• What is a vector quantity?

  A quantity that has both magnitude and direction
• Examples of contact forces:

  Friction

  Air resistance

  Tension
• Examples of non contact forces:

  Gravity

  Electrostatic forces

  Magnetic forces
• Examples of vector quantities:

  Velocity

  Displacement

  Force
• Examples of scalar quantities:

  Time

  Mass

  Distance

  Energy

  Speed
• What is meant by an objects centre of mass?


  The single point where an objects weight can be considered to act through
• What is resultant force?

  The overall force acting on an object, taking into account both magnitude and direction
• What does it mean if a force is said to do ‘work’?

  The force causes an object to be displaced through a distance
• What occurs when work is done against frictional forces?

  A rise in temperature of the object

  Kinetic energy is converted to heat
• 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 be inelastic 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
• What can extension be replaced with in the equation for spring force?

  Compression
• What is an alternative name for the turning effect of a force?


  A moment
• What distance measurement is used when calculating a moment?

  The perpendicular distance from the pivot to the line of action of the force
• What determines the moment of a gear wheel?

  The size of the wheel
• Describe the moment and speed of a larger gear being driven by a smaller gear:

  The larger wheel will rotate more slowly but will produce a larger moment
• In any fluid, at what angle do the forces due to pressure act on a given surface?

  At right angles to the surface
• What happens to the density of the atmosphere with increasing altitude?

  The atmosphere becomes less dense as altitude increases
• Explain why atmospheric pressure decreases with an increase in height?

  > Pressure is created by collisions of air molecules

  > The quantity of molecules (and so weight) decreases as height increases

  -> This means atmospheric pressure decreases
• 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 remains constant, 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 is upthrust always equal to?

  The weight of the fluid that the object displaces
• Explain why an object with a density greater than that of water can 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 the object will sink
• What are the typical human walking, running and cycling speeds?

  Walking: 1.5 m/s

  Running: 3 m/s

  Cycling: 6 m/s
• Why can an object travelling at a constant speed in a circle not have a constant velocity?

  > Speed is a scalar quantity

  > Velocity is a vector quantity which means it can only be constant if the direction is constant

  > In a circular motion, the direction is constantly changing
• How can speed be calculated from a distance time graph?

  The speed is equal to the gradient of the graph
• What must be done to calculate speed at a given time from a distance time graph for an accelerating object?

  Drawing a tangent to the curve at the required time

  Calculating the gradient of the tangent
• How can the distance travelled by an object be calculated from a velocity time graph?

  It is equal to the area under the graph
• State Newton’s first law:

  An object at rest remains at rest, and an object in motion remains in motion at a constant speed and in a straight line unless acted in by an unbalanced force
• What is inertia?


  The tendency for the motion of an object to remain unchanged
• What is Newton’s second law?

  An objects acceleration is directly proportional to the resultant force acting on it and inversely proportional to its mass
• What is inertial mass?

  > A measure if his difficult it is to change a given object‘s velocity

  > The ratio of force over acceleration
• State Newton’s third law:

  When two objects interact, the forces that they exert are always equal and opposite
• What is stopping distance equal to?

  The sum of thinking and braking distance
• Describe the energy transfers that take place when a car applies its brakes?

  > Work is done by the friction force between the brakes and wheel

  > Kinetic energy of the wheel is converted to heat and is dissipated to the surroundings through the brake disks
• State two consequences of a vehicle undergoing very large decelerations:

  > Kinetic energy converted to heat is very high causing brakes to over heat

  > Loss of control of the vehicle
• In a closed system, what can be said aviation the momentum before and after a collision?

  The total momentum before is equal to the total momentum after
• Explain how a seatbelt improves a passenger‘s safety during a collision:

  The seatbelt stretches, increasing the time over which force is applied, reducing the rate of change of momentum

  -> Therefore reducing the force experienced