P5 - Forces

• What is a scalar?


  A quantity with magnitude but no direction
• What is a vector?

  A vector is a quantity that has both magnitude and direction.
• What is the equation for weight?

  Weight = Mass x Gravitational Field Strength

  W = m g
• Where does weight act?

  Centre of mass
• What is the equation for work done?

  Work = Force x Distance
• What is a joule?

  1 Watt per second

  The unit of energy
• Why does work done against the frictional forces acting on an object cause a rise in the temperature of the object?

  The energy is transferred to the kinetic energy store of the object, but if there is friction on the object then work is done against the frictional forces. This means there is a transfer to thermal energy and hence there is a temperature rise in the object.
• Why does there need to be more than one force applied on a stationary object to change its shape?


  Applying a single force causes movement. When a single force is applied to a stationary object, the object will move in the direction of the force. There is no change of shape, since only one force has been applied. Applying multiple forces causes change of shape.
• What is the difference between elastic deformation and inelastic deformation caused by stretching forces?

  Elastic deformation: reversible, material returns to original shape. Inelastic deformation: permanent, material does not return to original shape.
• Describe the extension of an elastic object below the limit of proportionality

  Below the limit of proportionality, force and extension are directly proportional and we call this relationship Hooke’s law. The deformation will be elastic, meaning that if the force is removed, the object will return to its original shape. However, beyond the limit of proportionality, the relationship is no longer linear.
• State hooke's law

  Force = Extension x Spring Constant

  F = k e
• State the equation for elastic potential energy

  Elastic Potential Energy = 0.5 Spring Constant x Extension squared

  E = 1/2 k e2
• A body in equilibrium must experience equal sums of clockwise and anticlockwise moments.

  Moment = Force x Distance
• A body in equilibrium experiences an equal total of clockwise and anticlockwise moments about any pivot.
• The distance, d, must be taken as the perpendicular distance from the line of action of the force to the pivot. This is because:

  • A bigger force needs to be applied far away from the pivot point for a greater moment.
  • The moment depends on the size of the force and the perpendicular distance from the pivot to the line of action of the force.
  • If the line of action of the force passes through the pivot, the moment will be zero.

  
• How do levers and gears transmit the rotational effects of forces?


  • Levers: They rely on the principle of moments to act as force multipliers. When using a lever, an input force (effort) around a pivot moves an object (load) by increasing the distance over which it acts.
  • Gears: They also transmit turning effects and multiply force. When using gears, force is exerted on the smaller wheel (load), and the rotational effect is transmitted to the larger wheel through the teeth of each gear.

  
• What is a fluid?


  Either a liquid or a gas

  Pressure in a fluid causes a force to act at right angles (normal) to the surface of its container
• State the equation for pressure

  Pressure = Force / Area

  p = F / A
• Why does pressure at a point in a fluid increase with the height of the column of fluid above?


  Pressure increases as the depth increases. The pressure in a liquid is due to the weight of the column of water above. Since the particles in a liquid are tightly packed, this pressure acts in all directions.

  
• State the equation for pressure at a point in a fluid


  Pressure = Height x Density x Gravity Field Strength
• What is up thrust?

  Buoyancy
• Why does the density of the fluid have an effect on the up thrust experienced by an object submerged in it?


  If the upthrust is less than the weight of the object, the object will sink. If the density of the object is greater than the density of the fluid, the object can never displace enough fluid to create an upthrust that will hold its weight up so it sinks.

  
• Describe the earth's atmosphere and atmospheric pressure

  The atmosphere is a thin layer (relative to the size of the Earth) of air round the Earth. The atmosphere gets less dense with increasing altitude.Air molecules colliding with a surface create atmospheric pressure. The number of air molecules (and so the weight of air) above a surface decreases as the height of the surface above ground level increases. So as height increases there is always less air above a surface than there is at a lower height. So atmospheric pressure decreases with an increase in height.
• Define distance


  Distance is how far an object moves. Distance does not involve direction. Distance is a scalar quantity.
• Define displacement


  Displacement includes both the distance an object moves, measured in a straight line from the start point to the finish point and the direction of that straight line. Displacement is a vector quantity.
• Typical speed for walking


  1.5 m/s
• Typical speed for running


  3 m/s
• Typical speed for cycling


  6 m/s
• Speed does not involve direction. Speed is a scalar quantity.The speed of a moving object is rarely constant. When people walk, run or travel in a car their speed is constantly changing.The speed at which a person can walk, run or cycle depends on many factors including: age, terrain, fitness and distance travelled.
• It is not only moving objects that have varying speed. The speed of sound and the speed of the wind also vary.A typical value for the speed of sound in air is 330 m/s.
• The velocity of an object is its speed in a given direction. Velocity is a vector quantity.Motion in a circle involves constant speed but changing velocity.
• If an object moves along a straight line, the distance travelled can be represented by a distance-time graph.The speed of an object can be calculated from the gradient of its distance-time graph.If an object is accelerating, its speed at any particular time can be determined by drawing a tangent and measuring the gradient of the distance-time graph at that time.
• State the equation for acceleration


  Acceleration = Change in velocity / time
• The acceleration of an object can be calculated from the gradient of a velocity-time graph.The distance travelled by an object (or displacement of an object) can be calculated from the area under a velocity–time graph.
• An object falling through a fluid initially accelerates due to the force of gravity. Eventually the resultant force will be zero and the object will move at its terminal velocity.
• Newton's first law of motion

  If the resultant force acting on an object is zero and:

  • the object is stationary, the object remains stationary
  • the object is moving, the object continues to move at the same speed and in the same direction. So the object continues to move at the same velocity.

  So, when a vehicle travels at a steady speed the resistive forces balance the driving force.So, the velocity (speed and/or direction) of an object will only change if a resultant force is acting on the object.
• Newton's 2cnd law of motion

  The acceleration of an object is proportional to the resultant force acting on the object, and inversely proportional to the mass of the object.

  As an equation:

  Force = acceleration x mass

  F = a m
• Inertial mass


   inertial mass is a measure of how difficult it is to change the velocity of an object

  • inertial mass is defined as the ratio of force over acceleration.
• Newton's third law

  Whenever two objects interact, the forces they exert on each other are equal and opposite.
• Stopping distance


  The stopping distance of a vehicle is the sum of the distance the vehicle travels during the driver’s reaction time (thinking distance) and the distance it travels under the braking force (braking distance). For a given braking force the greater the speed of the vehicle, the greater the stopping distance.