KS4 Effects of Forces

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Effects of Forces
Learning Objectives

Candidates should use their skills, knowledge and understanding of how science works:

  • A force or a system of forces may cause an object to rotate.
  • Students should be able to describe examples in which forces cause rotation.
  • The turning effect of a force is called the moment of the force. The size of the moment is defined by the equation: moment of a force = force × distance (M = F d), where distance, d, is the perpendicular distance from the pivot to the line of action of the force
  • If an object is balanced, the total clockwise moment about a pivot equals the total anticlockwise moment about that pivot.
  • Students should be able to calculate the size of a force, or its distance from a pivot, acting on an object that is balanced.
  • A simple lever and a simple gear system can both be used to transmit the rotational effects of forces.
  • Students should be able to explain how levers and gears transmit the rotational effects of forces.
  • Students should be able to explain qualitatively, with examples, that motion in a circle involves constant speed but changing velocity.
  • Students should be able to explain qualitatively how:
    • for circular orbits, the force of gravity can lead to changing velocity but unchanged speed
    • for a stable orbit, the radius must change if the speed changes.
  • Students should be able to:
    • give examples of the forces involved in stretching, bending or compressing an object
    • explain why, to change the shape of an object (by stretching, bending or compressing), more than one force has to be applied – this is limited to stationary objects only
    • describe the difference between elastic deformation and inelastic deformation caused by stretching forces.
  • The extension of an elastic object, such as a spring, is directly proportional to the force applied, provided that the limit of proportionality is not exceeded.
  • Force = spring constant × extension (F = k e)
  • This relationship also applies to the compression of an elastic object, where ‘e’ would be the compression of the object.
  • A force that stretches (or compresses) a spring does work and elastic potential energy is stored in the spring. Provided the spring is not inelastically deformed, the work done on the spring and the elastic potential energy stored are equal.
  • Students should be able to:
    • describe the difference between a linear and non-linear relationship between force and extension
    • calculate a spring constant in linear cases
    •  interpret data from an investigation of the relationship between force and extension
  •  calculate work done in stretching (or compressing) a spring (up to the limit of proportionality) using the equation: elastic potential energy = 0.5 × spring constant × extension2 (Ee = ½ k e2)
  • Students should be able to calculate relevant values of stored energy and energy transfers
  • A fluid can be either a liquid or a gas.
  • The pressure in fluids causes a force normal (at right angles) to any surface.
  • The pressure at the surface of a fluid can be calculated using the equation:
  • pressure = force normal to a surface/ area of that surface  (P = F/A)
  • The pressure due to a column of liquid can be calculated using the equation:
  • pressure = height of the column × density of the liquid × gravitational field strength [ p = h ρ g ] (In any calculation the value of the gravitational field strength (g) will be given.)
  • Students should be able to explain why, in a liquid, pressure at a point increases with the height of the column of liquid above that point and with the density of the liquid.
  • Students should be able to calculate the differences in pressure at different depths in a liquid.
  • A partially (or totally) submerged object experiences a greater pressure on the bottom surface than on the top surface. This creates a resultant force upwards. This force is called the upthrust.
  • Students should be able to describe the factors which influence floating and sinking.
  • 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.
  • Students should be able to:
    • describe a simple model of the Earth’s atmosphere and of atmospheric pressure
    • explain why atmospheric pressure varies with height above a surface.

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