Unit 4: Mechanics and materials

• Impulse: The change of momentum of an object when a force acts on it. It is equal to the product of the force acting on the object and the length of time over which it acts.
• Breaking Stress: The maximum stress that an object can withstand before failure occurs.
• Inelastic Collision: A collision in which the total kinetic energy of the system before the collision is not equal to the kinetic energy of the system after the collision.
• Newton’s Second Law: The sum of the forces acting on an object is equal to the rate of change of momentum of the object.
• Newton’s First Law: An object will remain in its current state of motion, unless acted on by a resultant force. An object requires a resultant force to be able to accelerate.
• Newton’s Third Law: Every action has an equal and opposite reaction. If an object exerts a force on another object, then the other object must exert a force back, that is opposite in direction and equal in magnitude.
• Plastic Behaviour: If a material deforms with plastic behaviour, it will not return to its original shape when the deforming forces are removed. The object will be permanently deformed.
• Scalar: A scalar quantity is one that only has a magnitude. Examples include length, mass and temperature.
• Stiffness: A measure of how difficult it is to stretch a given object.
• Tensile Strain: The ratio of an object’s extension to its original length. It is a ratio of two lengths and so has no unit.
• Conservation of Energy: Energy cannot be created or destroyed - it can only be transferred into different forms.
• Terminal Speed: The maximum speed of an object that occurs when the resistive and driving forces acting on the object are equal to each other.
• Vector: A vector quantity is one that has both a magnitude and a direction. Examples include velocity, displacement and acceleration.
• Conservation of Momentum: The total momentum of a system before an event, must be equal to the total momentum of the system after the event, assuming no external forces act.
• Elastic Behaviour: If a material deforms with elastic behaviour, it will return to its original shape when the deforming forces are removed. The object will not be permanently deformed.
• Elastic Collision: A collision in which the total kinetic energy of the system before the collision is equal to the total kinetic energy of the system after the collision.
• Elastic Limit: The force beyond which an object will no longer deform elastically, and instead deform plastically. Beyond the elastic limit, when the deforming forces are removed, the object will not return to its original shape.
• Hooke’s Law: The extension of an elastic object will be directly proportional to the force applied to it up to the object’s limit of proportionality.
• Equilibrium: For an object to be equilibrium, both the resultant force and resultant moment acting on the object must be equal to zero.