Forces - Definitions

• Acceleration: The rate of change of velocity
• Atmosphere: The thin layer of air surrounding the Earth, which gets less dense with increasing altitude
• Braking Distance: The distance a vehicle travels under the braking force
• Centre of Mass: The single point through which the weight of the object can be said to act
• Changes of Momentum: When a force acts on a moving object, a change of momentum will occur
• The force is equal to the rate of change of momentum
• Contact Forces: A force that occurs when objects are physically touching
• Displacement: A measure of how far an object moves in a given direction
• Distance: A measure of how far an object moves, that does not depend on direction and is therefore a scalar quantity
• Elastic Potential Energy: The energy stored in a spring when it is stretched or compressed
• Equilibrium: An object is in equilibrium if the resultant force and resultant moment are both equal to zero
• The upthrust acting on the object is greater than its weight
• Fluid: A liquid or gas
• Forces: A push or pull that an object experiences due to the interaction with another object
• Force is a vector quantity
• Inertia: The tendency of an object to remain in its same state of uniform motion or rest
• Defined as the ratio of force over acceleration
• Moment: The turning effect of a force
• Momentum: The product of an object’s mass and velocity
• Newtonmeter: A calibrated spring-balance used to measure weight
• Newton’s First Law: If a stationary object’s resultant force is zero, the object will remain stationary
• If a moving object’s resultant force is zero, the object will continue to move at the same speed, and in the same direction
• Newton’s Second Law: An object’s acceleration is directly proportional to the force applied to it, and inversely proportional to its mass
• Newton’s Third Law: The forces that two objects exert on each other when they interact are equal and opposite
• Non-Contact Forces: A force that occurs when objects are physically separated
• Pressure in a Column: The pressure in a column of liquid is equal to the product of the liquid’s density, the height of the column and the gravitational field strength
• Resolution of Forces: All forces can be resolved into two perpendicular components that have the same effect as the single force
• Resultant Force: The single force that can replace all the individual forces acting on an object, and have the same effect
• Resultant Moment: The single moment that has the same effect as the sum of all the other clockwise and anticlockwise moments acting on an object
• Scalar Quantities: Quantities that only have a magnitude, not a direction
• Sinking: An object will sink if the volume of liquid it displaces has a lower weight than that of the object itself
• The upthrust acting on the object is lower than its weight and so there is a resultant downwards force
• Speed: A scalar quantity that is a measure of the rate of increase of distance
• Spring Constant: A measure of a spring’s stiffness
• The constant of proportionality for a spring’s extension
• The higher the spring constant, the smaller the extension is for a given force
• Stopping Distance: The sum of the thinking and braking distances
• Thinking Distance: The distance a vehicle travels during the driver’s reaction time
• This reaction time may be affected by tiredness, drugs or alcohol
• Upthrust: The upward force acting on an object in a fluid