3.2 Forces in action Flashcards
What is the equation for net force?
Net force = mass x acceleration (F = ma)
What is the definition of 1 Newton?
The force needed to make a mass of 1 kg accelerate by 1 ms^-1.
What is the equation for weight?
Weight = mass x acceleration of free fall (W = mg)
What are the four fundamental forces?
Gravitational forces, electromagnetic forces, strong force, weak force.
What is Newton’s First Law?
An object will travel at the same velocity unless a net force acts on it.
What is Newton’s Second Law?
Force is proportional to the rate of change of momentum.
What is Newton’s Third Law?
If object A exerts a force, F, onto object B, object B exerts a force, -F, onto object A (equal in magnitude and opposite in direction).
What are features of dynamics?
Net F = ma, in motion.
What are features of statics?
Net F = 0 (also at constant velocity), stationary.
What is tension?
Force in string, cable, rope, etc which acts along it.
What is normal reaction force?
Force that acts perpendicular to the point of contact of a body and the surface with which it is in contact.
What is friction?
Force that occurs between two surfaces in contact with each other, caused by inter-atomic and inter-molecular forces, acts in the opposite direction to the direction the object is moving in or trying to move in.
What is upthrust?
Force that a fluid exerts on a body floating in it due to the water displaced, equal to the weight of the fluid displaced, always acts up.
What is drag?
A combination of friction and air resistance (or water resistance).
What does a plateau in a velocity time graph mean?
The object has reached its terminal velocity.
What is centre of mass?
The point at which the mass appears to act.
What is the centre of gravity?
The point at which the weight appears to act.
What is the moment of a force?
The product of a force and the perpendicular distance of its line of action from the point or axis.
What is the equation for the moment of a force?
Moment = force x perpendicular distance
What are moments measured in?
Newton-metres (Nm)
What is the principle of moments?
When a body is in equilibrium, there is no net moment.
What are the two conditions of equilibrium?
No net force and no net moment.
What is the equation for pressure?
Pressure = force / area (P = F/A)
What is the equation for pressure in a column of liquid?
Pressure = density x gravitational field strength x height of column (P = ρxgxh)
What is the equation for density?
Density = mass / volume (ρ = m/V)
How do you calculate density?
Find mass using a balance, find volume by LxWxH or measuring diameter and using 4/3πr^3 for a sphere, or using a water displacement can for irregular objects.
What is the definition of density?
Mass per unit volume.
What is a resultant force?
A single force which has the same effect as the sum of all forces acting on a body.
What is the weak interaction force responsible for?
The process of radioactive decay.
What is the strong interaction force responsible for?
Holding subatomic particles together in the nucleus of atoms.
What are three examples of non-contact forces?
Gravitational, magnetic, and electrostatic forces.
What are five examples of contact forces?
Tension, normal contact force, upthrust, friction and drag.
What two upwards forces will a body falling through any liquid experience?
Drag and upthrust.
What should a free body diagram contain?
The body on which the forces act, the direction of application of each force, the type of each force, the size of each force.
What do most frictional forces not depend on?
The velocity of the object.
What frictional force does depend on velocity?
The frictional force of drag that an object experiences when moving through a fluid.
How can the force of drag be calculated?
Force of drag = 0.5 x cross-sectional area of the moving object x the density of the fluid x the coefficient of drag x (the velocity of the moving object)^2 (Fd = 1/2 x ρ x Cd x A x(v)^2
What is the force of drag directly proportional to?
Velocity squared.
How does a falling object reach terminal velocity?
Initially, the downwards gravitational force is greater than the resistive forces, so the object accelerated, causing the drag on it to increase, eventually drag becomes equal to weight and the resultant force on the object is zero, so falls at a constant velocity (terminal velocity).
What are the two ways the resultant of two forces acting on a body can be worked out?
By scale drawing, by calculation from a right angled vector triangle.
What is torque due to a couple?
the product of one of the forces and the perpendicular distance between them.
When does a couple occur?
When two equal anti-parallel (parallel but acting in opposite directions) forces act to produce a rotation, no linear motion occurs.
