Mechanics and Materials (3): Newton's Laws of Motion Flashcards
State Newtons’ first law of motion
Object’s either stay at rest (acted on by no forces) or move with a constant velocity (forces acting on the object are in equilibrium) unless acted on by a force
What is the equation that relates force, mass and acceleration?
F = ma
What are the units for force?
Newtons, N
What are the SI units for force?
kgms^-2
What is Newtons second law of motion?
F = ma for a constant mass
What is the equation for the weight of an object?
W = mg
State Newtons’ third law of motion?
If object A exerts a force on object B, then object B will exert an equal and opposite force of the same type on object A
What is the definition of inertia?
The resistance of an object to changes in motion (affected by the mass of the object) e.g it requires a stronger force to give a heavier object a certain acceleration than to give an object with less mass that same acceleration
What is the resultant force on a car of mass, M, pulling a trailer with a mass, m, with tension in the tow bar being T?
F - T = Ma
What is the force on a trailer of mass, m, be pulled by a car with a mass, M, with tension in the tow bar being T?
T = ma
What is the equation for engine thrust, T of a rocket with a mass, m, and an acceleration upwards?
T - mg = ma
T = ma + mg
What is the general equation for tension, T, in a lift cable when taking: upwards as positive and m as the total mass of the lift and its occupants?
T - mg = ma
where m is the total mass of the lift and its occupants
What is the equation for tension, T in a lift when it is moving at a constant velocity?
T = mg
What is the equation for tension, T in a lift when it is moving up and accelerating?
T = mg + ma
ma > mg
What is the equation for tension, T in a lift when it is moving up and decelerating?
T = mg + ma
ma < mg
What is the equation for tension, T in a lift when it is moving down and accelerating?
T = mg + ma
(if taking downwards as negative)
What is the equation for tension, T in a lift when it is moving down and decelerating?
T = mg - ma
(of taking down as negative, therefore, a will be +ve)
The tension in a cable of a lift is less when…?
- The lift is moving up and decelerating
- The lift is moving down and accelerating
The tension is a cable of a lift is more when…?
- The lift is moving up and accelerating
- The life is moving down and decelerating
What force does any object moving in a fluid experience?
A drag force
What does the drag force acting on a object in a fluid depend on?
- The shape of the object
- The speed of the object
- The viscosity of the fluid (a measure of hos easily the fluid flows past a surface)
More viscous fluids flow…?
More slowly
Less viscous fluids flow…?
Less slowly
Give a standard response to a question about the terminal velocity of a falling
- Initially, only weight acts on the the object, therefore the object experiences a large resultant force acting downwards and so accelerates downwards
- As the velocity of the object increases the drag force on the object acting upwards increases
- This means that the resultant force downwards decreases and so the object acceleration decreases
- ## Terminal velocity is obtained when the drag force acting upwards on the object is equal and opposite to the force of weight acting downward on the object