3 Dynamics Flashcards
State Newton’s First Law of Motion. What does it imply?
A body continues in its state of rest or uniform motion in a straight line unless a resultant external force acts on it. It implies that the state of rest or uniform motion requires no resultant force to maintain.
State Newton’s Second Law of Motion. What does it imply?
The rate of change of momentum of a body is directly proportional to the resultant external force acting on it and the direction of the change is in the direction of the force.
F = dp/dt = dmv/dt = m(dv/dt) + v(dm/dt) (which is - when mass is constant) = ma
It implies that a body with a larger momentum will require a larger force to stop it in the same time or a longer time to stop it using the same force. This is because, to stop an object, F = -mv (initial)/change in time.
State Newton’s Third Law of Motion. What constitute an action-reaction pair?
If body A exerts a force on body B then body B exerts an equal and opposite force on body A.
For an action-reaction pair, the two forces must:
(i) be equal in magnitudes
(ii) be opposite in direction
(iii) be of the same type
(iv) act on different bodies (and therefore do not cancel each other out)
Define weight. Is it an intrinsic or extrinsic property? Where are its origin and direction?
Weight is the force experienced by a mass in a gravitational field. It is an extrinsic property of a body as it does not just depend on the mass of the body but also the gravitational field strength at the point where the body is where W = mg. It acts vertically downwards and originates from the centre of gravity of a body.
What are the conditions necessary for the equilibrium of a body?
Resultant force on the body must be zero (Stems from N1L where the net force acting on a body must be zero in order to ensure that translational motion of a body remains unchanged).
Resultant torque on the body about any axis must be zero (Comes about when dealing with a body that can rotate. The net torque acting on the body must be zero such that rotational motion remains unchanged).
Define mass. Is it an intrinsic or extrinsic property?
Amount of substance in a body.
The intrinsic property of a body that resists change in motion (a measure of inertia). For the same force, when applied to a body of larger mass will cause a smaller change in motion.
What is an example of static equilibrium?
The person standing on the floor where the normal contact force is equal in magnitude and opposite in direction to her weight. (W = N)
(If a body is at rest and remains at rest, then the equilibrium is said to be static equilibrium.)
What is an example of translational equilibrium?
Plane moving horizontally with constant velocity.
(Thrust = Friction, Lift = Weight)
(If the net force acting on a body is zero, then the body is said to be in translational equilibrium.)
Define friction. Where are its origin and direction?
Force resisting the relative motion or impending motion between surfaces in contact. It originates and acts along contact surfaces, opposite to the direction of motion or impending motion of the body of interest.
Define normal contact force. Where is its origin and direction? What happens to normal contact force when objects are just touching?
The force that exists when a body is in contact with another body. It originates from the contact surface and acts perpendicular from the contact surface through the body of interest. Normal contact force becomes 0 when objects are just touching.
Define viscous/drag force. In which direction does the force act? When does the force act?
The force that resists the relative motion between the body and fluid (exists when a body is moving relative to a liquid or gas). It acts opposite to the direction of motion. Viscous force does not act when both the body and fluid are stationary relative to each other.
Define tension. Where are its origin and direction? What is the assumption made?
Tension is the force transmitted through a rope, string or wire when it is pulled by forces acting from opposite sides. It originates where the string touches the body of interest and acts over the length of the string. It pulls equally on the bodies at both ends (away from the body). The assumption made is that the string is massless and the tension along the string is constant.
Define lift. In which direction does it act?
The force exerted by the fluid flowing around the surface of an object. Lift is the component of the force that is perpendicular to the oncoming flow direction. It acts perpendicular to the surface of wings/blades.
Define momentum. What is the S.I. unit of momentum? Is momentum a scalar or velocity?
The momentum p of a body is defined as the product of its mass, m, and its velocity, v. Its S.I. unit, is kg m s^-1 or N s. Since velocity is a vector, momentum is a vector and its direction follows the direction of velocity.
How do you calculate the total momentum p of a system?
It is the vector sum of the momenta of the individual bodies.
