Dynamics and Forces Flashcards
What is Newton’s first law of motion?
Law of inertia: a body remains at rest or continues to move in a straight line with a uniform velocity unless a net external force acts on it.
What is Newton’s second law of motion?
The rate of change of momentum of a body is proportional to the net force producing it, and it takes place in the direction of this net force.
What is Newton’s third law of motion?
The forces of a action and reaction between interacting bodies are equal in magnitude, opposite in direction and act on different bodies.
What is the difference between mass and weight?
Mass is a measure of inertia in a body while weight is the effect of a gravitational field on a mass.
Relate weight, mass and acceleration of free fall.
W=mg
What is linear momentum of a body?
It is the product of its mass and velocity.
What is force?
It is the rate of change of momentum of the body acted upon (N2L)
What is impulse? (J)
Product of the force and time interval which the force acts.
The impulse of a variable force is equivalent to ____.
The impulse of a constant average force in the same time interval.
What is force?
It is the rate of change of momentum.
Acceleration and net force are always ____.
In the same direction.
Under Newton’s third law, the equal but opposite forces act on _____.
Different bodies.
Weight does not have a _____.
Horizontal component.
State the principle of conservation of linear momentum.
The total linear momentum of an isolated system(no net external force) remains constant.
Total momentum before collision = total momentum after collision
Thus, rearrange the equation to obtain the changes in momentum of each mass.
m1u1+m2u2=m1v1+m2v2
m1(v1-u1)=-m2(v2-u2)
In all collisions, momentum is conserved but kinetic energy is only conserved in _____.
Elastic collisions.
In an elastic collision, relative velocity of approach= ?
-relative velocity of separation
In elastic collisions, if the bodies have the same mass, their velocities are _____.
Exchanged upon collision.
For in elastic collisions compare the relative speeds of approach and separation.
Approach > separation assuming no explosive separation.
If two colliding bodies coalesce after impact, why is the collision in elastic?
There is zero separation. Which means the relative speed of separation is 0, which is less than approaching speed.
What is an explosive separation?
It is when two or more bodies separate due to a rapid release of energy.
Total momentum before the separation has to be 0.
What is the ration of kinetic energies of bodies undergoing explosive separation?
v1/v2 and hence m2/m1 since the momentum after the separation is equal.
How is the force derived when mass is varying but velocity is constant?
F=vδm/δt.
When is Newton’s 2nd law applied?
When there is a change in momentum ie change in mass or velocity.
What is Hooke’s law?
Within elastic limit, the magnitude of tension in an elastic body is directly proportional to its extension.
What is the equation for Hooke’s law?
T=-kx
Tension of a spring is always _____ in direction to the extension.
Opposite.
For a given elastic material, spring constant k is _____ to cross sectional area A and _____ to its natural length lo.
Directly proportional, inversely proportional.
The net force on a mass suspended from a light spring at equilibrium is _____.
Zero.
What is a force field?
If is a region of space within which force is experienced.
A uniform field is represented by _____ lines.
Parallel.
What is gravitational field strength g at a point? State the equation for gravitational force.
It is the gravitational force per unit mass acting on a small stationary test mass placed at that point.
F=mg
What is electric field strength at a point?
It is the force per unit positive charge acting on a small stationary test charge placed at that point.
F=QE
What is the magnitude of magnetic force in a conductor of length l carting a current I in a magnetic field with magnetic field strength B?
F=BIlsinθ where θ is the angle to the field.
What is the magnitude of magnetic force on a charge Q moving at velocity v at an angle θ to the magnetic field of field strength B?
F=BQvsinθ
What is pressure?
It is the force per unit area.
What is density?
Mass per unit volume.
What is the pressure p at depth h of a fluid of density ρ?
p=hρg
Derive p=hρg from p=F/A.
p=F/A=mg/A=ρVg/A=hρg
What is the actual pressure in a fluid?
Sum of pressure due to the fluid and any pressure subjected on the fluid.
What is up thrust?
It is the upward force exerted by a fluid on a body when it is partially or fully immersed in the fluid.
State Archimedes’ principle.
When a body is immersed in a fluid, it experiences and up thrust equal to in magnitude to the weight of fluid displaced.
Upthrust acts through the _____.
Center of gravity of the fluid before it was displaced and is always upwards.
How does up thrust arise?
Pressure in a fluid increases with depth so the pressure on the lower surface is greater than on the upper surfaces, resulting in a net upward force.
If an object floats while partially immersed in a fluid, weight of floating body= ?
Weight of fluid displaced.
What are the directions of frictional force and normal contact forces?
Normal force: perpendicular to Surface.
Friction: parallel to surface, resisting the motion between Teo solid surfaces.
What are viscous forces?
They are forces in a fluid that resist the relative motion of a body through the fluid.
What is the difference between friction and viscous forces?
Viscous forces are 0 when relative velocity is 0 while there is always a magnitude of static friction.
If Teo or more forces acting on a point are in equilibrium, the vector sum of all forces must be _____.
Zero.
Geometrically, when vectors representing the forces are connected head-to-tail, the final point of the last vector _____ of the first vector.
must be the initial point
What is center of gravity?
It is the point at which the whole weight of the body appears to act.
What is the moment of a force?
It is the product if the force and the perpendicular distance between the line of action and the pivot point.
What is a couple?
It is a pair of forces, equal in magnitude but opposite in direction and so fends go produce rotation only.
What is a torque?
It is the product of one of the forces and the perpendicular distance between the lines of action.
The torque is _____ of the position of the axis of rotation.
Independent.
What is the difference between torque and moment?
Moment refers to the turning effect of a force while torque refers to turning effect without translational effect.
What are the possible directions of moment and torque and their axes of rotation?
Moment: clockwise and anti clockwise.
Axes: determined using right hand grip rule.
A body is in equilibrium if _____.
Σall forces=0 and Σall moments about any point = 0.
What is the principle of moments?
Σclockwise moments=Σanti-clockwise moments about the same point.
If only three Non-parallel Coplanar forces maintain a body in equilibrium, their lines of action must be concurrent. Explain what is meant by concurrent.
They all must pass through one point.
For a body placed on a surface to be in equilibrium, the line of action of its weight must pass within the _____.
Contact area.
