Force And Motion Flashcards
(26 cards)
Newton’s first law of motion
Objects will remain rest, or move with a constant velocity unless acted on by a resultant force
This means if the resultant for e acting on an object is 0:
-object will remain stationary
-object will continue to move at same constant velocity
Newton’s second law
The acceleration of an object is proportional to the resultant force acting on it and inversely proportional to an object’s mass
-an object will accelerate in response to a resultant force
-the bigger this resultant force, larger the acceleration
-for a given force, the greater the object’s mass, the smaller the acceleration experienced
Calculating force and acceleration
Force= mass x acceleration
F=ma
F=newtons
Mass=kg
Acceleration= m/s*2
Newton’s third law
Whenever two bodies interact, the forces they exert on each other are equal and opposite
All the forces arise in pairs-if object a exerts a force on object b, object b exerts an equal and opposite force on object a
Force pairs are of the same type-if object a exerts a gravitational force on object b, then object b exerts an equal and opposite gravitational force on object a
Inertia
The tendency of an object to continue in its state of rest, or in uniform unless accepted upon by a external force
-in other words, interia is an object’s resistance to change motion:
-if an object is at rest, it will remain at rest
-if an object is moving at constant velocity it will continue to do so
Inertia and mass
Interial mass is the property of an object which describes how difficult it is to change its velocity
Interial mass=force/acceleration
M=f/a
M=kg
F=N
A=m/s*2
This equation shows that for a given force, interial mass is inversely proportional to acceleration
-larger interial mass will experience small accelerations
-smaller interial mass will experience larger accaleration
Weight
The force acting on an object due to gravitational attraction
Because of wieght:
-object will stay firmly on ground
-object will fall to ground
-satellites are kept in orbit
Mass is related to amount of matter in an object-the more mass an object has the larger the weight force it will experience
Weight that an object experiences depends on:
-object mass
-the mass of planet attracting to it
Centre of mass
The point through which the weight of object acts
-for a symmetrical object of uniform density, the centre of mass is located at the point of symmetry
-the centre of mass in an irregular object can be found by locating its balance point
Calculating weight
Weight=mass x gravity
W=mg
Free fall:
-an object in free fall is falling solely under the influence of gravity
-on earth, all free-falling objects accelerate towards earth at rate of 9/8 m/s2
-in absence of air resistance, all bodies near earth will fall with same acceleration regardless of their mass
Mass v weight:
-an objects mass will always remain the same,however, its weight will differ depending on strength of gravitational field on different planets
USE TERMINAL VELOCITY TO HELP (p9)
Thinking and braking distance
Stopping distance-the total distance travelled during the time it takes for a car to stop in response to some emergency
Stopping distance=thinking distance + braking distance
Thinking distance-distance travelled in the time it takes the driver to react in metres
Braking distance-the distance travelled under the braking force in metres
Stopping distance-the sum of both in metres
-for a given braking force, the greater the speed of vehicle, the greater the stopping distance
Reaction time
A measure lf how much time passes between seeing something and reacting to it
-person holds a ruler 30cm vertically, such as the bottom end of ruler hovers over the top of the hand of person b
-a person should release ruler unexpectedley
-as soon as person b sees ruler move, they should close hand,catching it
-the ruler marked at the point at which it was caught by person B
-this gives a measurement of the distance the ruler fell-greater distance,longer the reaction timr
Thinking distance and factors affecting it
The distance travelled by a car from when a driver realises they need to brake to when they spply the brakes
Reaction distance= speed of car x driver’s reaction time
Thinking distance increased by:
Tiredness
Distractions
Intoxication p-alcohol or drugs
Factors affecting break distance
Braking distance is the distance travelled by a car under braking force
The main factor affecting the braking distance lf s car is its speed
Additional factors that affect braking distance:
-vehicle condition-worn tyres or brakes
-road condition-wet or icy roads making it hard er to decelerate
-vehicle mass
Breaking and friction and speed
-when a driver applies the brakes, there is a frictional force between the brakes and the wheels of car
-this frictional force does work on brakes-i.e transfers energy from car to brakes
-therefore the kinetic energy of the car decreases and the thermal energy of the brakes increase
-this means the car decelerates
The greater the speed of vehicle, the greater the braking force required to bring the vehicle to a halt for given distance
-since the braking force would need to be larger, the deceleration of vehicle will be large as well
-large decelerations could lead to breaks overheating or loss of control of vehicle
Estimating decelerating force
Braking force x braking distance= 1/2 x mass x velocity*2
Shows that:
-work done is the transfer of kinetic energy
-the braking distance is proportional to the speed squared
Calculating momentum
A moving object has momentum so p= mv
P=momentum=kg m/s
M=mass=kg
V=velocity=m/s
This means that an object at rest has no momentum
Momentum keeps an object moving in the same direction,making it difficult to change the direction of an object with a large momentum
-since velocity is a vector this means that the momentum of an object also depends on direction of travel
-momentum can be either positive or negative
Therefore the momentum of an object will change if:
-the object accelerates or decelerates
-vhanges direction
-mass changes
Conservation of momentum
In a closed system, the total momentum before an event is equal to the total momentum after the event
-a closed system means that energy within the system is constant and the absence of external forces
-in other words:
Total momentum before collision=total momentum after collision
-a system is a certain number of objects under consideration
-since the momentum is a vector,a system of objects moving in opposite directions at the same speed will have an overall momentum of 0 since they will cancel out:
Momentum id always conserved
Before collision:
-the momentum is only mass which is moving
-if the right is taken as positive direction, the total momentum of system is m x u
After collision:
-mass also has momentum
-the velocity of m is now-v and the velocity of M is now V
-the total momentum is now the momentum of M+ momentum of m
-this is (MxV) - (m x v)
Collision
Objects will either:
-collide and move in opposite-this is an elastic collision
-collide and move in same direction-inelastic collision
-when the object move in opposite directions:
Each object will have different velocity depending on its mass snd initial momentum of the system
-when the objects move in same direction together:
They will have combined mass and velocity
Kf an exam question ask you to analyse a collision:
Always consider the motion before and after the collision and state:
-velocities of object
-the direction each object moves
State wether the collision was elastic or inelastic and explain
-in a perfectly elastic collision, the kinetic energy is the same before and after
-in a perfectly inleastic collision, the two objects stick together after colliding
Describe any energy transfers that occur if kinetic energy is not conserved:
-e.g, may be concerted into Heat,sound,elastic potential energy
Changing shape
-for stationary objects,more than one force has to be applied to change their shape-stretching,bending,compressing
Compression:
-an example of compression is placing a mass on top of a spring placed on a flat surface
Two forces are:
-weight of mass and reaction force from surface of spring
Stretching:
Example is placing a mads on bottom of a vertically hanging spring
Tw forces are:
-weight of mass
-tension in spring
Bending:
-example is a diver board bending when a swimmer stands at far end
Two forces are:
-weight of swimmer
-resction force from block to diving board
Inelastic and elastic defomration
Elastic formation:
-when object returns to their original shape when stretching force is removed
-examples of materials are rubber bands,fabrics and steel springs
Inelastic for ation:
-when object remains stretched and do no return completely to their original shape even when stretching force is removed
-example is plastic,clay,glass
Hookes law
-states that th extension of an elastic object is directly proportional to force applied,up to limit of proportionality
-directly proportional means that as force is increased, extension increases:
-if force is doubled, then extension will double
-if force is halved,extension will halve
-the limit of proportionality is the point beyond which the relationship between force and extension is no longer directly proportional
Using hooke’s law
F=ke
-e can represent either extension or compression of an elastic object
-spring constant represents how stiff a spring is
Extension can be calculated by:
Final length-original length
Linear and non linear relationship in hookes law
Hooke’s law is the linear relationship between force and extension-represented by straight line
-materials that do not obey hooke law they have a non-linear relationship-represented by curve
Calculating sprint constant
K=F/e
K=n/m
F=N
e=m
-equation shows that the spring constant is equal to force per unit extension needed to extend the spring, assuming that the limit of proportionality is not reached
-stiffer the spring, greater the spring constant
