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Chapter 8 Newton's laws of motion (8.1 (Force and Motion (Use dynamics…
Chapter 8
Newton's laws of motion
8.1
Motion without force
Friction
- force opposing the motion of a surface that moves or tries to move across another surface
Newton's 1st law
Objects either stay at rest or move with constant velocity unless acted on by another force
Object moving at constant velocity is either
Acted on by no forces
all forces acting on it are balanced
Force and Motion
Use dynamics trolley and a motion sensor connected to a computer- computer processes signal from motion sensor and displays graph velocity change against time
Trolley pulled along sloped runway using an elastic band- runway sloped to compensate for friction
Velocity-time graph should be straight line through origin showing constant acceleration
Newton's 2nd law
- F=ma - the amount of force that will give an object of 1kg an acceleration of 1ms-2
Weight
Object in equilibrium- support force is equal and opposite to weight
g= gravitational field strength
Mass- measure of inertia- resistance to change of motion
Scale on balances are calibrated for convenience in g or kg
8.2
Two forces in opposite directions
Object acted on by two unequal forces in opposite directions- moves in direction of larger force
Towing a trailer
Car pulls trailer forwards- trailer pulls car back
Resultant of car=
F-T=Ma
Force of trailer due to tension-
T=ma
Combine equations=
F=(M+m)a
Rocket
T=Thrust
Resultant when a is +ve = T-mg
T-mg=ma
Lift
3 scenarios
a=0 stationary or constant velocity
T=mg
a>0 accelerating up or decelerating down
T>mg
a<0 decelerating up or accelerating down
T<mg
T-mg=ma
Pulleys
2 masses m and M (M>m) on pulley
Resultant force on M, Mg-T=Ma
Resultant force on m, T-mg=ma
Total= Mg-mg= (M+m)a
Block on a slope
Component of weight down slope= mgSinθ
Resultant-
ma=mgsinθ-Fr
8.3
Objects moving through fluid experience a drag force- depends on:
Shape of object
Speed
Viscosity of fluid
Faster it travels the greater drag
Motion of an object falling in a fluid
Speed increases as it falls so drag increases
Resultant force is difference between gravity and drag
As drag increases resultant force decreases
At terminal speed- drag force is equal and opposite to weight
Initial acceleration= g as speed= 0 so drag=0
At terminal speed potential energy of an object is transferred as it falls, into internal energy of the fluid by the drag force
Motion of a powered vehicle
Top speed depends on engine power and vehicle shape
If motive force=FE and resistive force= FR then resultant= FE-FR
Max. speed= resistive force is equal and opposite to engine force and a=0
8.5
Measuring impacts
Can express an acceleration or deceleration in terms of
g
- acceleration due to gravity- makes force easily relatable to weight
e.g.
Vehicle hits wall- acceleration of -30ms-2= -3
g
so impact force must be 3x its weight
Contact time and impact time
Contact time
If object collide ad bounce off they are in contact for equal time
Shorter contact time= greater impact force
If objects don't separate they exert a force on one another until they're moving at the same velocity
Impact time
Duration of impact force
If mass of vehicle is known you can calc. impact force as well as time
8.4
Stopping distances
Braking
Distance travelled in time taken to stop after brakes 1st applied
Stopping
Thinking + Braking
Thinking
Distance travelled in time for a driver to react
Affected by distractions drugs and alcohol
Average Speeds
30mph= 30ft TD=45ft BD
50mph= 50ft TD=125ft BD
70mph= 70ft TD= 245ft BD
Practical: Testing friction
Measure friction between block and surface
Pull block with increasing force until it slides
Friction= to pull on the block just before sliding occurs
Change mass of block to see how it affects friction
