Section 1: Forces and Motion Flashcards
Scalar quantities
Scalars have a magnitude (size) only
e.g temp and speed
Moments definition
Moment = the turning effect of a force
Principal of moments
If the object is in equilibrium:
sum of clockwise moments = sum of anti-clockwise moments
Centre of mass
The point where the whole of the mass of the object appears to be concentrated
Vector quantities
Vectors have both magnitude and direction
e.g force and displacement
Resultant force
The sum of all forces acting
Reaction / contact force
Force created by a body as a a reaction to a force being applied
e.g a book on a table
Upthrust
Upward force on a body caused by the fluid (water or gas) being displaced around it
Gravitational force
Force due to gravitational attraction. Can act at a distance
Magnetic force
Force caused by magnetic attraction. Can act at a distance
Electrostatic force
Force caused by attraction between charges. Can act at a distance
Engine force
Forward force e.g created by a car engine
Tension
Force tending to stretch or elongate something
Lift
Upward force on a airplane
Friction
Resistant force that opposes the motion of an object
Air/water resistance / Drag
Frictional force caused by particles colliding with the body that opposes motion
Newton’s 1st law
An object will remain stationary or continue at a constant velocity, unless acted upon by a resultant force
Newton’s 2nd law
F = m x a
Force (N) = mass (kg) x acceleration (m/s2)
Newton’s 3rd law
For every action there is an equal and opposite reaction
Stopping distance
Stopping distance = Thinking + Braking distance
Factors affecting thinking distance
- age
- tiredness
- influence of alcohol/ drugs
- speed of car
What is thinking distance
Distance travelled between when the driver has seen the hazard and puts his foot on the brake
thinking distance = reaction time x speed
What is braking distance
Distance travelled when driver puts foot on brake and car stops
Factors affecting braking distance
-condition of road (icy, slippery)
-tyres
-quality of brakes
-speed of car
(if speed doubles, braking distance is 4 times greater)
Equation for average speed
Average speed = total distance / Time
m/s
Equation for acceleration
acceleration = change in velocity / time taken for change
m/s2
Equation for velocity
velocity = displacement / time
(m/s) + direction
Velocity - time graphs
(see ipad)
-gradient is acceleration (rise/run)
-area under graph is distance travelled
Area = 1/2 x base x height
distance = 1/2 x a x t2
= 1/2 x acceleration x time2
What is a force?
A push or pull
Gradient of graphs
rise/run
y/x
Displacement - time graphs
(see ipad)
- positive velocity is moving away from point
- negative velocity is moving towards point
independent and dependent variables and their axis
independent variables : x axis
dependent variables : y axis
Draw a v/t graph for a parachutist from jumping to landing
Check iPad
Equation for moments
Moments = force x perpendicular distance from the pivot
Equation relating weight, mass and gravity
Weight = mass x gravity
kg) (N) (10N/Kg
Resultant force diagrams
- Square or circle in the middle
- Find the difference between forces, use arrow to denote which direction the resultant force is acting
Describe the forces acting on a parachutist from 0 - 40 seconds
When a parachutist jumps out a plane:
- 0-2 s: The parachutist accelerates very quickly towards he ground due to his weight being greater than drag. The resultant force and velocity is very large
- 0-8 s: The parachutist accelerates slower towards the ground due to increase in drag. The acceleration and resultant force is lower
- 10-15s: The weight and the drag balances out and the resultant force is zero. This means that the parachutist has reached terminal velocity and is no longer accelerating
- 15-17s: The parachutist opens his parachute and decelerates very quickly. The air resistance increase dramatically and the weight stays the same so the upward resultant force increases. The velocity decreases.
- 17-20s: The parachutist decelerates slower and as weight and drag balance out. The resultant force and velocity is smaller. The acceleration is slower.
- 21-39s: The weight and drag balances out again and the resultant force is zero. The parachutist reaches a new terminal velocity and is no longer accelerating.
How to calculate weight?
w = mass x gravity
what do distance-time graphs show?
speed
time on x axis
distance on y axis
gradient = velocity
How is average speed calculated ?
total speed / total time
Define a moment
a turning effect of a force
How to calculate a moment
moments (N/m) = force(N) x perpendicular distance from a pivot (m)
How does a an object’s weight act?
throughout the centre of mass
What is the purpose of a counterweight of a crane
- To prevent the crane from toppling over, concrete blocks are suspended at the other end of the load arm
- Act to create a moment that opposes moment of load
Describe an experiment to find the COM of an object
- Drill a hole and hang up the object
- Hang a plumb line from the suspension point
- Mark the vertical line
- Drill another hole in a different position and draw a line where the plumb line suspends. Repeat
- The point where the lines meet is the centre of mass
Describe the changes in support on bridge as a vehicle moves from one side to the other
- When the lorry is at the middle of the bridge its weight is equally supported by column A and B
- When the lorry moves to column A then all upward force will be at column A
- When the lorry is 1/4 of the way, the column nearest to him will support 3/4 of his weight
Draw force diagrams of a vehicle on a bridge
Check on iPad
what is Hooke’s law
The extension of an elastic object (e.g spring) is directly proportional to the force applied, provided the limit of proportionality is not exceeded
Identify Hooke’s law graphically
A straight line until near the end where it starts to curve
Equipment for investigating Hooke’s law
- spring
- meter ruler
- clamps
- retort stand
- 10 g slotted masses
Define elasticity
Ability to return to original size and shape after having been deformed
How to get accurate data when investigating Hooke’s law?
Exclude anomalous results
Describe the relationship of a load v extension graph that obeys Hooke’s law
Provided the elastic limit is not reached:
- straight line graph passing through the origin
- directly proportional relationship
After the elastic limit:
- stretches more for each successive increase in force
- shape is permanently changed
Draw a diagram of apparatus used in the Hooke’s law experiment
check on iPad
What error might occur during the Hooke’s law experiment?
Random error: reading not taken at eye level
Systematic error:
using ruler with zero error
spring not in good condition, passed elastic limit already
Describe Hooke’s law experiment
- Set up apparatus
- Add 10g mass to the holder and record the spring length
- Add another 10g and record new spring length
- Take away previous spring from current to calculate the extension
- Repeat until 100g is reached
improve by: repeating several times with different springs
decrease increments by which weight is added, more precise
continue adding weight beyond 100g
Describe experiments to investigate the motion of everyday objects such as toy cars or tennis balls
- Attach one end of ticker tape to a toy car
- The ticker tape is pulled through the machine as the car moves
- Cut up the tape in length representing equal time
5 dots equal 0.1 seconds, if the tape is pulled quickly the dots are further apart, tape pulled slowly, dots closer together - Make a speed time graph
note: most school ticker tapes make 50 dots per second
