1 Forces and Motion Flashcards
distance time graphs
A constant gradient represents constant speed
The gradient of the line represents the speed:
A very steep gradient means the object is moving at a large speed
A shallow gradient means the object is moving at a small speed
A flat, horizontal line means the object is stationary (not moving)
Objects might be accelerating - this is represented by a curve
In this case, the gradient of the line will be changing
If the gradient is getting steeper, the speed is increasing (accelerating)
If the gradient is getting shallower, the speed is decreasing (decelerating)
The speed of a moving object can be calculated from the gradient of the line on a distance-time graph
speed, distance and time equation
speed = distance / time
practical: investigate the motion of everyday objects such as toy cars or tennis balls
Measure out a height of 1.0 m using the tape measure or metre ruler
Drop the object from this height, which is the distance travelled by the object
Use the stop clock to measure how long the object takes to travel this distance
Record the distance travelled and time taken
Repeat steps 2-3 three times, calculating an average time taken for the object to fall a certain distance
Repeat steps 1-4 for heights of 1.2 m, 1.4 m, 1.6 m, and 1.8 m
acceleration, time taken and change in velocity equation
acceleration = change in velocity / time taken
velocity time graphs
The gradient of the line represents the magnitude of acceleration
A steep gradient means large acceleration
A shallow gradient means small acceleration
A positive gradient (upward slope) shows increasing velocity -> acceleration
A negative gradient (downwards slope) shows decreasing velocity -> deceleration
A horizontal line means the acceleration is zero so the object is moving with a constant velocity
how to find acceleration from a velocity time graph
calculated from gradient of the line
how to find the distance traveled on a velocity time graph
find the area under the line
what does kg measure
mass
what does N/kg measure
gravitational field strength or acceleration due to gravity
what are the effects of forces between bodies
When a force acts on an object, the force can affect the object in a variety of ways
The object could:
change speed
change direction
change shape
types of forces
Gravitational (or weight) - the force between any two objects with mass (like the Earth and the Moon)
Electrostatic - the force between any two objects with charge (like a proton and an electron)
Thrust - the force pushing a vehicle (like the push from rocket engines on the shuttle)
Upthrust - the upward force on any object in a fluid (like a boat on the surface of a river)
Air resistance (or drag) - the force of friction between objects falling through the air (like a skydiver in freefall)
Compression - forces that squeeze an object (like squeezing a spring)
Tension - forces that stretch an object (like two teams in a tug-of-war)
Reaction force - the force between any two objects in contact (like the upwards force from a table on a book)
what is the difference between vectors and scalers
Scalars are quantities that have magnitude but not direction
Vectors are quantities that have both magnitude and direction
examples of vectors
displacement
force
weight
velocity
acceleration
momentum
what is a force that opposes motion
friction
Frictional forces always act in the opposite direction to the object’s motion
Friction occurs when two (or more) surfaces rub against each other
At a molecular level, both surfaces contain imperfections - i.e. they are not perfectly smooth
These imperfections push against each other
examples of scalers
distance
speed
mass
energy
temperature
force, acceleration and mass equation
force = mass x acceleration
weight, mass and gravitational field strength equation
weight = mass x gravitational field strength
what is the stopping distance
thinking distance + braking distance
what factors affect the vehicles stopping distance
speed
mass of car
condition of cars breaks
road condition
reaction time
forces acting on falling objects
Weight
Air resistance
The force of air resistance increases as the object’s speed increases
This is because the object collides with air particles as it moves through the air
The faster the object is travelling, the more collisions it has with the air particles
terminal velocity
Terminal velocity is the fastest speed that an object can reach when falling
Terminal velocity is reached when the upward and downward acting forces are balanced
The resultant force on the object reaches zero
The object no longer accelerates and a constant terminal velocity is reached
skydiver example of terminal velocity
At the instant the skydiver steps out of the plane, the support force of the plane is no longer acting on the skydiver, but they are not yet falling, so the only force exerted them is the weight force
There is a downward acting resultant force on the skydiver
The resultant force is equal to the weight force
The skydiver accelerates downward at maximum acceleration
As the skydiver begins to fall, the force of air resistance is very small because the skydiver’s speed is small
There is a downward acting resultant force on the skydiver
The resultant force is equal to the weight force minus the force of air resistance
The skydiver accelerates downward but the acceleration decreases
As the skydiver accelerates, their speed increases, so the force of air resistance increases
There is a downward acting resultant force on the skydiver
The resultant force is equal to the weight force minus the force of air resistance
The skydiver accelerates downward but the acceleration continues to decrease
As the skydiver’s acceleration decreases, their speed increases at a slower and slower rate
Eventually, the skydiver reaches a speed at which the force of air resistance is equal to the force of weight
The forces are balanced, so the resultant force is zero
The skydiver no longer accelerates and a constant velocity is reached
This is terminal velocity
equation with initial speed, final speed, distance moved and acceleration
(final speed)^2 = (initial speed)^2 + (2 x acceleration x distance moved)
how to calculate the resultant force that acts along a line
largest force - smallest force
Practical: investigate how extension varies with applied force for helical springs, metal wires and rubber bands
- set up a clamp and stand with a G-clamp holding it to the desk so the clamp doesn’t fall over
- strap a ruler to the stand so the 0 is aligned with the bottom of the spring when it is hanging
- add a 100g mass onto the spring
- record the mass (kg) and the extension of the spring (cm)
- repeat 3+4 until all masses have been added
- remove all masses and repeat whole process 3x and calculate mean for average length of extension
extension is the distance the bottom of the spring has moved from where the bottom originally was
NEED TO WEAR GOGGLES incase spring snaps
don’t stand below the weights incase they fall
Hooke’s Law
The extension of an elastic object is directly proportional to the force applied, up to the limit of proportionality
force - extension graphs
Hooke’s law on a graph is only the part where the line is straight and goes through the origin (proportional)
when the line is no longer straight then the object does not obey Hooke’s law and it has gone beyond its limit of proportionality
elastic behavior
the ability of a material to recover its original shape after the force causing the deformation has been removed
elastic deformation
where the object does return to its original shape after the force has been removed
inelastic deformation
where an object does not return to its original shape after force is removed
formula with momentum, mass and velocity
momentum = mass x velocity
formula with force, change in momentum and time taken
force = change in momentum / time taken
unit for momentum
kg m/s
principle of conservation of momentum
the total momentum before an interaction is equal to the total momentum after an interaction if no external forces are acting on the objects
the interaction can either be a collision where two objects collide with each other or an explosion where a stationary object explodes into two or more parts
how to calculate mass, velocity or momentum of objects
calculate original momentum of each object then add together to get total momentum before
as total momentum before = total momentum afterwards calculate momentum of objects after
add momentum of objects after together and this number must =momentum before
how to explain safety features using momentum
to reduce the force experienced by passengers extend the contract time as force = change in m / time so as change in momentum is constant by changing the time you can change the force.
cars have front and back crumple zone to absorb energy and extend the time taken for the impact which then also increases the time taken when the force acts which increases the time taken for the change in momentum to act reducing the force and risk of injury
airbags also deploy and are compressible so when you hit them your head compresses them so you are in contact with them for longer so the time taken is longer which decreases the force and deceleration over a longer time
seatbelts prevent you hitting the steering wheel and increases the time for you to stop so therefore decreases the force
newtons third law
for every force that acts there is an EQUAL sized reaction force of the SAME type, SAME size but OPPOSITE direction
each force acts on DIFFERENT objects
for example, i am attracted to the earth by weight and the earth is attracted to me by weight
formula with moment, force and perpendicular distance from the pivot
moment = force x perpendicular distance
so by opening a door far away from its hinge it is much easier as the distance is larger
where does the weight of an object act through
its centre of gravity
principle of moments
total moments clockwise = total moments anticlockwise in equilibrium
unit for moment
Nm newton metre
how to calculate force or distance from pivot using principle of moments
calculate the total moments clockwise and also total moments anticlockwise
if in equilibrium these will equal each other
