05 Forces - 3rd Form Flashcards
What is the difference between the force upthrust and reaction force?
Up thrust is created when an object displaces a fluid, such as a boat displacing water.
Reaction force is the upward force on a block which is resting on a solid surface.
What is the SI unit for force?
newton or N
What type of force is created between two magnets?
magnetic force
This force can be attractive or repulsive
If a person has a mass of 100 kg on Earth. What will their weight be?
The gravitational field strength of the Earth is 10 N/kg
W=mg
W = 100 kg x 10 N/kg
W = 1000 N
Masses are attracted to each other. Planets are attracted to each other. Name the force acting between them.
Gravitational force
If an object has a 50 N force acting North and a 50N force acting south, what is the resultant force?
50 N - 50 N = 0 N
F= ma
Make acceleration the subject of the equation
a = F/m
Which of the following quantities are vectors?
mass
acceleration
force
speed
velocity
time
acceleration
force
velocity
What type of force is reduced if an object is streamlined?
air resistance
drag
Rockets can push gas out and downward and the rocket moves upward. What is the name of this upward force on the rocket?
Thrust
What can forces do to an object?
- change the speed of the object (accelerate or decelerate)
- change the direction of the object
- change the shape of the object
If a book is a rest on a table, what two forces are acting on it.
What can be done to reduce friction when an object is sliding on a surface?
- use oil or grease on the surfaces
- make the surfaces smoother
- blow air between the surfaces
What type of force acts upward on the wing of an aeroplane?
Lift
In the equation W=mg, what is g (gravitational field strength or gravity) measure in?
gravity g is measure in N/kg
It is the force which acts on every kg of mass
F= ma
Make mass the subject of the equation
m = F/a
What is used to measure force?
A force meter
Name three scalar quantites
time
mass
speed
distance
energy
What is the equation which links weight, mass and gravitational field strength?
Weight = mass x gravitational field strenght
W = mg
What is Newton’s second law?
F = ma
An object will accelerate or decelerate if an unbalanced force is acting on it.
F is the resultant force
What force is created in the rope of a swing?
tension
This upward force on an object is equal the weight of the fluid (water) the object displaces. Name the force.
Upthrust
What type of force is created between two charged particles?
electrostatic force
This can be attractive or repulsive
What is the difference between a vector and scalar quantity?
scalar quantities have magnitude only
vector quantites have both magnitude and direction
If a person has a mass of 100 kg on Earth. What will their weight be?
The gravitational field strength of the Earth is 10 N/kg
W=mg
W = 100 kg x 10 N/kg
W = 1000 N
How is mass different from weight?
Mass is the amount of matter an object is made of
(measured in kg or g)
Weight is the gravitational pull on the object which depend on the gravitational field strength of the planet
(measured in Newtons or N)
Why does the moon have less gravity (or gravitational field strength) than the Earth?
The moon has less mass than the Earth
If a person has a mass of 100 kg on Earth. What will their mass be on the moon?
The gravitational field strength of the moon is 1/6th that of the Earth
100 kg
Mass the amount of matter an object is made of. It does not change when on another planet
What is the relationship between force, mass and acceleration?
Force = mass x acceleration
F = ma
This is called Newton’s second law
F= ma
Make mass the subject of the equation
m = F/a
What type of force is created between two magnets?
magnetic force
I am a type of force that is created from a fluid being displaced.
What force am I?
Up thrust
Why are arrows perfect for drawing forces (a vector) quantities?
The arrow size can represent the magnitude of the force. The arrow direction can represent the direction the force is acting in.
If spring A is a single spring, spring B must be..
1) stiffer spring
2) weaker spring
Answer: 2) weaker spring
B has a smaller gradient
It requires less force to stretch by the same amount.
Below is a velocity-time graph for a parachute jump.
What portion/s of the graph represents the constant speed? How do you know?
portion c and e
The velocity is constant. The graph is horizontal at c and e
gradient is zero = zero acceleration or constant speed
The parachutist has reached terminal velocity at c and again at e (after he opens his parachute)
Below is a force extension graph for a wire. Does the wire obey Hook’e Law?
The wire obeys Hooke’s Law during the straight part of the graph. If the force were removed, the wire behaves elastically and would go back to its original length.
For larger forces the wire no longer obeys Hooke’s Law, the wire behaves plastically and will not go back to its original length.
Define thinking distance
Thinking distance is the distance a car travel after you see the hazard to the point you put your foot on the brake - the distance travelled while you are reacting to the hazard
Below is a velocity-time graph for a parachute jump.
What portion/s of the graph represents the greatest deceleration?
portion d
The velocity is decreasing at d and the gradient is very steep representing a very large deceleration.
If one spring needs 5 N to stretch 10 cm. How far would this spring system stretch with 5 N pulling on it.
There are three springs in the series system. Each spring will stretch 10 cm. The total stretch of the series spring system would be 30 cm.
What factors affect thinking distance of a vehicle?
tiredness
drinking or drug taking
speed of vehicle
distractions such as mobile phone
Define braking distance
The distance a vehicle travels while the brakes are applied and the vehicle comes to rest.
How do you find the stopping distance from thinking distance and braking distance?
stopping distance = thinking distance + braking distance
If the speed of a vehicle doubles, what happens to the braking distance?
As speed double the braking distance more than doubles- it actually quadruples!
If the single spring needs 2 N to stretch 10 cm, how many centimetres will the series spring system stretch with the same force?
There are two springs in series with 2 N hanging on them. They will both stretch by 10 cm.
The overall stretch will be 20 cm.
Which spring is stiffer or has the greater spring constant?
Spring A needs more force to stretch the same distance as spring B.
Gradient for spring A is greater.
What factors affect braking distance of a vehicle?
tread depth of tyres
icy or wet roads
mass of vehicle
condition of brakes
speed of vehicle
Which spring is stiffer or has the greater spring constant?
Spring A needs more force to stretch the same distance as spring B.
Gradient for spring A is greater.
Below is a force vs length graph for a spring.
a) What is the original length of the spring?
b) What is the extension at 4 N?
Read the length at 0 N
Therefore the original length is 17.5 cm
extension = length - original length.
extension = 40.0 - 17.5 = 22.5 cm
If the speed of a vehicle is doubled, what happens to the thinking distance.
As speed doubles, thinking distance doubles- you cover double the distance in the same reaction time.
What equipment is needed to study how the force on a spring affects its extension?
How would this be carried out accurately?
- read the length of the spring at eye level to avoid a parallax error
- allow spring to stop bouncing up and down before taking reading.
- repeat readings AND take an average by taking readings as the spring is loaded then again as it is unloaded- do not go beyond its elastic limit.
Below is a force-extension graph for a spring.
(a) In which portion of the graph does the spring obey Hooke’s Law?
(b) Where is the elastic limit (E) of the spring- where the spring would be permanently deformed?
The straight region of the graph shows that the force is proportional to the extention. Therfore, it obeys Hooke’s Law
If the spring is stretched beyond the elastic limit it would no longer go back to its original length.
How would you investigate how the force on a wire affects its extension?
- wire must be over 1 metre
- attach tape to wire with a pen line- read length at eye level
- add mass carefully to wire without pulling on it
If the single spring needs 6 N to stretch 20 cm, how much force must hang on the parallel spring system to stretch it by the same amount (20 cm)?
There are two springs in parallel. Each spring needs 6N to stretch 20 cm. The overall force needed is 12 N to stretch the parallel system 20 cm
Below is a velocity-time graph for a parachute jump.
Describe what happens to the
acceleration from a to c
- The gradient of the graph is decreasing.
- gradient of a v-t graph is the acceleration
- acceleration is decreasing
If a spring stretches by x centimeters with 1N hanging on it. How far will it stretch with 2N?
The spring will stretch 2x centimetres.
- Springs obey Hooke’s Law.*
- If force is doubled, extension doubles.*
- Force is proportional to extension*
Below is a velocity-time graph for a parachute jump.
Explain the change in
acceleration from a to c
- at the start- only downward weight is acting on the person. Large resultant force downward. F=ma, large force –> more acceleration
- as speed increases at b, air resistance increases. Resultant force is smaller, F=ma, smaller force, less acceleration
- at c- upward air resistance = downward weight. Forces are balanced. Resultant force = zero, F=ma, no resultant force, no acceleration- constant speed and terminal velocity has been reached
Does a rubber band obey Hooke’s Law?
How do you know?
The force extension graph for the rubber band is not a straight line through the origin.
Force is not proportional to extension
It does not obey Hooke’s Law
The graph below is the force-extension graph for a spring. Explain how you know that the spring obeys Hooke’s Law.
The force-extension graph is a straight line through the origin.
As the force doubles the extension doubles
Force is proportional to extension
