SP2: Forces and Motion Flashcards
How do you calculate the resultant force?
- If the two forces are in the same direction, you add them and state their direction
- if the two forces are in opposite directions, subtract the smaller force from the bigger one and state the direction of the bigger one
When are the forces on an object balanced?
When the resultant force is 0
What is Newton’s first law?
An object remains in the same state of motion unless acted upon by a resultant force
What does it mean if the resultant force on an object is 0?
- stationary objects remain stationary
- a moving object continues to move at the same velocity
What is Newton’s Second Law?
Force = mass x acceleration
What does Newton’s Second Law show?
It shows the acceleration of an object is:
- proportional to the resultant force of the object
- inversely proportional to the mass of the object
What is inertial mass?
How difficult it is to change the velocity of the object. It is defined as the ratio of force over acceleration
Define mass
The quantity of matter in an object
What is used to measure force?
A forcemeter
What are the units of force?
Newtons
What is weight
The measure of the pull of gravity
How do we calculate weight?
mass x gravitational field strength
What is the unit for gravitational field strength on Earth?
N/kg
What is the quantity of gravitational field strength on Earth?
9.8 N/kg
What does the acceleration of the resultant force depend on?
- the size of the force (for the same mass, the bigger the force, the larger the acceleration)
- the mass of the object ( for the same force, the more massive the object, the smaller the acceleration)
What is Newton’s third law?
Whenever two objects interact, they exert an equal and opposite force onto each other
What do the forces have to do for Newton’s third law to prove true?
- act on two different types of objects
- are the same type of force (eg contact force)
What are action-reaction forces?
A pair of forces that:
- act on two different bodies
- forces exerted are of equal size
- act in opposite directions
- same size
- the same type of force (contact/ non contact)
- Act in the same line of action
What is the equation to measure inertial mass?
force/ acceleration
Define normal contact forces
The force that surfaces exert to prevent solids from passing through each other. This only happens on contact
Define momentum
The tendency of an object to keep moving, or how hard it is for it to stop moving
What are the two factors momentum is dependant on?
mass and velocity
What is the equation for momentum?
mass x velocity
What are the units for momentum?
kg m/s
What is the conservation of momentum?
The total momentum before a collision is the same as the total momentum after the collision
When is the conservation of momentum possible?
In a closed system
How is momentum conserved?
In collisions and explosions
Using Newton’s third law, describe the collision of two marbles and the effect they will have on each other
- two marbles will collide
- the force exerted by marble a onto marble b will be the same as that exerted by marble b onto marble a
How can we combine Newton’s second law with momentum?
force = (change in momentum)/ time
Define stopping distances
Sum of breaking and thinking distance
Define breaking distance
Distance the driver travels when the breaks have been applied
Define thinking distance
Distance driver travels when acknowledging the situation
What is a reaction time?
Time between a person detecting a stimulus and their reaction
What factors affect reaction times?
- distractions
- fatigue
- illness
- consumption of drugs or alcohol
How can we measure someone’s reaction time?
Using the ruler drop test
Describe the ruler drop test
Dropping a ruler between someone’s thumb and forefinger, with the 0 cm facing downwards. The higher the reaction time needed to grasp the falling ruler, the further the ruler falls before being stopped.
Give some examples of factors affecting thinking distance
- speed of car
- tiredness
- drugs and alcohol
- medication
What can increase the braking distance of a car?
- poor road and weather conditions
- poor vehicle conditions, eg: worn tires or brakes, which reduces the friction between the tire and brake
- more mass in the vehicle, the brakes have to travel a large distance to remove the large kinetic energy
Define work done
The amount of energy it takes to do a task
What are the units for work done?
Joules (j)
How are speed and braking distance related?
The greater the speed, the greater the braking distance
What is the danger of a large deceleration?
The force can cause injury
Describe a large deceleration during a crash in terms of momentum
- Before the crash, you have a large momentum, due to a large velocity
- After you crash, you are stagnant and therefore have no momentum
- so a greater force is felt
What is the equation for work done?
force x distance
What safety features are present in cars to reduce the effects of a large force during a crash?
- seat belts
- traction control
- anti-block brakes
- crumple zones
- electric windows
- safety cage
- paddle shift controls
- air bags
What are active safety features?
These actively occur at a car crash, such as seatbelts, air bags and crumple zones
How do active safety features work?
They change shape during the crash to absorb all the energy. This:
- reduces injuries
- reduces the rate of change of momentum
- force exerted from the driver decreases
What is work done in a vehicle related to?
The initial kinetic energy of the vehicle
What is the average mass of a car?
1500kg
Define large deceleration
Slowing down or a negative acceleration
What are the effects of a large deceleration?
- cause the brakes to overheat
- driver may lose control of the brakes
