Acceleration Flashcards
What is a force and what unit are they measured in?
A force is a push or pull upon an object in a particular direction. They result from the object’s interaction with another object. Force is measured in newtons, N.
Give three examples of forces.
Contact:
Push, pull, buoyancy/upthrust, drag/air resistance, friction, reaction force, tension
Non contact:
Electrostatic, magnetic, gravitational (weight)
List the possible effects of a resultant force acting on an object.
A resultant force is a single vector representing the net (overall) effect of all of the forces acting on an object
Acceleration (change speed or direction), Change shape.
what is resultant force?
This is the size of the overall force acting on an object
When the forces are balanced the resultant force is zero.
When the forces are unbalanced the resultant force is equal to the size of the unbalanced force.
What happens to an object if the forces are balanced?
It maintains a constant velocity (doesn’t accelerate).
If it isn’t moving it will stay stationary.
If it’s already moving it will continue to move in the same direction at a constant speed.
What happens to an object if the forces are unbalanced?
It will accelerate:
Speed up
Slow down
Change direction.
Newton’s first law:
Key point:
A resultant force will cause acceleration.
This will cause an object to:
change speed
and/or change direction
what factors affect acceleration?
mass
Newton’s Second Law
This states that the acceleration of an object is:
Directly proportional to the resultant force on the object.
a ∝ F
(Double the force = double the acceleration)
Inversely proportional to the mass of the object.
a ∝ 1/m
(Double the mass = half the acceleration)
acceleration equation
acceleration = force /mass
How does a resultant force affect velocity?
If resultant force is acting in the same direction as velocity then the object will accelerate and the velocity will increase.
If resultant force is acting in the opposite direction as velocity then the object will decelerate and the velocity will decrease.
Inertia
The tendency of an object to continue on the course its on (i.e., move at a constant velocity) is called its inertia.
It will be more difficult to make an object with lots of inertia to accelerate than one with less inertia.
The inertial mass of an object is a measure of how difficult it is to change the object’s velocity.
inertia equation:
Inertial mass = force / acceleration
what is mass
The mass of an object is related to the amount of matter it contains.
Mass is not changed by gravitational forces.
Mass is measured in kilograms (kg) using a balance
what is weight
The weight of an object is the force due to gravity acting on it.
If the gravitational field changes, the weight changes. (The moon has a lower gravitational field strength so you would have less weight there!)
Scientists measure weight in newtons (N) using a newtonmeter.
what is weight equal to?
weight = mass x gravitational field strength (9.8)
what is falling due to
Falling is due to weight - the force caused by gravity acting on the mass of an object.
In the vacuum both objects only had weight acting on them.
Drag
If an object is moving through a fluid (liquid or gas) there is a drag force acting on it.
This force is caused because the object has to push the fluid particles out of the way
The fluid particles therefore push on the object in the opposite direction with the same amount of force (Newton’s third law).
3 things increase the size of the drag force:
Speed of object
Shape of object - wide flat shapes cause more drag
Viscosity of the liquid (how ‘thick’ it is)
Speed - The drag force increases as the speed increases, because the faster the object moves the more particles it hits every second.
The acceleration of the object decreases as it falls because
The drag force increases as the object speeds up.
So the resultant force on the object decreases so the rate of also acceleration decreases.
When drag force = weight…..
the resultant force is equal to zero. The object stops accelerating, it has reached terminal velocity.
The stopping distance of a car can be split into 2 parts:
Thinking distance
Braking distance
what is thinking distance
The thinking distance is the distance travelled during the reaction time of the driver.
what is braking distance
The braking distance is the distance travelled while the brakes are applied.
What factors affect reaction time?
Tiredness
Drugs / alcohol
Age
Distractions (eg mobile phone)
factors that affect thinking distance:
Initial velocity
Reaction time
what is thinking distance equal to?
Thinking distance = initial velocity x reaction time
factors that affect braking distance?
Initial velocity
Mass of vehicle
Size of braking force applied
Condition of vehicle (esp tyres, brake pads and discs)
Road surface (eg water, ice, gravel)
If the braking force applied is too big it can lead to:
Skidding and loss of control
The brakes overheating - they could even catch fire
This energy is transferred by friction to maximise we need:
Good brake pads and discs
Good road conditions
Tyres with good grip
Calculating Acceleration
a = v2 – u2/ 2s
In a closed system momentum is always ………..
In a closed system momentum is always conserved
This means the total momentum in a system before an event is equal to the total momentum after the event.
momentum equation
Momentum (p) = mass (m) x velocity (v
Closed system
Closed system - a group of objects whose total mass remains constant and that are not subject to any external forces.
If an external force acts on a system energy will be transfers to or from it and momentum will not be conserved.
