FORCES AND MOTION Flashcards
Definition of Displacement
The distance of an object from a fixed point in a specified direction.
Definition of Velocity
The rate of change of displacement.
Definition of Acceleration
The rate of change of velocity.
Equation linking speed, distance and time
Speed = Distance/Time
Gradient of distance time graph and curved lines
Gradient = Velocity
Curved lines = Acceleration
Definition of braking distance
The distance travelled between the brakes being applied and the vehicle coming to stop.
- Affected by road conditions and vehicle
Definition of free fall
An object is Said to be in free Fall when the only force acting on it is the force of gravity
Definition of Instantaneous speed
The exact speed of an object at a specific given point
Definition of thinking distance
Distance travelled in the time it takes for the driver to react.
- Thinking distance = initial speed x reaction time
Definition of stopping distance
The sun of thinking distance and braking distance for a driven vehicle
How is speed related to thinking distance?
Speed is directly proportional to thinking distance
v~x
How are braking distance and the initial speed of the car related?
-Braking distance is proportional to initial speed of the car squared
x~u^2
Definition of Archimedes’ Principle
The upthrust acting on an object (fully or partially) submerged in a fluid is equal to the weight of fluid it displaces
Definition of Centre of Gravity
The single point through which the object’s weight appears to act
Definition of centre of mass
The single point through which all the mass of an object can be said to act
Definition of a couple
Two equal and opposite parallel forces that act on an object through different lines of action
OR
A pair of forces that together cause a resultant moment but not a resultant force on an object
Definition of density
The mass per unit volume of a material. Density = mass/volume
Definition of Drag
Drag is the frictional force that an object experiences when moving through a fluid
Definition of equilibrium
For an object to be in equilibrium, both the resultant force and resultant moment acting on the object must be equal to zero
Newton’s First Law
An object will remain at rest or constant velocity unless acted on by a resultant force
Definition of work done
Force multiplied by distance moved in the direction of the force
W=Fx
Definition of Power
The rate at which work is done
Definition of a watt
1 watt is 1 joule per second
Efficiency equations (write both)
Efficiency=(Useful energy output/total energy input )x100
Efficiency=(Useful power output/total power input)x100
Definition of average speed
Average speed = total distance travelled/ time
Newton’s Second Law
Resultant force is directly proportional to the rate of change of momentum
Newton’s Third Law
When one body exerts a force on another, the other body exerts an equal and opposite force on the first body
The principle conservation of momentum
For a closed system, the total momentum is constant (total momentum before = total momentum after)
Definition of linear momentum
Linear momentum= mass x velocity (p=mv)
Definition of a Newton
1 Newton is the force required to cause a mass of 1kg to have an acceleration of 1ms^-2 in the direction of the force
Definition of Impulse
Impulse = Force x time for which the force acts
Area under a force -time graph
=Impulse =Change in Momentum
Definition of Impulse
Change in momentum
m(v-u)
Definition of terminal velocity
The maximum velocity of an object that occurs when the resistive and driving forces acting as on the object are equal in magnitude to each other
Definition of a moment
Moment of a force = force x perpendicular distance from pivot
Definition of torque
Torque of a couple = one of the forces x perpendicular distance between the forces
What is the principle of conservation of energy?
The total energy of a closed system remains constant.
What is the moment about P due to a force acting at Q, a distance x from P and at an angle theta to PQ?
Moment = Fx sintheta
What equation gives the resultant force on an object in terms of its mass and acceleration due to the force?
F = ma
In which direction does upthrust act?
Vertically upwards
How can you measure the volume of an irregularly shaped object?
Displacement method:
-> Fully submerge object in water, measure displaced water with a measuring cylinder
-> Volume of object = Volume of water displaced
How do you find displacement using a velocity-time graph?
Area under the line
In projectile motion with negligible air resistance describe the horizontal component of motion:
Horizontal acceleration is zero.
Horizontal velocity is constant so speed = distance / time (v = s/t)
Explain a floating object in equilibrium in terms of density:
An object of lower density than a fluid will be in equilibrium when the weight of the object is equal to the weight of fluid displaced by the object.
What is meant by the resultant force on an object?
The vector sum of all the individual forces acting on the object.
If three coplanar forces add up to zero, what can be said about their vector addition diagram?
The three forces will form a triangle.
Define the Newton
The force required to accelerate an object of mass 1kg at 1ms^-2
(1N = 1kgms^-2)
List 4 factors that increase the size of the drag force acting on an object:
-> Increased surface area of the object
-> Increased relative speed
->Increased fluid viscosity (viscosity is a measure of a fluid’s resistance to flow. HIGH VISCOSITY=LESS FREE FLOWING, LOW VISCOSITY=FREE FLOWING)
-> Less aerodynamic shape
How should you approach projectile motion problems?
Consider horizontal and vertical motion separately
In which direction does the force of weight act?
Vertically downwards
Archimedes’ Principle
The magnitude of the upthrust on an object fully or partially submerged in a fluid is equal to the weight of the fluid displaced.
What is meant by free fall?
Motion of an object when the only force acting on it is its weight, acceleration is g vertically downwards.
Give three differences between mass and weight:
Mass:
-Amount of matter
-Unit is kg
-Remains constant
Weight:
-Gravitational force
-Unit is N
-Depends on location
Explain sinking in terms of density:
An object of higher density than a fluid cannot displace a weight of fluid equal to its weight and so upthrust will be less than the weight of the object.
Describe the free body diagram for a falling object at terminal velocity:
Drag vertically upwards equal to weight vertically downwards (resultant force is zero)
What does the gradient on a velocity-time graph represent?
Acceleration
In which direction does friction act?
Opposite direction to the movement causing it
In which direction does a normal contact force act?
Perpendicular to the surface
Instantaneous Speed
Rate of change of distance at a given point in time
Give the quantities and their units for the equation p=hpg
-> p = Pressure (Pa)
-> h = Vertical depth (m)
-> p(roe) = Density (kgm^-3) of the fluid the object is submerged in
->g = Gravitational field strength (Nkg^-1)
Describe the changes in resultant force on a falling object from rest to terminal velocity
Start: Only weight
Middle: Drag increases as speed increases, resultant force decreases
End: Drag = weight, resultant force is zero
What does the gradient on a displacement-time graph represent?
Velocity
What can be said about the total moment acting on an object in equilibrium?
It is zero
(total clockwise moment = total anticlockwise moment)
What is the moment about a point due to a force acting through that point?
Zero, because moment = Fx and x=0
What is one pascal equivalent to?
1Pa = 1Nm^-2
Give the quantities and their units for the equation torque=Fd:
Torque = Moment due a couple (Nm)
F = F is the size of one of the forces (N)
d = Perpendicular distance between the forces (m)
What is a free body diagram?
A diagram with forces on an object represented by arrows giving magnitude and direction.
Under which conditions can suvat equations be used?
Uniform acceleration (constant magnitude and constant direction)
In projectile motion, what is the vertical component of the velocity at maximum height?
Zero
How do you find velocity at a point using a displacement-time graph?
Gradient (for curved lines, gradient of the tangent at that point)
What can be said about the resultant force acting on an object in equilibrium?
It is zero
Give the horizontal and vertical components of the initial velocity u for a projectile launched at an angle theta to the horizontal
Horizontal = ucostheta
Vertical = usintheta
Describe the changes in acceleration for a falling object from rest to terminal velocity
Start: Acceleration is g
Middle: Acceleration decreases as speed increases and resultant force decreases
End: Acceleration is zero as resultant force is zero
In projectile motion with negligible air resistance describe the vertical component of motion:
Vertical acceleration is g downwards, suvat equations can be used as a is g is uniform
What conditions must be met for a body to a body to be in equilibrium?
Resultant force acting on it is zero, and total moment acting on it is zero.
State the value of g
9.81 ms^-2
Give the quantities and their units for the equation moment = Fx
Moment = (Nm)
F = Force (N)
x = Perpendicular distance between line of action of force and pivot (m)
