Chapter 4 - couples, torque, density etc. Flashcards
What happens when a resultant force acts on an object
There is an acceleration
Equation for force and units for force
F = ma
Newton’s (kgms^-2)
Difference between mass and weight and equation for weight
Mass is the amount of stuff- it is constant for a given object (kg)
Weight is the force exerted by an object and is measured in Newtons
W = mg
What is a centre of mass
The centre of mass of an object is the point through which any externally applied force produces straight line motion but no rotation
Finding centre of mass
- a freely suspended object will come to rest with its centre of mass vertically below the suspension point
- this can be found by hanging a plumb line from the suspension point and drawing the line that it hangs along
- repeat for different points of suspension and the intersection of the lines is the centre of mass
Free-body diagrams
Can be used to model all of the forces acting on an object where forces are shown as vector arrows
Define Weight
The gravitational force acting on an object, through its centre of mass
Define friction
The force that arises when two surfaces rub together forming a resistive force opposite to motion
define drag
The resistive force acting on an object travelling through a fluid
Define Tension
The force within a stretched cable or rope
Define up-thrust
The upward buoyancy of acting on an object in a fluid equal to the weight of fluid displaced
Define normal contact force
The force acting when an object rests against another object acting at a 90 degree angle to the plane of contact
Main factors affecting size of drag
The speed of the object
Drag directly proportional to speed squared
The cross-sectional area of the object
What is terminal velocity
The point in an objects free fall where it’s weight is equal to drag causing there to be 0 acceleration
Drag = mg
Techniques to determine terminal velocity
How to improve it
- fill a cylinder with a viscous liquid
- release a ball bearing and start a timer simultaneously
- mark the position of the ball at regular time intervals and when the distance travelled between lines is equal it will have reached terminal velocity
Attach the ball to a pulley system with a ticker tape on the other side
Define a moment of a force
Give the equation for the moment of a force
Why might it be necessary to resolve forces
A moment is the turning effect of a force and is defined by
Moment = Fx
Where moment is in Nm
F = force (N)
x = perpendicular distance from pivot
It might be necessary to resolve forces if they aren’t acting perpendicular to the pivot
What is the principle of moments
For a body in rotational equilibrium the sum of the clockwise moments is equal to the sum of the anti-clockwise moments
What is a couple
A couple is a pair of forces that have equal magnitude and are parallel but act in opposite directions
i.e. they only produce a rotational force and no translational force
What is the torque of a couple
The torque of a couple is defined as the product of the magnitude of one of the forces multiplied by the perpendicular distance between them
What is the result if 3 forces form a triangle where the arrows are end to end
The resultant force is 0
Define density and the equation
Density is the mass per unit volume of a substance
P = m/v
Methods of determining density
For regular objects it can be measured with a ruler and weighed
For irregular objects an Archimedes can could be used or displacement where the volume of the object is equal to the water displaced
Define pressure and the equation for pressure
Pressure is the normal force exerted on a surface per unit cross sectional area
P = f/a
What is atmospheric pressure
About 100,000 pa
What is pressure of a vertical column of fluid
P = hpg
What is Archimedes principle
The upthrust exerted on a body immersed in fluid is equal to the weight of the fluid displaced
define centre of gravity
The point through which the weight (appears to) act
why does an object experience upthrust
The object experiences a greater pressure at its bottom surface compared
with its upper surface.
This difference in pressure produces an upward force (upthrust).
The upthrust is equal to the weight of the water displaced – Archimedes’
principle.
what to remember when working with F = MA and gravity
if the force acts upwards, remember to take away the weight of the object first
what is the component of weight of an object on a slope
still the vertical component, given by wsin(theta), theta is the angle between the slope and the horizontal
what is a good way to define the resultant force acting on an object
the force obtained when ‘the vector sum’ of all the forces acting on the object is taken
why can seat belts and airbags help prevent injuries in crashes
- increases time period for which change in momentum occurs, therefore decreases force
- increases surface area (airbag), therefore decreases pressure
- stops whiplash
- prevents driver hitting windshield
why can we not use F = ma for objects travelling close to the speed of light
the mass of objects travelling close to the speed of light INCREASES
