Mechanics Flashcards
What is a vector?
A vector is a quantity that has both magnitude and direction
What is a scalar?
A scalar is a quantity that has only magnitude
Give examples of scalar quantities
Distance Length Speed Time Energy Power Pressure
Give examples of vector quantities
Momentum Impulse Displacement Velocity Acceleration Force Moments
What is the equation for resolving a vector horizontally?
A=Hcos(angle)
What is the equation for resolving a vector vertically?
O=Hsin(angle)
What is the definition of equilibrium?
Equilibrium is when all the forces acting on a body are balanced in each direction
When is a body in equilibrium?
There is no resultant force
Acceleration = 0
The motion of the body is unchanged
The body has a constant velocity
What is the equation used to resolve the vertical component of weight in inclined planes?
F = mgcos(angle)
What is the equation used to resolve the horizontal component of weight in inclined planes?
F=mgsin(angle)
What is a moment?
A moment of a force about any point is defined as the force * perpendicular distance from the line of action of the force to the point
What is the equation for calculating moments?
Moment = force * perpendicular distance
What are the units for moments?
Nm
What is the principle of moments?
If a body is acted upon by more than one force and it is in equilibrium, then the turning effects of the force must equal to zero
What is equation for calculating non-perpendicular moments?
Moment = Fcos(90-angle) * perpendicular distance
What is mass?
Mass is the quantity of matter which a body contains
What is inertia?
Inertia is an object’s resistance to change in velocity
What happens to inertia as mass is increased?
As mass increases, inertia increases
What is weight?
Weight is the force experienced by a mass due to a gravitational field
What is the equation for calculating the weight of an object?
W=mg
What is the centre of mass?
The centre of mass of an object is the single point that you can consider its whole weight to act through
Where is the centre of mass of a uniform, regular solid?
In the middle
How can you find the centre of mass of an object?
- Use symmetry
- Hang the object freely from a point. Draw a vertical line downwards from the point of suspension(use a plumb bo to get the line exactly vertical). Hang the object from a different point and draw a vertical line down. The centre of mass is where the two line cross.
What factors affect the stability of an object?
The centre of mass
Base area of the object
What is a stable equilibrium?
The object can be released from equilibrium and it will return to its equilibrium position
What is an unstable equilibrium?
The object when released from equilibrium, it moves away from its equilibrium position
What is a neutral equilibrium?
The centre of gravity of the system is always at the same level and it remains in equilibrium after it has been displaced
When will an object tilt?
When the line of action is outside the base area because there is a resultant moment that provides a turning force
When will an object be most stable?
When it has a low centre of mass and a wide base area
What is the equation to calculate the max force that can be applied to an object before it tilts?
Force * height = weight*(base area/2)
What is the equation for calculating the max angle of an object on a slope before it topples?
tan(angle) = (base area/2)/height
What is speed?
How fast an object is moving, regardless of the direction
What is displacement?
How far an object travels from its starting point in a given direction
What is velocity?
The rate of change of an object’s displacement
What is acceleration?
The rate of change of an object’s velocity
What is the equation for calculating the average velocity?
average velocity = total displacement / total time
What is the gradient of a displacement-time graph?
Velocity
What does the negative quadrat in a velocity=time graph mean?
It means that the object is moving in the opposite direction
What is the gradient of a velocity-time graph?
Acceleration
What is the area under a velocity-time graph?
Displacement
What is the area under an acceleration-time graph?
Change in velocity
What is an object’s motion when its acceleration = 0?
It is moving at a constant velocity
What is free fall?
Free fall is defined a the motion of an object undergoing acceleration of g.
What is the only force acting on an object in free fall?
Its weight
All objects in free fall accelerate at the …..
same rate
What is a projectile?
An object acted upon only by the force of gravity
What are the 3 principles of projectile motion?
- Acceleration is always equal to the gravitational field strength
- The horizontal velocity is constant because acceleration doesn’t have a horizontal component
- Horizontal and vertical motions are treated separately
What are the equations for calculating vertical projection?
v = u-gt s = ut-1/2gt^2
What is the equation for calculating horizontal projection?
s=ut
What is Newton’s First Law?
An object will remain at rest or keep travelling at a constant velocity unless it is acted on by an external force
What is Newton’s Second Law?
The resultant force acting on an object is directly proportional to the rate of change of momentum
Also known as acceleration is proportional to the force F= ma
What is Newton’s Third Law
If an object A exerts a force on object B, then object B exerts an equal but opposite force on object A.
What does it take for two forces to be called force pairts?
They must be: opposite in direction act on different bodies be of the same type equal in size
What is momentum?
Momentum is the product of the mass and velocity of an object
What is the equation for momentum?
Momentum = mass * velocity
What are the units for momentum?
kgm/s
What is 1 Newton?
I Newton is the force required to accelerate 1kg at 1ms^-2
Why would momentum be changing?
Momentum could be changing because either:
- the mass is changing OR
- the velocity is changing
What are the two types of collisions?
Elastic
Inelastic
What happens in an elastic collision?
Momentum and kinetic energy are both conserved
What happens in an inelastic collision?
Momentum is conserved but kinetic energy is transferred into other forms of energy during the collision
What is impulse?
Impulse is the change in momentum
What is the area under a force-time graph?
Impulse
What happens to the impact force when the impact time reduces?
As the impact time decreases, the impact force increases
Give an application example of how impact time is increased
Vehicle safety features
List vehicle safety features used to reduce the impact forces and increase the impact time
Crumple zones
Seat belts
Air bags
What is friction?
A force that oppose motion
What is dry friction?
Dry friction occurs between solid surfaces
What is fluid friction?
A force that depends on the viscosity of the fluid(liquid or gas) and the shape of the object moving through the fluid
When does lift occur?
Lift occurs when the object’s shape causes the fluid flow to change direction
Lift is …… to fluid flow
Perpendicular
What is terminal speed?
The maximum speed reached by an object when the frictional force is equal to the driving force
What are the conditions to reach terminal velocity?
A constant driving force
An increasing frictional force
What are the three main stages to reach terminal velocity?
- An object accelerates from rest with a constant driving force
- As the speed increases, so do the frictional forces. The resultant force on the object decreases and the acceleration decreases
- Eventually, the frictional forces will be equal to the driving force. There is no resultant force and no acceleration. Therefore, the object continues at a constant speed.
How can a vehicle’s max speed be increases?
Increase the driving force( increase the engine size)
Reduce the frictional forces(make the vehicle more streamlined)
How can a vehicle’s max speed be increases?
Increase the driving force( increase the engine size)
Reduce the frictional forces(make the vehicle more streamlined)
What is work?
Work is the amount of energy transferred from one form to another
What is work done?
Work done is equal to the energy transferred from one form to another
What is the equation for calculating work done?
Work done = Force * distance travelled in the direction of the force
What is 1 Joule?
I Joule is the work done when a force of 1N moves an object through a distance of 1 metre
What is thearea under a force-displacement graph?
Work done
What is power?
Power is the rate at which work is done(amount of energy transferred per second)
What is 1 Watt?
1 Watt is defined as the rate of energy transfer equal to 1 Joule per second
What is the principle of conservation of energy?
Energy cannot be created or destroyed. Energy can be transferred from one form to another but the total amount of energy in a closed system will not change
