Mechanics and Materials Flashcards
Forces in equilibrium, On the move, Newton's laws of motion, Force and momentum, Work, energy and power, Materials.
State two conditions for a rod supported by two wires to be in equilibrium (2)
Resultant force zero
Resultant moment about any point zero
State three vector quantities (3)
Velocity
Acceleration
Force
State three scalar quantities (3)
Speed
Distance
Mass
State the three forces that will act on a helicopter that is moving horizontally (3)
Lift
Air resistance
Weight
How can force vectors be arranged to show that an object has constant velocity? (2)
Closed triangle of vectors
Forces are in equilibrium
What is the difference between a vector quantity and a scalar quantity? (1)
Vector has a direction
Scalar does not
What is meant by centre of gravity? (1)
The point in a body where the weight of the object appears to act
Define the moment of a force (1)
Product of the force and the perpendicular distance from the line of action of the force to the point
State the principle of moments (2)
Sum of the clockwise moments about a point is equal to the sum of the anticlockwise moments for a system in equilibrium
State and explain a feature of a crane that helps prevent it from falling over when lifting heavy things (2)
Counterweight
Provides a sufficiently large moment in the opposite direction
Why would a thin card give a more accurate measurement of g compared to dropping a tennis ball? (2)
Ball’s acceleration will decrease
Air resistance affects cards less
What feature of a speed against time graph would show a constant acceleration? (1)
Constant gradient
Why does an object first accelerate after being dropped then reach a terminal velocity? (4)
Weight greater than air resistance
Therefore resultant force downwards
Air resistance increases with speed
Until resultant force is zero
Why would an apple hit the ground first if dropped with a leaf? (2)
Air resistance depends on surface area
Air resistance less on apple, therefore greater velocity
How can using an inclined ramp rather than free fall produce valid data on investigating a falling object’s motion? (2)
Freefall is too fast
No accurate method to time freefall
On a distance against time graph, how would speed at a certain time/distance be worked out? (1)
Draw a tangent work out the gradient
For impulse and force, state whether each one is a vector or a scalar (2)
Impulse - vector
Force - vector
The rate of change of momentum of a body in free fall under gravity is equal to its.. (1)
Weight
On a force against time graph, what is the area under the graph equal to? (1)
The change in momentum of the body or impulse
In an inelastic collision, which quantity is not conserved? (1)
Kinetic energy
Use Newton’s laws of motion to explain how molecules of gas exert a force on the wall of a container (3)
Force is exerted on molecule by wall
To change its momentum
Molecule must exert an equal but opposite force on wall
In accordance with Newton’s third law
Describe how a trolley moving down a sloped track can have its speed measured experimentally (3)
Place piece of card on trolley of measured length
Card obscures light gate just before trolley strikes block
Calculate speed from length card and time the laser was obscured
Define density of a material (1)
Mass divided by volume
State Hooke’s law (2)
The force needed to stretch a spring is directly proportional to the extension up to the limit of proportionality
What is meant by the elastic limit of a wire? (2)
The maximum amount that a material can be stretched and still return to its original length when the force is removed
Define a material’s Young modulus and state the units (2)
Ratio of tensile stress to tensile strain
unit: Pa
Define tensile stress (1)
Force per unit cross-sectional area
Define tensile strain (1)
Extension per unit length
A wire is stretched beyond its elastic limit, explain why when the wire is unstretched, the line is parallel to original extension but does not return to zero (2)
Plastic deformation has produced permanent extension
Gradient is the same because identical forces between bonds
A pellet hits a wooden block hung by a string and is embedded in it, the wooden block reaches a certain height. A steel block is used and the pellet rebounds. Comparing energy and momentum changes, which block will reach higher (4)
As pellet rebounds, change in momentum of pellet is greater so change in momentum of block is greater
Initial speed of steel block is greater
Initial kinetic energy of steel block is greater
Therefore the height reached by the steel block is greater than with the wooden block
Explain how and why the maximum range of an electric wheelchair on level ground is affected by: the mass of the user and the speed at which the wheelchair travels (4)
Increasing mass: Reduces range More energy used accelerating user to final speed Increasing speed: Reduces range Air resistance increases with speed
Define a couple
a pair of equal and opposite coplanar forces
On a velocity - time graph how can the acceleration be found?
Draw a target and work out the gradient
On a velocity - time graph how can the displacement be found?
Area under the graph
On an acceleration - time graph how can the velocity be found?
Area under the graph
When can the equations of motion be used
With constant acceleration
What is the initial rate of acceleration on earth? Why does it decrease?
9.8
Air resistance increases reducing the resultant force
Why does a kicked football move in a parabolic curve/projectile motion
Horizontal motion is constant speed
Vertical motion is constant acceleration downwards due to gravity
Define termina velocity
The maximum velcoity an object will fall at when the force of weight is balanced by the force of air resistance
State Newton’s first law of motion
Is there is no resultant force acting on an object it will either stay at rest or move with a constant velocity or constant speed in the same direction
State Newtons second law of motion
If there is a resultant force acting on an object it will accelerate in the direction of the larger force
State Newtons third law
If object A exerts a force on object B, object B will exert and equal force on A but in the opposite direction
State the units of momentum
kg m/s
Using ideas of momentum explain how an air bag/ seat belt/crumple zone keeps you safe.
The increase the time to stop the person.
For the same change in momentum
as smaller force can be used
Force = change in momentum/time
How can the young modulus for a material be found from a stress strain graph?
The gradient - only up to the elastic limit
Define the conservation of momentum
The principle of conservation of momentum states that, in any direction, in the absence of external forces the total momentum of a system remains constant
Total momentum before = total momentum after
Define an elastic collision
No momentum OR kinetic energy is lost during the collision. Both quantities are conserved (i.e. the same) before and after the collision
Define an inelastic collision
Momentum is conserved
Total energy is conserved but kinetic energy is lost during the collision
Define Power
Rate of change of energy, measured in watts (W)
Define a W
1 joule of energy transferred per second
Define efficiency
ratio of useful output energy to total input energy, often express as a percentage
With Newtons third law what factors do the pair of forces have in common?
same magnitude
same type of force
With Newtons third law what factors do the pair of forces have that are different?
Act in opposite directions
Act on different bodies
