3.4 Mechanics Flashcards
Define vector and scalar.
Vector = direction and magnitude Scalar = magnitude
Give examples of vectors.
Velocity, acceleration, force.
Give examples of scalars.
Mass, density, volume, energy, distance.
What are the two methods for the addition of vectors?
Scale drawing or calculation.
How do you add vectors by scale drawing?
- Decide on suitable scale (1N=1cm)
- Draw 1st vector in correct direction and length (protracted and ruler)
- Draw 2nd vector with its tail on the nose of the 1st (correct direction and length)
- Result is found by drawing a line from the tail of 1st to nose of 2nd
Does the order you draw vectors for scale drawing matter?
Nah.
For what vectors can you calculate instead of drawing?
Vectors at right angles.
How do we calculate the addition of vectors?
Pythagoras.
How do we resolve vectors into two components?
Use SohCahToa.
What is equilibrium?
Forces are balanced, stationary or constant velocity. Resultant force = 0.
Define moment.
The force x the perpendicular distance from its line of action to the pivot
For an object in equilibrium the sum of what around a point = the sum of what around the point?
Clockwise and anti-clockwise movements.
What is a couple?
Consists of a pair of coplanar forces equal in magnitude and opposite in direction who do not act along the same line of action.
What is the moment of a couple?
Force x perpendicular distance between the lines of action of the forces.
Define centre of mass.
The point where all off the mass can be considered to be concentrated. A force applied through this point won’t cause rotation, or a resultant movement.
Define centre of gravity.
The point where all of the weight of an object can be considered to act through.
What do the COM and COG share?
The same point.
Define uniform.
Mass is evenly distributed.
How can the COM be found for uniform objects?
Using lines of symmetry.
How can the COM be found for non uniform objects?
Balance it on a knife’s edge, COM would be directly above the support.
How do you find the COM for an irregular lamina?
- Push a pin through the sheet and allow it to swing freely
- It comes to rest with the COM directly below the point of suspension
- Draw a plumb line, the COM is somewhere along this line
- Repeat from a different position
- Where lines cross is COM
v = ?
∆s/∆t
a = ?
∆v/∆t
On a s/t graph what is the gradient? What do straight lines mean?
Velocity, constant velocity.
What does it mean if an s/t graph goes into the negative quadrant? What is the x axis?
Moving in opposite direction to starting point. The starting point.
What are the two velocities you can find from a curve an s/t graph? How do we find them?
Instantaneous velocity of a point (draw tangent), average velocity (take gradient between top and bottom point).
On a v/t graph what is the gradient? What do straight lines mean?
Acceleration, constant acceleration.
What does a negative gradient mean on a v/t graph?
Deceleration.
What does it mean when a v/t graph goes into the negative quadrant?
Negative velocity moving in the opposite direction (and getting faster).
What are the two accelerations you can find from a curve on a v/t graph? How do we find them?
Instantaneous acceleration (tangent), average acceleration (top and bottom points).
What is the area of a v/t graph?
Displacement.
What happens on an s/t graph for a bouncing ball?
Starts at s, slopes down to x axis, slopes back up but does not reach original height.
What happens on a v/t graph for a bouncing ball?
Starts at x axis, straight line down into negative quadrant, vertical line up to positive quadrant, straight line (with same grad. as first) goes down reaching x axis.
What happens on an a/t graph for a bouncing ball?
Straight, horizontal line at -9.81.
If 0 air res. what happens to Vx and Vy in projectile motion? What effect does this have?
Vx is constant. Vy increases due to acceleration due to gravity. Angle to the horizontal increases.
Horizontal and vertical motion are ………..?
Independent of each other.
If there is air res. what happens to Vx and Vy in projectile motion?
Vx decreases over time (eventually reaching 0). Vy increases over time (however acceleration isn’t constant it decreases eventually reaching 0 - terminal velocity).
What is terminal velocity?
When an object moves at a steady speed in a constant direction because the resultant force acting on it is zero.
How do we change terminal velocity?
Increasing SA, eg. parachute.
What is drag?
A resistive force that acts parallel, but in the opposite direction, to the direction of motion.
What factors effect drag?
Size and shape of the object.
Properties of the fluid (density, viscosity).
Velocity of the object.
What is lift?
A force that acts perpendicular to the direction of motion. When an object moves through a fluid.
What three factors effect lift?
Size and shape of the object.
Properties of the fluid (density, viscosity).
Velocity of the object.
What is friction?
A force that acts against the direction of motion or resultant force on a stationary object.
What is friction controlled by?
Type of surfaces in contact (eg. ice vs carpet).
Perpendicular forces between the surfaces (normal forces).
How do we increase max speed (I knew a lad called Max Speed) of a vehicle?
Changing thrust (more/bigger engines). Changing drag (changing the size and where in the atmosphere).
What is Newton’s 1st law of motion?
An object at rest, remains at rest, and a moving object continues with a uniform velocity (same speed straight line), unless acted on by a resultant force.
If a resultant force acts on an object which what does it do and in which direction?
Accelerates in the direction of the force.
What is Newton’s 2nd law?
The resultant force on an object is equal to the rate of change of momentum.
What is the equation for Newt’s 2nd law?
F = ∆(mv) / ∆t
What is the equations for Newt’s 2nd law where the mass is a constant?
F = ma
What is Newton’s 3rd law?
If object A exerts a force on object B, object B exerts a force equal in magnitude and opposite in direction back onto object A.
What is the condition of Newt’s 3rd law?
Must be two different objects.
Must be the same type of force (eg. Both gravitational).
Same line of action.
Momentum = ?
Mass x velocity
In a collision or explosion between 2 objects, if no external forces act, the linear momentum before the event is what?
Equal to the momentum after the event.
Define linear momentum.
Momentum from an object moving in a straight line with any orientation.
What is impulse equation?
The force x the time taken for the collision. Ft = ∆p
Define impulse.
Impulse is the change in momentum during a collision or explosion.
On a force time graph what is the area under the graph?
Impulse.
What happens in an elastic collision?
The Ek before and after a collision is =, in an isolated system.
What happens in an inelastic collision?
The Ek is not equal before an after a collision, it will be lower in an isolated system.
How to prove inelastic collision?
Calculate Ek (0.5mv^2) before and after to show not =.
What are examples of elastic collisions?
Ball with velocity v hits stationary ball what the same mass, ball 1 stops and ball 2 continues with velocity v.
Two balls come at each other with different masses and speeds, collide, move away in opposite direction with negative velocity.
Why do we want to reduce the force experienced in a collision, what do we do?
Reduce risk of death and injury, we make the collision longer - reducing impulse.
How do we reduce impulse? What are these things designed to do?
Crumple zones, steel motorway barriers, tyre walls (not just for the aesthetics).
Designed to deform to elongate time taken.
What is work done, W?
The energy transferred to an object by the components of the force parallel to the direction of motion when it’s moved across a distance.
What is the equation for work done?
W = Fscosθ
Power = ?
∆Energy / ∆t
Define power.
Rate of energy transfer or rate of work done.
What is the area under a force displacement graph?
Work done.
Efficiency =
Useful energy / total energy
OR
Useful power / input power
What is conservation of energy?
Energy cannot be created or destroyed, only transferred between sources. Energy before = energy after.
For the mark scheme what do you need to do to equations before subbing in values?
Rearrange.
Why won’t real value reach calculated value?
Work done by friction/ resistive forces. Energy converted to internal/thermal energy.
Define gravitational field strength, scalar or vector?
Force per unit mass, vector.
State and explain one use for satellite travelling in orbit.
GPS, several satellites needed to fix position on earth.
Show magnitude of rf acting on a object.
Find differences between ups and downs, find magnitude of these two values.
