Y2, C5 - Forces & Friction Flashcards
What direction does a normal reaction act
Perpendicular to the surface
What direction does weight always act
Vertically downwards
What is Newton’s first law
An object will remain at rest or continue to move with constant velocity unless acted upon by an external force
Why are forces resolved perpendicular to each other
Because perpendicular forces do not interact with each other
If a particle is static what do:
a) forces left equal?
b) forces down equal?
a) forces left = forces right
b) forces down = forces up
What does it mean if a bead is smooth on a string
The two parts of the string can be considered the same and thus the tension is the same throughout
What does it mean in terms of tension if a particle (bead) cannot smoothly move along a string
The tension is different both sides (e.g. T1, T2)
What steps should you always take when solving statics problems (2)
1) Resolve horizontally
2) Resolve vertically
What is Newton’s second law
F = ma
An object will accelerate if there is an overall resultant force on the object
When shown a diagram with lots of forces that need resolving, what can you do
Draw a new diagram with all the forces resolved horizontally and vertically
How can you use vectors to combine forces
Separate forces into i and j components
Write in vector form
Resultant force = (i, j) where all i components are added and all j components are added
Magnitude = Pythagoras
arctan(j / i) = angle
How should you resolve forces on inclined planes in relation to the plane
Parallel and perpendicular to the plane
With inclines plane problems can you draw a new diagram with forces resolved parallel and perpendicular to the plane
Yes
When there is no pulling force (tension T = 0), what is the frictional force and is the system in equilibrium
μR = 0
No friction
Equilibrium as not moving
When there is a pulling force (tension T < μR(max)), what is the frictional force and is the system in equilibrium
μR = T
Friction is the same magnitude as the opposing force
Equilibrium as not moving
When there is a pulling force (tension T > μR(max)), what is the frictional force and is the system in equilibrium
μR = μR(max)
Friction has reached its maximum limit
No equilibrium, the forces are imbalanced so the object accelerates
When there is a pulling force (tension T = μR(max)), what is the frictional force and is the system in equilibrium (2 situations)
μR = μR(max)
Friction is limiting and has reached its maximum
Equilibrium - limiting equilibrium
OR
Equilibrium - object moves with constant velocity
What is the relationship between T and μR(max) if a force is applied to an object and it doesn’t move
T < μR(max)
What is the relationship between T and μR(max) if a force is applied to an object and the block accelerates
T > μR(max)
What is the relationship between T and μR(max) if a force is applied to an object and the block is on the point of slipping
T = μR(max)
What is the maximum friction between two surfaces equal to (formula)
Friction(max) = μR
Can you have a negative coefficient of friction (μ) value
NO
What direction does friction act
Opposite to motion
Can friction be less than it’s maximum value
YES, if the force opposing it is small
How do you use the assumption that a pulley is smooth in calculations
Tension both sides of it is equal
Friction acts up a slope to stop an object from slipping. What direction does the friction act if a horizontal force is applied to the box (into the plane) given that the box remains in equilibrium
Friction changes direction and acts down the slope, opposing the direction of motion up the slope
How can you find the maximum value P that can work against a friction while keeping an object in equilibrium
μR is at a maximum
Resolve and solve simultaneously to find P
How would you find a range of possible values for tension that keep an object at equilibrium on a rough plane
Solve 2 equations for T
Slipping UP (friction works down)
Slipping DOWN (friction works up)
ans = Smallest T <= T <= Biggest T
Where smallest T is the limiting equilibrium to move down the plane and biggest T is the limiting equilibrium moving up the plane
How do you determine of an object will move on a rough plane
If F > Frictional force
Compare F with μR
If F > μR it will move
(where F is the force up or down the plane opposing the friction)
What should you take as the weight of an object if you are not given the mass
mg
If a particle is projected up a slope, what is the force upwards
0
There is no force acting on it as it is projected, all forces will work to stop it from going up
You have a block (P) on a rough plane attached to a smooth pulley with a block on the other end (Q) hanging freely. When the hanging block is released, how do you go about calculating the acceleration of the system
Write equations of motion for both P and Q separately using F = ma
Solve equations simultaneously to find acceleration and force (tension in string)
How do you use the fact that a string is inextensible in calculations
Acceleration is the same in particles at both ends
What is the equation for the resultant force exerted on a pulley by a string (over an inclined plane)
2 * T * cos((90 - α) / 2)
Where T is tension
α is the angle of incline of the plane
What direction does the resultant force exerted on a pulley by a string (over an inclined plane) act
Directly down the middle into the incline plane (splitting the angle of the corner in half)
