Y2, C5 - Forces & Friction

Question 1

What direction does a normal reaction act

Answer 1

Perpendicular to the surface

Question 2

What direction does weight always act

Answer 2

Vertically downwards

Question 3

What is Newton’s first law

Answer 3

An object will remain at rest or continue to move with constant velocity unless acted upon by an external force

Question 4

Why are forces resolved perpendicular to each other

Answer 4

Because perpendicular forces do not interact with each other

Question 5

If a particle is static what do:
a) forces left equal?
b) forces down equal?

Answer 5

a) forces left = forces right
b) forces down = forces up

Question 6

What does it mean if a bead is smooth on a string

Answer 6

The two parts of the string can be considered the same and thus the tension is the same throughout

Question 7

What does it mean in terms of tension if a particle (bead) cannot smoothly move along a string

Answer 7

The tension is different both sides (e.g. T1, T2)

Question 8

What steps should you always take when solving statics problems (2)

Answer 8

1) Resolve horizontally
2) Resolve vertically

Question 9

What is Newton’s second law

Answer 9

F = ma
An object will accelerate if there is an overall resultant force on the object

Question 10

When shown a diagram with lots of forces that need resolving, what can you do

Answer 10

Draw a new diagram with all the forces resolved horizontally and vertically

Question 11

How can you use vectors to combine forces

Answer 11

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

Question 12

How should you resolve forces on inclined planes in relation to the plane

Answer 12

Parallel and perpendicular to the plane

Question 13

With inclines plane problems can you draw a new diagram with forces resolved parallel and perpendicular to the plane

Answer 13

Yes

Question 14

When there is no pulling force (tension T = 0), what is the frictional force and is the system in equilibrium

Answer 14

μR = 0
No friction
Equilibrium as not moving

Question 15

When there is a pulling force (tension T < μR(max)), what is the frictional force and is the system in equilibrium

Answer 15

μR = T
Friction is the same magnitude as the opposing force
Equilibrium as not moving

Question 16

When there is a pulling force (tension T > μR(max)), what is the frictional force and is the system in equilibrium

Answer 16

μR = μR(max)
Friction has reached its maximum limit
No equilibrium, the forces are imbalanced so the object accelerates

Question 17

When there is a pulling force (tension T = μR(max)), what is the frictional force and is the system in equilibrium (2 situations)

Answer 17

μR = μR(max)
Friction is limiting and has reached its maximum
Equilibrium - limiting equilibrium
OR
Equilibrium - object moves with constant velocity

Question 18

What is the relationship between T and μR(max) if a force is applied to an object and it doesn’t move

Answer 18

T < μR(max)

Question 19

What is the relationship between T and μR(max) if a force is applied to an object and the block accelerates

Answer 19

T > μR(max)

Question 20

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

Answer 20

T = μR(max)

Question 21

What is the maximum friction between two surfaces equal to (formula)

Answer 21

Friction(max) = μR

Question 22

Can you have a negative coefficient of friction (μ) value

Answer 22

NO

Question 23

What direction does friction act

Answer 23

Opposite to motion

Question 24

Can friction be less than it’s maximum value

Answer 24

YES, if the force opposing it is small

Question 25

How do you use the assumption that a pulley is smooth in calculations

Answer 25

Tension both sides of it is equal

Question 26

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

Answer 26

Friction changes direction and acts down the slope, opposing the direction of motion up the slope

Question 27

How can you find the maximum value P that can work against a friction while keeping an object in equilibrium

Answer 27

μR is at a maximum
Resolve and solve simultaneously to find P

Question 28

How would you find a range of possible values for tension that keep an object at equilibrium on a rough plane

Answer 28

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

Question 29

How do you determine of an object will move on a rough plane

Answer 29

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)

Question 30

What should you take as the weight of an object if you are not given the mass

Answer 30

mg

Question 31

If a particle is projected up a slope, what is the force upwards

Answer 31

0
There is no force acting on it as it is projected, all forces will work to stop it from going up

Question 32

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

Answer 32

Write equations of motion for both P and Q separately using F = ma
Solve equations simultaneously to find acceleration and force (tension in string)

Question 33

How do you use the fact that a string is inextensible in calculations

Answer 33

Acceleration is the same in particles at both ends

Question 34

What is the equation for the resultant force exerted on a pulley by a string (over an inclined plane)

Answer 34

2 * T * cos((90 - α) / 2)
Where T is tension
α is the angle of incline of the plane

Question 35

What direction does the resultant force exerted on a pulley by a string (over an inclined plane) act

Answer 35

Directly down the middle into the incline plane (splitting the angle of the corner in half)