Lecture 2 - Kinematics & Newton's Laws

Question 1

Define “kinematics

Answer 1

Study of simple description of motion only of objects (not the cause of the motion).

Kinematics is a type of mechanics. Mechanics is a branch of physics.

Question 2

Define “dynamics”

Answer 2

Study of the causes of motion - governed by Newton’s Laws of Motion

Dynamics is a type of mechanics. Mechanics is a branch of physics.

Question 3

Define “distance”

Answer 3

Distance is if an object travels from point S(initial) to S(final).

Distance = scalar quantity

Formula:
|dS->| = |S->(final) – S->(initial)|

Question 4

Define “displacement”

Answer 4

Displacement is both direction and distance that the object travels.

Displacement = vector quantity with direction.

Negative (-) sign indicates that the object travelled in the opposite direction of a defined positive (+) direction.

Formula:
dS->| = |S->(final) – S->(initial)

Question 5

What’s the difference between “distance” and “displacement”?

Answer 5

Distance calculates only distance travelled.
Distance = scalar quantity.

Displacement calculates both distance travelled AND direction of movement.

Displacement = vector quantity with direction.

i.e.
Moving object only east by 5km (+) = positive direction

Moving same object west by 7km (-) = negative direction
Total distance = 12 m
displacement = 5 – 7 = -2 m

Question 6

Define “speed”

Answer 6

Speed is the rate of change of distance over time.

Question 7

Define “velocity”

Answer 7

Velocity is same as speed but with a specific direction

Question 8

What’s the difference between “speed” and “velocity”?

Answer 8

Speed is the rate of change of distance over time (no direction) = Scalar quantity

Velocity is same as speed but with a specific direction = Vector quantity

Question 9

Define “acceleration”

Answer 9

Acceleration is the rate of change of velocity with respect to time

• Represents how much the velocity of an object **changes over time AND direction.

Acceleration due to gravity
- g = 9.81 m/s^2
- Gravity always has downward force

Average acceleration formula (we will only deal with constant acceleration)

i.e. - Acceleration is the same (no change in velocity over time) = 10m/s

Time vs Velocity
0 s | 0 m/s
1 s | 10 m/s
2 s | 20 m/s
3 s | 30 m/s
4 s | 40 m/s
5 s | 50 m/s

Question 10

List the five kinematic equations, and how to use them…

Answer 10

4. 5.

Question 11

Example:

You are transporting pita bread to packaging facility on a conveyer belt. The belt is accelerating at 20m/s^2.
Starting from rest, how much time is required in order to move the bread to the packing facility located at 1000m from the starting point?

Answer 11

S = v0 x t + 1/2 a x t^2

Known variables:
s (distance) = 1000 m
a (acceleration) = 20m/s^2
v0 (initial velocity) - 0 m/s; starts from stationary

=Solve for T (time)

T = 10 seconds = time it will take for pita bread to move from A –> B.

Question 12

Define “Force”

Answer 12

Force is the strength of either push or pull action.

Force = vector quantity = has direction

Question 13

Define “Mass”

Answer 13

Mass dictates how easy or difficult it is to change an object’s velocity.

Velocity is same as speed but with a specific direction = Vector quantity

Question 14

What’s the difference between “Force” and “Mass”? How are they similar?

Answer 14

Force is the strength of either push or pull action.

Mass dictates how easy or difficult it is to change an object’s velocity.

BOTH are considered vector quantities = have direction.

Question 15

Describe Newton’s First Law of Motion

Answer 15

AKA “Law of Inertia” = maintaining state of motion or rest with no external force added

“in the absence of any net external force an object will keep moving at a constant speed in a straight line; or remain at rest”

Inertia = an object resisting a change in its “state of motion.”

Formula:
If (Fnet–>) = 0, then dV = 0

• i.e. state of motion will continue = throwing a ball up in the air (with NO external forces affecting the ball)
• i.e. state of rest will continue = not throwing a ball (and NO further force applied externally)
  i.e. a planet’s orbit will continue if no external force is applied

Question 16

Describe Newton’s Second Law of Motion

Answer 16

When there are external (unbalanced) forces acting on object.

“When an **external, unbalanced force **acts on an object, the object will accelerate in the same direction as the force.”

“Acceleration varies in two ways:
- directly proportional to the force * more F = more A; moving in the same direction)*
- inversely proportional to the mass (as object mass increases, the applied force is the same, but the acceleration will decrease)”

Formula:
Sum of F–> = ma–>

i.e. pen will stay on table b/c forces are balancing
gravitational force AND normal force
i.e. “–>” on up ^ and down v = “moves more –>”
i.e. If 10 N–> = 10 m/s^2, then “directly proportional”
i.e.
- if 10 N applied to a 20kg mass = 0.5 = A
vs
- if 10 N applied to a 50kg mass = 0.2 = A

Question 17

Define “weight”

Answer 17

Weight is the gravitational force exerted on an object by the planet Earth

Formula:
W–> = m x g–>
weight equals mass x gravitational constant

Units in Newton, N

Question 18

Describe Newton’s Third Law of Motion

Question 19

What’s the difference between “gravitational force” and “normal force”

Answer 19

Gravitational = downward force

Normal force = upward force

Question 20

What are the three main points of “normal force”?

Answer 20

1. Exerted by a surface on an object (upward force)
2. Acts perpendicular to, and away from, the surface at the point of contact
3. Never “pulls”; always “pushes”

i.e. a mass resting on a surface
i.e. putting a heavy block on a table
i.e. surface strength will depend on the table quality/structure = normal force upwards of table holding up heavy block on table (“will the object break through the table?”)
i.e. Free-body diagram: object is on Y-axis, N is positive; W is negative

Question 21

What are “Free-body diagrams?”

Answer 21

Free-body diagrams are used in physics theory to show the relative magnitude AND direction all forces; upon an object in a given situation.

4 Forces acting on object:
Weight (W) = downward
Normal (N) = upward
Applied (?) = rightward –>
Frictional(?) = <– leftward

i.e. Free-body diagram: object is on Y-axis, N is positive; W is negative
i.e. if (Fnet–>) = 0; means no net movement

Question 22

In Free-body diagrams, what are the four forces acting on object; which direction do they go?

Answer 22

Forces acting on object:
Weight (W) = downward
Normal (N) = upward
Applied (?) = rightward –>
Frictional(?) = <– leftward

Question 23

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