Topic 2- Motion and Forces

SP2

Scalar Quantities

Scalar Quantities

Distance

Speed

Mass

Energy

Vector Quantities

Displacement

Velocity

Acceleration

Force

Weight

Momentum

acceleration formula

acceleration = change in velocity over time (m/s^2₎

free fall (acceleration towards the earth in the absence of air resistance)

10 m/s²

typical acceleration of:

1. family car

2. falling object

3. rocket

4. formula 1 car

5. fighter jet

in m/s²

1. 2-3

2. 10

3. 30

4. 50

5. 90-120

formula to use with constant accelerations (time taken not known)

v2 - u2 = 2ax

(x = distance travelled) (a = acceleration)

gradient of a velocity-time graph =

acceleration

area under a velocity-time graph

distance travelled

typical speeds of:

1. walking

2. running

3. cycling

4. car

5. passenger aeroplane

6. sound

in m/s:

1. 1.5

2. 3

3. 6

4. 10 to 30

5. 200-250

6. 330-340

using 2 light gates to measure speed

• gates are a set distance apart

• timer starts when object passes through 1st light gate

• timer stops when object passes through 2nd light gate

single light gates to measure speed

• timer measures how long the light beam is blocked for

• use the length of the marker as the distance

Newton’s First Law

Objects that are at rest will remain at rest, and objects that are moving will continue moving at a constant velocity, unless acted on by a resultant force

when the resultant force on a body is not zero

the speed and direction of the object can change

Newton’s Second Law

the acceleration of an object is proportional to the resultant force acting on it and inversely proportional to the object’s mass

the bigger the resultant force…

the larger the acceleration

the greater the object’s mass…

the smaller the acceleration

Newton’s Second Law FORMULA

f = ma

Newton’s Third Law

whenever 2 objects interact, the forces they exert on each other are equal and opposite

Difference between Newton’s first and third laws

Newtons 1st law: forces act on one object

Newton’s 3rd law: forces act on two objects

weight (not formula)

the force acting on an object due to gravitational attraction

how to measure weight directly

newtonmeter

how to measure weight indirectly

find mass, then multiply by g (~10)

circular motion:

constant speed?

constant velocity?

is there acceleration?

constant speed

changing direction, therefore changing velocity

acceleration is a vector therefore yes

when an object is moving in a circle, there must be acceleration. if there is acceleration, there must be a resultant force. What is this called?

centripetal force

centripetal force

the resultant perpendicular force towards the centre of the circle required to keep a body in uniform circular motion (can be any force keeping an object moving in a circle)

examples of centripetal force

tension

friction

gravitational

momentum formula

p = mv

p = momentum (kgm/s)

m = mass

v = velocity

what is momentum?

what keeps and object moving in the same direction

momentum changes if:

• the object accelerates or decelerates

• changes direction

• changes mass

elastic collision (what happens to mass and velocity)

after a collision objects move in opposite directions

they have different velocities

inelastic collision (what happens to mass and velocity)

after a collision objects move in the same direction

they have a combined mass and velocity

momentum is always ___ in a collision

conserved

to analyse a collision in an exam:

• state velocity and direction of each object before and after

• state inelastic/elastic

• describe energy transfers

conservation of momentum

in a closed system, The total momentum before a collision = The total momentum after a collision

Newton’s 3rd law in collisions

when two objects collide, they exert force of each other (that is equal and opposite)

this means that one object will speed up and one object will slow down

therefore acceleration will be different

what is the formula relating force and change in momentum? (explain!)

Force = change in momentum / time

A force acting on an object makes it accelerate, so its momentum changes. The force is the rate of change in momentum.

what is inertia?

an object’s resistance to a change in motion (the tendency of an object to continue in its state of rest, or in uniform motion unless acted upon by an external force)

what is inertial mass?

how difficult it is to change the velocity of an object (the ratio between the force applied to it and the acceleration it experiences)

inertial mass formula

inertial mass = f/a

the inertial mass applied to an object is ____ to its acceleration as a result of a force

inversely proportional

dangers of large decelerations

• overheating of brakes (due to friction)

• loss of control causing collision and/or injury

how to measure reaction times

dropping ruler

typical reaction time for someone who is alert

0.2-0.9 seconds

factors affecting braking distance

• speed

• vehicle condition

• road condition

• vehicle mass

factors affecting thinking distance

• tiredness

• distractions

• intoxication

reaction distance =

speed of car x drivers reaction time

for a given braking force, the stopping distance of a car depends on the…

speed of the vehicle

braking distance formula

breaking distance = (½ x mass x velocity²) / breaking force