Motion

There are four sections to this….

1. Speed and Acceleration

2. Forces

3. Work

4. Power

Click on the section you want.

1. Speed and Acceleration

Speed

S

D

TxS =

Speed =distance

time

D

T

Speed tells you how fast something is travelling

A turtle travels 2 metres in 300 seconds. What speed

is it travelling?

S

D

Tx

S = D

T

S =2 m

300 s

S = 0.007 m/s

Distance

S

D

TxS =

Speed =distance

time

D

T

Distance = speed x time

D = S x T

A 747 travels from L.A. to Auckland at 700 km/hr. The trip takes 9 hours. How far

has the plane travelled?

D = S x T

S

D

TxD = 700 x 9

D = 6,300 km

Time

S

D

TxS =

Speed =distance

time

D

T

Distance = speed x time

D = S x T

Time = distance

speed

T =D

S

A train travels from Wellington to Auckland at an average speed

of 110 km/hr. The distance it covers is 550km. How long does

the trip take?

S

D

Tx

T =D

S

T =

550 km

110 km/hr

T = 5 hours

Units

The Standard units are :

• distance = metres = m

• time = seconds = s

• speed = metres per second = m/s

Be careful!

• Look at the units given to you in a question.

•Convert all parts of a question into the same units.

•Remember to include the units in your answer.

Distance – Time Graphs

Time (s)

Distance

(m)

A.

Time (s)

Distance

(m)

B.

Time (s)

Distance

(m)

C.

Constant speed Constant speed (but slower than A)

Stationary

The gradient of the line is the speed.

The steeper the line, the faster the speed!

Distance – Time Graphs

Time (s)

Distance

(m)

A. Constant speed

Time (s)

Distance

(m)

B.Acceleration

Time (s)

Distance

(m)

C.Deceleration

If the line on a distance – time graph is curved then the object

must be changing speed.

i.e. getting faster or slower.

Acceleration

Acceleration =change in speed (m/s)

time (s) a

s

tx

Units = m/s

s= metres per second squared.

= change

= m/s2 = ms-2

When the parachutist jumps from the plane his speed downwards goes from 0m/s to 20m/s in just 2 seconds.

20 m/sa =2s

= 10 m/s2

a

s

tx

His change in speed is from 0m/s to 20 m/s

So his acceleration is…….

(10 m/s2 is the acceleration due to gravity)

Speed – Time graphs

speed

m/s

Time (s)

Increasing speed = acceleration

Steady speed = no acceleration

Decreasing speed = deceleration

Acceleration can also be shown on distance – time graphs:

Distance (m)

Time (s)

Acceleration

Deceleration

2. Forces

A Force is a:

push

pull

twist

Forces may be:

Contact Non-contact (field)

friction upthrust

buoyancyair-resistance

magnetic electrical

gravitational

Forces always act in pairs.

ActionReaction

ThrustDrag

Gravity Buoyancy (Upthrust)

If the forces are balanced

the object will not

change its speed,

direction or shape

Unbalanced forces however….

Gravity buoyancy

… will make an object

change speed, shape or direction.

The Stages of a Parachute

Jump 2. Freefall

3. Deployment

4. Descent

There are different forces acting at each

stage. Try to work them out before going to the next

slide.

1. Exit

Exit

The force pulling the man down is his weight.

At first this is far more than the upthrust (air resistance) pushing him upwards

… so he accelerates downwards!

Freefall

As he accelerates down his weight stays the same

… but the upthrust (air resistance) increases rapidly.

Once they are the same he stops accelerating and falls at a steady speed (terminal

velocity).

Deployment

As he opens his parachute his weight

is still the same.

… but the upthrust due to air resistance is suddenly

huge!

So he decelerates

rapidly.

Descent

Once again, his weight hasn’t

changed

… but as he slows the upthrust

decreases until it equals his weight

again.

So, once again he descends at a steady (but slower) speed.

3. Work

Work done = Force x Distance Moved

W

F x D

•Work is actually measuring the amount of energy that has been converted from one form to another……

….. So its units is JOULES

1.5m

2kg2kg

How much work is done to lift the two bricks on to the table?

Work = Force x Distance moved

Force (weight) = mass x acceleration

= 4kg x 10 m/s2

= 40N

40N x 1.5m = 60J

1.5m

2kg2kg

How much work is done to lift the two bricks on to the table?

Work = Force x Distance moved 40N x 1.5m = 60J

It takes 1 joule of energy to do move a force of 1N for 1 metre

So in this example the work done is 60Joules

1.5m

How much work is done to lift the two bricks on to the table one at a time?

2kg2kg

2kg => 20N 2kg => 20N

20N x 1.5m = 30 J Two bricks needing

30J of work each = 60J

The work done is the same!

1.5m

2kg2kg

Lifting the bricks separately means less force each time, but twice as much distance moved overall.

2kg2kg

1.5m1.5m

Some machines work this way …..

….. Using a small force over a large distance to move a larger force over a smaller one.

60J 60J

1.5m

3m

The ramp is an inclined plane. It increases the distance moved so it decreases the force needed.

(small force , big movement big force, small movement)

20N10N

Levers Levers are usually used as force multipliers.

A small force moving a long distance

Lifts a large force a short distance

Here are some more force multipliers.(small force, big movement big force, small movement)

….. Alternatively levers can be distance

multipliers

Big force, small distance

Small force, big distance

For instance:

4. Power (Watts)Power measures how fast work is being done.

P

w

t

Power =work done

time taken

A weightlifter lifts 350kg up 2.5 metres in 2 seconds.

How much power does he use?

Force (weight) = 350 kg x 10 m/s2 = 3,500N

Force = mass x acceleration

Work done = force x distance

Work done = 3,500 N x 2.5 m = 8,750 J

Power `=work done

time taken

Power = 8,750 J

2 s= 4,375 W

END