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ESO 2 UNIT 1: Forces and movements


WHAT IS MOVEMENT?


Which of these things move?


Great example of movement and systems of reference

For people on the big wheel, the boat moves but to people on the boat, it will be at rest.


We say that an object is moving when it changes its position with respect to another one that we consider as fixed.


Don't forget that even if you appear to be standing still, the Earth is moving around the Sun, and the Sun is moving around our galaxy.


Everything in the universe moves. Movement can be slight or slow, but it still happens.


MOTION ELEMENTS Trajectory. Position. Distance travelled Time taken Speed Acceleration.


TRAJECTORY Trajectory is the path that a moving object follows through space. It can be rectilinear, circular, elliptical, parabolic, etc.


POSITION Position is where an object is located. If we want to describe the movement of a person who is running, we measure the distance from the person to a point that we have taken as reference. This distance is called position and is represented by the letter “s�.


Positions on the left of the reference system are often taken as negative and on the right as positive.

1m

s0

s1

-

+

s2

s3

R

The positions of the ball would be: s0 = - 6 m

s2 = + 3 m

s1 = - 4 m

s3 = + 8 m


DISTANCE TRAVELLED The distance travelled is equal to the subtraction between two positions, the final position minus the initial position if there is no change in the sense of the movement. 1m

s0

s1

-

+ R

s2

s3


Example

1m

s0

s1

-

+

s2

s3

R The distance travelled by the ball from the initial position to the position 3, would be: distance travelled = s3 - s0 = 8 m – (-6 m) = 8 m + 6 m = 14 m Imagine that the ball goes from the position 3 to the position 2. The distance travelled would be: s2 - s3 = + 3 m – 8 m = - 5 m

However, we will say that the distance travelled is 5 m. The position can be either a positive or a negative number but the distance travelled is always a positive number.


SPEED Speed is how fast an object is moving. It is the ratio between the distance travelled and the time elapsed to do it. It is represented by the letter “v”.

The mathematical expression of the speed is:

Speed (v)

Distance

Time

The SI unit of speed is m/s REMEMBER SIDOT Speed Is Distance Over Time


Example

A sheep is running down a farmer’s track. It takes exactly 10 seconds to move between two fence posts, 10 meters apart. What’s the sheep`s speed?

Step 1) Write down what you know: Distance: 10m Time: 10 seconds Step2) We want to find speed: Speed (v) = Distance (e) / Time (t) v= 10/10= 1m/s


• A car is moving on a straight road at an average speed of 72km/h. Calculate the distance covered in one and a half hours. Use SI units. Speed (v)

Distance Time

Distance= Speed*Time e= 72km/h*1,5h=108h

108.000m


An athlete can run long distances at 4 metres per second. How far can she run in? a) 50 seconds --------------------------200m b) 3 minutes---------------------------- 720m

c) 1 hour--------------------------------- 14400m d) 2 hours and a half----------------- 36000m


A dog can run long distances at 3 metres per second. How long can it run in? a) 120 meters ------------------------ 40 s b) 1200 meters-------------------------400s

c) 2100 meters ----------------------- 700s d) 5400 meters------------------------ 1800s


An athlete can run long distances at 8 meters per second. How far can he run in?

a) 10 seconds --------------------------80m b) 12 minutes----------------------------5.760m c) 1 hour and a half -------------------43.200m

d) 2 hours -------------------------------57.600m


Andrew rows at an average speed of 2 meters per second. a)How long does he take him to row: • 70 m---------------------------------------35s • 800 m ------------------------------------400s • 1 km and a half ------------------------750s

b) How far can Andrew row in: • 12 seconds-----------------------------24m • 3 minutes and a half-----------------420m • 4 hours-----------------------------------28800m


Use common sense with speed • Although it is very important use SI unit sometime has no sense use it. For instance: Average speed of the Continental Plates is 2cm per year 6,34*10-10 m/s

Average speed of halley’s comet is 100km/s 10*104m/s


A snail moves 5 m in 2 hours. If it moves at the same speed all the time, calculate:

a) The time it takes to move 20 m------------------------------- 8h or 28000s b) The time it takes to move 1 m --------------------- 0,4h 贸 24min 贸 1440s c) The distance it would move in 3 hours and a half ------------ 8,75m d) The distance it moves in 15 minutes -----------------------------0,625m


GRAPHS OF THE UNIFORM MOTION (I) Distance versus Time In the uniform motion the speed is always the same. The distance-time graph for constant speed is a straight line.


DO NOT FORGET

• The graph s/t doesn’t say to us anything about the trajectory


GRAPHS OF THE UNIFORM MOTION (III) (example) The steeper the straight line is, the faster the movement is

VB =4m/s

VA =2 m/s

vA

4 m/s

vB

2 m/s


Draw the distance-time graph for 6 m/s and 8 m/s constant speed

.


Draw the distance-time graph for 3 m/s and 10 m/s constant speed

.


GRAPHS OF THE UNIFORM MOTION (II) Speed versus Time In the uniform motion the speed is constant. The velocity-time graph for constant velocity is a horizontal line.

v (m/s)

t (s)

Speed (m/s)

Distance (m)

Time (s)

20m/s

20m

1s

20m/s

40m

2s

20m/s

60m

3s

20m/s

80m

4s

20m/s

100m

5s


ACCELERATION If speed does not change in a movement, it is an uniform motion and if it changes in a movement, it is an accelerated motion The acceleration is defined as the ratio between what the speed has changed and the time elapsed to occur that change. In other words Acceleration is defined as the rate at which an object changes its speed. An object is accelerating if it is changing its speed. It is represented by the letter “a�. The mathematical expression of the acceleration is: a

The SI unit of acceleration is m/s2

vfinal - vinitial tfinal - tinitial


Example A cyclist accelerates from 0 m/s to 8 m/s in 3 seconds. What is his acceleration ? a cyclist

vfinal - vinitial tfinal - tinitial

8 m/s – 0 m/s

2,6 m/s2

3s–0s

A car accelerates from 0 to 30 m/s in 8 seconds. What is its acceleration? a car

vfinal - vinitial tfinal - tinitial

30 m/s – 0 m/s

3,7 m/s2

8s–0s

A race car accelerates uniformly from 18.5 m/s to 46.1 m/s in 2.47 seconds. Determine the acceleration of the car a = 11.2 m/s2


FORCES

• A FORCE IS ANYTHING THAT CAN DEFORM A BODY OR CHANGE ITS STATE OF MOVEMENT OR REST


FORCES (II) • Forces are nearly always pushes and pulls. •

We need to use a force to get something moving.

• Small forces can be measured with a force meter in units called Newton (N).

• Forces have direction and we use arrows to .show the direction of a force


FORCES (III) • A force can • Change the direction

• Speed something up

• Slow something down

• Change shape


MASS versus WEIGHT MASS It measures the amount of matter.

Mass is measured in kilograms, kg, or grams, g.

The mass of an object doesn’t depend on its location.

We can measure it with balances.

WEIGHT It measures the gravitational force acting on an object. Weight is a force and forces are measured in Newton, N. The weight of an object weight depends on its location, because the gravitational force varies with the location. gEarth: 9,8 m/s2 gMoon: 1,67 m/s2; gMars: 9,8 m/s2;gMercury: 2,6 m/s2

We can measure it with force meter or dynamometers.

Weight and mass are linked to gravity, g, through the equation W=m.g


Exercise • Complete the following chart: Mass in the Earth

Weight in the Earth

Weight in the Moon

Weight in Mars

Weight in Mercury

gEarth: 9,8 m/s2

gMoon:1,67 m/s2

gMars: 3,71 m/s2

gMercury: 2,6 m/s2

10Kg 19,6N 66,8N 40,81N 52N


Archimedes´ Principle An immersed body is buoyed up by a force that is equal to the weight of the fluid that it displaces.

Forces and movements version pupils  

Force and movement 2º eso science