1. Density

2. Density
If a person is very dense, we mean they are a bit thick or stupid …

3. Density In science, if something is very dense it has …
a large mass for its volume

4. Density In science, if something is not very dense it has …
You don’t expect to see this!
a small mass for its volume

5. Density To measure the density of something, you need to know:
its mass
its volume

6. Density You need to know this equation: density = mass  volume
The density of a block of aluminium with a mass of 5.4 g and a volume of 2 cm3 is:
5.4  2 = 2.7 g/cm3

7. DENSITY Q) Which weighs more:-
A kilogram of feathers or a kilogram of iron?
DENSITY

8. Density is the Mass per unit Volume
What is Density?
If you take the same volume of different substances, then they will weigh different amounts.
Wood
Water
Iron
1 cm3
1 cm3
1 cm3
IRON
0.50 g
1.00 g
8.00 g
Q) Which has the greatest mass and therefore the most dense?
Density is the Mass per unit Volume

9. Density Equation: Density= m V m D V Mass Density = Volume Example:
g or kg
Density =
Mass m
Volume
gcm-3 or kgm-3
Density= m V
cm3 or kg3 D V
Example:
Q) Liquid water has a density of 1g/cm-3, while ice has density of gcm-3.
Calculate the volume occupied by 25 g of each.
V = m = 25 = 25 cm3
D 1
V = m = = 27.2 cm3 D

10. DENSITY OF A REGULAR SOLID
Find the Mass of the solid on a balance.
Measure the three lengths and calculate the Volume.
(ie V = l x w x h )
Calculate the Density.
m = 240 g
2.0 cm
4.0 cm
D = m = 240 =10.0 g/cm3 V
3.0 cm

11. DENSITY g/cm3 Aluminium 2.70 Iron 7.86 Brass 8.50 Wood 0.50 Slate 2.80
1 cm3 of each of the these substances have a mass of
DENSITY
g/cm3
Aluminium
2.70
Iron
7.86
Brass
8.50
Wood
0.50
Slate
2.80
Glass
2.50
Lead
11.3
Marble
2.70
Wax
0.89

12. DENSITY OF AN IRREGULAR SOLID
m = 360 g
 = m = 360 =12.0 g/cm3 V
Find the Mass of the solid on a balance.
Fill the Measuring Cylinder with Water to a known Volume.
Add the Object.
Work out the Volume of Water that is displaced.
Calculate the Density.
80 cm3
50 cm3

13. DENSITY OF AN IRREGULAR SOLID
OR use a Overflow Can to find the Volume.
Find the mass of the solid on a balance.
Add water until just overflowing.
Place a Measuring Cylinder under the spout.
Add the Object.
Collect the Water and read off the Volume.
Calculate Density
m = 440 g
Volume = 40.0 cm3
D = m = 440 =11.0 g/cm3 V

14. DENSITY OF A LIQUID Find the Mass of an empty Measuring Cylinder.
Add a certain Volume of Liquid.
Find the Mass of the Measuring Cylinder and Liquid
Calculate the Mass of Liquid.
How?
Calculate Density of Liquid.
25.0 g
45.0 g
Mass of Liquid = Mass of Measuring Cylinder and Liquid – Mass of empty Measuring Cylinder
Volume =20.0 cm3
45 – 25 = 20 g
D = m = 20 =1.00 g/cm3
V 20

15. Floatation How do we know if a body will float or not?
An object will float if it is less dense that the liquid it is put in.
The density of water is 1.0 g/cm3. Will Aluminium density =2.7 g/cm3 float?
Explain you answer.
What about wood density 0.8 g/cm3

16. Floatation Which blocks have a greater density than water?
Would they all float in Mercury? Why/Why not?
Note Mercury has a density of g/cm3

17. Floatation
Why does a large metal ship float? Metal is denser than water.
The density of the ship is the average density of the metal and the air trapped in it which is less than water so it floats.

18. Speed, velocity and acceleration
Motion
Speed, velocity and acceleration

20. What is motion?
When you move from say the classroom to the hall you could be described as being in motion. What two values are changing as you move form the classroom to the hall?
As you are walking from the classroom to the hall your distance from the classroom is changing.
As you make your way to the hall the time since you left the classroom is changing.

21. What is motion?
So that we can compare how quick two students reached the hall from different classrooms we need to measure\\ two different values. What are they?
We must measure the distance they travelled and the time it took them. How do we use this information?
Then if we divide the distance by the time the answer we get tells us which student was the quickest.

22. Speed Speed = Distance ÷ Time D_ S T
Example: A car travels 300km in 6 hours. What is the speed of the car?

23. Answer: Speed = distance ÷ time Speed = 300km ÷ 6 hours
Speed = 50km/hr

24. More practice
1. How far can a plane travel if it flies 800km/hr for 9 hours?
2. How long does it take a ship to go 500 km if it travels at a speed of 50km/hr?

25. Answer D
S T D
800km ▪ 9hrs = 7200km hr

26. Answer D
S T
500
50 T
500km ÷ 50km = 10 hrs hr

27. Velocity Speed in a given direction.
What is the velocity of a boat that travels from Dublin to Leixlip (14 km) in 15 minutes?

28. Answer Speed = distance ÷ time
Speed = m (14 km)÷ 900 s (15 minutes
Speed = m/s
Velocity = m/s West

29. Distance-time graphs
On your paper, graph the following:
Dist (m) Time (sec)

30. Distance (m)
time (sec)

31. Was your graph a straight line?
A distance-time graph which is a straight line indicates constant speed.
In constant speed, the object does not speed up or slow down. The acceleration is zero.

32. Graph the following on a distance-time graph:
D (m) T (s)
0 0
10 1
20 2
30 3
40 4
50 5
QUESTIONS
What distance has the object travelled after 2.5 s?
How long does it take to travel 46m?
What is the speed of the object?

33. Distance (m)
time (sec)

34. Answers After 2.5 s the distance is 25m. It takes 4.6s to travel 46 m.
Speed = distance  time e.g. 30  m/s

35. Was your graph a curve?
A straight line graph going up indicates a constant velocity on a distance-time graph

36. Distance-time graphs Describe the motion of the object as shown in the
From 0-8 sec, constant speed:
(25 m/sec);
From 8-12 sec, no motion;
From sec, constant speed
Find the speed for each section

37. answers Average Speed (m/s) Time (sec) ____ 0 to 8 ____ 8 to 1225
37.5

38. Understanding distance time graphsB
Distance/m
Distance/m
Time/s
Time/s C
The graphs show the movment of a car. Explain what is happening the car In each of the three graphs.
Distance/m
Time/s

39. answers In A the car is moving away from youwith a constant speed.
In B the car is moving towards younwith a constant speed
In C the car is not moving.

40. Acceleration Change in velocity
Can be change in speed or direction
Acceleration = Change in velocity Time V
a t

41. Acceleration problem
A roller coaster’s velocity at the top of a hill is 10m/s. Two seconds later it reaches the bottom of the hill with a velocity of 26m/s. What is the acceleration of the roller coaster?

42. Answer Acceleration = V/ T a = 26m/s – 10m/s 2 s a = 16m/s 2s
a = 8m/s/s or 8m/s2

43. More acceleration problems
1. A car accelerates at a rate of 20m/s. How long does it take to reach a speed of 80 m/s?
2. A car travels at 60 Km per hour around a curve. Is the car accelerating?
3. A car travels in a straight line at 60Km/hr. Is the car accelerating? 2

44. Answers: 1. V 80m/s a t 20m/s t t = 4 s
2. yes! Because it’s changing direction!
3. no! It’s not changing speed or direction! 2

45. Deceleration Negative acceleration
Example: A car slows from 60 m/s to 20 m/s in 4 seconds. What is its acceleration?

46. Answer: Acceleration = V/ T Acceleration = final V – initial V t
a = 20 m/s – 60 m/s
4 s
a = -40 m/s 4s
a = -10 m/s2

47. Speed Time Graphs Time/s Speed m/s2 5 10 20 15 25 40 50 30 605 10 20 15 25 40 50 30 60
Draw a speed time graph for the data shown opposite.
Find the speed after 22 s?
Find the time it takes to reach a speed of 55 m/s?
Find the acceleration?

48. Speed time graphs Answers After 22s the speed is 44 m/s
After 55 m/s the times is 27.5 s
The acceleration is speed / time
For 20 m/s the time is 10s therefore the acceleration is = 2m/s2
Speed m/s
Time/s

49. What does your graph look like?
Constant speed will be a horizontal line on a Velocity time graph.
If the Velocity decreases, the line will slant down.
If the Velocity increases, the line will slant up.

50. What do the following speed-time graphs depict?

51. Answers The first graph shows constant acceleration
The second graph constant velocity for the horizontal part of the the cuve and the second part shows constant deceleration.