1. May the Forces Be With You!
21-27
I will explain, calculate, and diagram forces.

2. FORCES 21. Forces are pushes or pulls (a combination is a twist).
Objects are stationary when forces are balanced
gravity is always acting but we don’t keep falling due to a support force
EXPT – egg drop or newton meter

3. BALANCED FORCES
22. An unbalanced force cause changes to objects motion (speed or direction), or shape.
23. A balanced force is when an object is stationary.
H/O A4 paper plane, drop from roof lift is proportional to how hard its thrown drag, flapping wings slows it so small wings better also slowed by air resistance ie streamlined is better. Air needs to hit the wing and be deflected down to provide lift therefore plane needs to be back heavy to tilt it into the air and provide lift which counteracts the weight
How long does it remain in the air?
All same mass
At roof they all have the same potential energy
Release and it turns into kinetik (movement) energy
Friction and support affect it UNBALANCED FORCE
When it reaches the floor support = gravity ie BALANCED FORCE

4. (S.N.) Types of Forces Gravity
Friction – the force that opposes motion
Magnetism
Tension – the force in rope, etc
Electrostatic
Support
Lift – in the air (planes/birds)
Bouyancy – in the water

5. (S.N) CONTACT FORCES
Some forces only act on contact, others can act from a distance. Differentiate the two (Which are which?).
Gravity
Contact
Distance
Magnetism
Tension
Friction
Electrostatic
Support

6. Force = mass × acceleration
UNBALANCED FORCES
*An unbalanced force results in acceleration.
The rate of acceleration depends on the mass of the object and force applied…
24. The formula for Force:
Force = mass × acceleration
(F = m × a).
EXPT egg drop or paper plane or eggs suck into bottle (hard boiled lots of lit matches into flask, egg point down) heating up air in flask = expansion = low pressure so UNBALANCED and egg is pushed by gravity into the bottle
CRUSH a bottle under the weight of atmosphere (10 tons per sq meter) hot water into bottle, screw top, shake, unscrew pour away recap. As air in bottle cools and contracts = low pressure UNBALANCED no air can rush in so high pressure outside crushes bottle.

7. An unbalanced force causes acceleration.
FORCE AND MOTION
What happens when you apply (using a Newton meter) a small constant force to a trolley and time it over a set distance?
Small constant force
Set distance
The trolley should accelerate because…
Do the expt and with the weight table on board
Can measure the acceleration using ticker tape 50x sec
EXPT cut up make graph
Shows SPEED vs TIME
An unbalanced force causes acceleration.

8. S.N.The larger the mass the slower the acceleration
FORCE AND MOTION
What happens when you apply (using a Newton meter) a small constant force to a trolley carrying a 1kg mass and time it over a set distance?
Small constant force
1Kg
Set distance
The trolley should accelerate but slower than previously because…
S.N.The larger the mass the slower the acceleration

9. Challenge: F m A a. 9kg 0.5ms-2 6N 0.2kg b. c. 800g 1.5ms-2 350N d.

10. FORCES AND ACCELERATION
Given the formula F = ma try the following questions.
1. What are the names and units of F, m and a?
2. Complete the table using 2 step equations….
3. The rider and cycle are 150kg:
a. What is the Net force?
b. What is the cyclist’s acceleration?
4. A bike accelerates at 10ms-2 using a force of 6000N. The rider is 70kg. What is the mass of the bike? F m A a.
9kg
0.5ms-2 6N
0.2kg b. c.
800g
1.5ms-2
350N d.
15ms-2 e.
1200kg
0.015ms-2
800N
150N
Previous H/O probs 1-5 and F,M,A questions from workbook
5. What happens to acceleration if the thrust is halved?

11. WEIGHT FORCE
25. Weight is a force. It is therefore measured in…Newtons (N).
An object’s weight depends on two things:
Gravity
varies depending where you are
10ms-2 or 10N/Kg on Earth
Mass
does not vary
measured in Kg
A man with mass of 75Kg on earth weighs 750N
BUT on the moon he weighs 125N
Gravity (on the moon your weight is 6x less than on the Earth – in space, where there is no gravity, you are weightless). Gravity accelerates things at 10m/s/s and adds 10x to every kg of mass
Mass. is the amount of matter in an object, which does not vary depending on where you are. It is measured in Kg
Definitons H/O

12. MASS AND WEIGHT a. What is mass? b. What is it’s unit?
2. a. What is Weight?
3. ON EARTH: 1N = kg 1kg = N
4. How would your mass and weight change if you went to Jupiter?
1a. Amount of matter
1b. Kg
2a. Force of gravity on mass
2b. Newton
3. 0.1Kg and 10Kg
4.Mass doesn’t change, weight would be heavier

13. FRICTION
26. Friction is a contact force that opposes motion, it causes heat, damage, wear and slowing
EXPT sliding on oil or sandpaper or bike stopping on dry, wet surface

14. Need for Speed

15. SPEED
27. Speed is the distance that an object travels in a period of time. Speed=d/t d t v
Units are meters and seconds (and therefore meters per second). However, sometimes km/hr is more sensible.
A cyclist travels 25 km in ½ an hour.
What is their speed - in kmhr-1
- in ms-1
Meters per second
= ms = m/s = ms-1
H/O activity 18.2 Running speeds
H/O Bubble gum champ
= 25km/0.5hr = 50km/hr
= 25000m/1800s = 13m/s

16. SPEED QUESTIONS What would these look like on a distance/time graph?
1. stopped
2. slow
3. fast
4. accelerating

17. Reaction Times

18. ACCELERATION
27. Acceleration is the change in speed in an object in a period of time. A=Change in Velocity/ Change in Time
Δ v a
Δ t
Units ms-2
It takes a cyclist 20 seconds to go from a standing start to 14m/s.
What is their acceleration?
What is 14m/s in km/hr?
a = v  t
a = m/s  s
a = m/s/s
=m/s2
= ms-2
H/O foot on accelerator or workbook exercises
a = Δv/Δt a = 14m/s / 20s a = 0.7ms-2
= 14 × 60s × 60min  1000m
= 50.4km/hr

19. SPEED/TIME GRAPHS
A runner travels at 4m/s for 10 seconds, then stops suddenly for 5 seconds, then accelerates for 5 seconds to get to 8m/s and continues for 10 seconds.
Plot this on a speed/time graph.
Speed
(m/s) 4 8
Using the speed/time graph:
In what part of the trip is the runner going the fastest?
What is the acceleration in part 4?
= part 5
Time (sec) 10 20 30
Part 5 = 8m/s
In a speed/time graph the slope of the line = the acceleration of the object.
a = Δv/Δt
a = 8m/s/5s
a = 1.6ms-2

20. SPEED/TIME GRAPHS
In a speed/time graph the distance covered = the area under the graph.
Speed
(m/s) 4 8
Part 2
Part 3
What distance is covered in part 1?
What is the total distance covered?
d = v × t
d = 4m/s × 10s
d = 40 m
Part 4
Part 1
Part 1 = 40m
Part 2 = 0m
Part 3 = v × t ×  = 8m/s × 5s ×  = 20m
Part 4 = v × t = 8m/s × 10 s = 80m
Total = =140m
Time (sec) 10 20 30
Calculate the average speed 140/30 =4.66m/s

21. ACCELERATION QUESTIONS
What would these look like on a speed/time graph?
1. stopped
2. slow
3. fast
4. accelerating

22. ACCELERATION QUESTIONS
What would these look like on a speed/time graph?
1. stopped
2. slow
3. fast
4. accelerating

23. ACCELERATION QUESTIONS
What would these look like on a speed/time graph?
1. stopped
2. slow
3. fast
4. accelerating

24. FORCE PAIRS
Forces act in pairs (e.g. thrust and friction, gravity and support).
Force diagrams show the forces acting on an object and whether they are balanced or unbalanced.
Arrow size represents force size if no measurements are available.
Arrows should touch the object and only the object should be drawn – no surrounding stuff.
Force diagrams practice. Draw force diagrams for the following:
An apple sitting on a table
A parachute jump freefall, deploy
H/O problems 1a,b,c,d