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Year 10
May the force be with you!
Types of Force
• A contact force requires objects to be
touching for the force to have an effect.
• A field force will act at a distance – the
field is the area within which the force has
an effect
Types of force
Contact
• Push
• Pull
• Twist
Field
• Magnetic
• Gravity
• Static electricity
Come up for a example for each.
Forces acting upon objects.
Gravity
Thrust
Friction
Support
Balanced Forces
• Results in either no
motion or constant
motion.
600N
600N
Unbalanced Forces
600N ↑
= 200N ↑
400N ↓
• Results in
acceleration; a
change in motion
(speeding up or
slowing down)
Calculating net force.
• The net force is the difference in the
amount of force acting upon an object.
• The object will move in the direction of the
bigger force.
• If all forces are balanced the net force will
be 0N.
10N
10N
10N
10N
• If the forces are unbalanced, there will be
a net force acting upon the object.
10N
50N
20N
10N
Net force 30N
• If the car is moving forward with 50N of
force and experiencing 20N of friction
pulling it backwards; the net force will be
30N forwards (acceleration).
10N
40N
50N
Net force 10N
10N
• If the car is moving forward with 40N of
force and experiencing 50N of friction
pulling it backwards; the net force will be
10N backwards (reversing).
• If the object has two net forces, the
movement will go in the direction of the
combined net force.
Net force 2N
Net force 45N
• Movement will be forwards and down.
Calculate the net force acting on
these objects.
A
B
10N
10N
10N
10N
C
20N
20N
25N
5N
D
10N
10N
15N
5N
5N
20N
55N
5N
10N
Distance/time graphs.
d/t graphs
Calculating Forces- using a
formula.
• To calculate the size of an unbalanced
force acting on an object, you need to
know its mass and how much it
accelerates (changes its speed) due to the
net force.
F
• F = ma
m
÷
a
x
You can figure out any of the
three factors.
• Using the triangle!
• Cover up the factor you want to calculate.
In this case force,
F
the triangle tells you to use;
m a
mxa
• Need to figure out mass?
Now you can use F ÷ a
F
m a
• Need to figure out acceleration?
Now you can use F ÷ m
F
m a
Calculate these equations.
1. An unbalanced force causes a 1200kg
car to accelerate at 2ms-2. What is the
size of the net force?
2. A 0.25kg soccer ball is kicked at 8ms-2.
What is the size of the net force?
3. A 50N force is acting on a skateboard
which is accelerating at 5ms-2. What is
the mass of the skateboard?
4. A cart with a mass of 50kg is moved with
a force of 100N, what will the acceleration
be?
5. A 3000N force is required to move a
cannon at 0.5ms-2, what is the mass of the
cannon?
Calculate these equations.
1.
2.
3.
4.
5.
2400N
2N
10kg
2ms-2
6000kg
Force = motion?
• So as we have seen both balanced and
unbalanced forces can result in motion.
• When an object moves it changes its
location in an amount of time.
• In order to work out how fast the motion is
occurring, we use the following formula;
speed = distance ÷ time
d
s
t
Speed of the trolley.
• Once we have recorded the distance the
trolley travelled every 0.1s, we can;
1. Create a d/t graph.
2. Calculate the overall speed of the trolley.
3. Calculate the instantaneous speed of the
trolley.
4. Create a v/t graph.
Units of speed.
• The two commonly used units for speed
are; metres per second (ms-1) and
kilometres per hour (kmhr-1).
• Sometimes you will need to convert your
measurements to fit one of the two units.
• E.g. 100km in 30 minutes becomes 100km
in 0.5hr= 100/0.5 = 200kmhr-1
• 5km in 30s becomes 5000m in 30s=
5000/30 = 166.6ms-1
Use formula to solve…
1. A car travels 50km in 30 minutes, what is
its speed?
2. A bus travels 100 km at 50kmhr-1, how
long does it take?
3. A bike travels at 5ms-1 for 5 minutes, how
far has it travelled?
Use the formula to solve…
• 100kmhr-1
• 2 hours
• 1500m or 1.5km
Distance/Time Graphs
• When drawing distance/time graphs:
• Time always goes along the horizontal (x)
axis.
• Distance always goes up the vertical (y)
axis
• A flat line (slope = 0) means the object is
stationary.
d
t
• A slope means the object is moving.
The slope gives the speed of the object.
d
t
• A curved line means the object is
accelerating
d
(speeding up or slowing down).
t
Drawing a distance time graph.
Distance (m)
Time (s)
0
0
10
5
20
15
30
20
40
25
50
30
60
45
70
55
80
70
• The distance time
graph will show the
motion of this car
race.
Motion of a car in a race.
Working out speed from a
graph.
• You can calculate the speed of an object
by reading the graph gradient, e.g. rise
over run for squares and rectangles.
• You can also use the area under the graph
for triangles by using ½ base x height.
• It’s a bit complex, let’s break it down
now…
Experiment; motion of a cart.
• Set up the ticker tape timer and physics
trolley along the bench.
• Measure out 1m of ticker tape timer tape
and attach to the trolley.
• Accelerate the trolley down the bench and
remove the tape.
• Rule the tape into 4 dot sections thusly;
C
V
C
V
C
V
C
V
C
V
C
V
C
V
C
V
C
V
C
V
Start of tape
• Number each section from the start of the
tape.
Create a graph.
• Cut each section down the line (through
each 5th dot).
• Line up along the bottom of the page
along the time axis & glue.
• Measure the length of each section and
record it on the strip.
What does that mean?
• You have created a distance time graph
for your trolley’s journey.
• Now you have the distance for each strip
you can calculate the speed for each part
of the journey (each paper section took
0.1s).
• Use the formula to work out the speed for
each section of the journey.
What next?
• Speed time graphs!
• Speed time graphs show how fast objects
are moving over time.
s
t
Speed/time Graphs
• Time always goes along the horizontal (x) axis
• Speed or velocity always goes up the vertical (y) axis\
• A flat line (slope = 0) means
the object is travelling at a
constant speed
(no acceleration)
s
t
• A slope means the object is
accelerating. The slope gives
the acceleration of the object
s
• A curved line means the
rate of acceleration is
increasing or decreasing
s
t
t
s/t
Speed (ms-1)
Time (min) Speed
(ms-2)
25
0
0
1
5
2
10
3
15
4
20
5
5
20
0
6
20
7
20
8
20
9
5
10
0
20
15
10
1
2
3
4
5
6
Time (min)
7
8
9
10
11
Turn your trolley data into a s/t
graph.
Speed (ms-1)
Time (min)
Exercise.
• Complete the 2 speed/time graphs for the
motion of the skateboard and the train.
Acceleration.
• Any change in speed is acceleration.
• Acceleration = Final speed – Initial speed
Time
• Speeding up; positive number
• Slowing down; negative number
Exercises: show all workings.
1. A bike starts at 0ms and speeds up to
10ms in 10 seconds. What is the
acceleration? 1ms-2
2. A car starts at 100ms and brakes to 2ms
in 20 seconds. What is the acceleration?
-4.9ms-2
Force, Mass & Acceleration
• When an unbalanced force acts on an object, it accelerates in
the direction of the net force.
• The equation is:
force = mass x acceleration
Fnet
= ma
• The same equation also applies to the effect of gravity on
mass (ie. weight).
weight force = mass x acceleration due
to gravity
Fw = mg
= m x10N (if mass is in kg)
• The unit for weight force is the Newton (N).
Pg 19
F= ma
Mass (kg)
Acceleration
(cms-1)
1
0.6
1.2
5.2
1.4
16.3
1.6
21.3
1.8
23.4
2
26.2
Force (N)
Friction.
Friction is a force that opposes motion.
It is created when objects rub against each other,
releasing energy as heat.
Friction between an object and air or water is
called drag.
Friction can be:
• useful – e.g. brakes, tyres
• undesirable – e.g. engine wear
Friction can be reduced by using:
• lubricants
• bearings
Experiment; friction blocks.
Aim:
To investigate the friction force between different surfaces on a friction block
and the bench.
Hypothesis:
Which side of the block will create the most friction force?
On which surface?
Method:
1.
Attach a force meter to the hook on the block.
2.
Put a 1kg mass in your block.
3.
Place your block on the bench with the wooden side down.
4.
Pull on the force meter and record the force at
which your block starts to move. Repeat 3 ×.
5.
Flip block to second side and repeat, repeat for 2 remaining sides.
5.
Calculate the average force used for each block side
on each surface.
Results: record your results in a table.
Graph: create a graph
Conclusion: write a conclusion that links your results to your aim.
Force (N)
Block surface
Experiment; shoes.
Aim:
To investigate the friction force between your shoe and different surfaces.
Hypothesis: which surface will create the most friction?
Method:
1.
Attach a force meter to your shoe.
2.
Put a 1kg mass in your shoe.
3.
Place your block on the lino and record the force to move 1m.
4.
Repeat twice more.
4.
Repeat for carpet & concrete.
5.
Calculate the average force used for each shoe on each surface.
6.
Repeat whole experiment with your lab partners shoe.
Results: record your results in a table.
Graph: create a graph.
Conclusion: write a conclusion that links your results to your aim.
Graph: averages only
50
45
40
35
30
25
20
15
10
5
0
Lino
Concrete
Carpet
Shoe 1 Shoe 2
Force and pressure
• Pressure is created when a force is applied to an area.
• The formula is; Pressure = force / area
P=F
A
• The greater the force and the smaller the area, the greater the
pressure.
10N
8cm 2
P = 10/8
P=1.25Nm-2
2
cm2
P= 10/2
P= 5Nm-2
High heels.
• A 50kg woman stands in a pair of high
heels.
• The area of each heel is 2cm x 2cm and
she stands with the front of her foot off the
ground.
• How much pressure is going into the
ground?
Steps to solve;
8cm2 is the area of the 2 shoes.
Convert to m2; 0.0008 m2
Calculate her force; F=mg
50 x 10 = 500N
P = F/A
P = 500/ 0.0008
P = 625,000Nm-2
Air pressure as shown on an isobar map.
Machines
Machines make work easier by spreading the
same effort over a greater distance. For example,
you can either lift a load straight up or wheel it up
a ramp, but because the ramp is a greater
distance, it takes less effort.
Examples of machines include:
• gear wheels
• pulleys
• ramps
• levers
Levers.
• Levers transform small forces into big
forces.
• The effort force is the one you apply,
the load force is the one that lifts the
object.
fxd
Load force
=
fxd
Pivot
Effort force
Levers 2.
• The effort force will be smaller than the
load force
Turning forces.
• A turning force is used to make an object
rotate around a fixed point.
• The fixed point is called the fulcrum.
Ramps.
• A ramp partially supports object’s weight,
making the amount of force needed to
move something, less.
Support force
Net force
Weight force
Pulleys.
• Can magnify a weak force.
• The more pulleys the less effort.
1N
2N
Load
Number of
Pulleys
Effort
20N
1
20N
20N
2
10N
20N
4
5N
20N
5
4N
20N
10
2N
Gear wheels
• As one cog turns, it
turns the other.
• The gear ratio is the
number of teeth on
the big cog : number
of teeth on the small
cog.
• E.g. 28:7 becomes…
• 4:1
Levers
Ramps
Pulley
Gears
Force/change of state activity
• Mini steam engine.
• http://www.muychingon.com/2007/04/07/st
eam-engine-from-a-votive-candle/
Ek & E p
• Energy and force are often related, at it
takes an unbalanced force to begin motion
and all moving objects have kinetic
energy.
• Objects which move from a raised place
also have gravitational potential energy.
How do we calculate Ek & Ep?
• Potential energy = mass x gravity x
change in height
ΔEp = mgΔh
• Kinetic energy = ½ mass x velocity
squared.
Ek = ½ mv2
• Mass (kg), Velocity (ms-1)
Using these calculations.
• Raising the ramp!
Car & ramp experiment.
• Follow the car and ramp experiment and
complete the calculations.
Calculating energy.
• Complete the experiment to calculate the
energy of a toy car.
Ramp height
(cm)
10
20
30
40
50
Observations
Ep (J)
V (ms-1)
Ek (J)
• If the car weighed 0.0288kg, gravity is
10ms-2 and it was set off from 10cm the
equation would be;
•
•
•
•
Ep = 0.0288 x 10 x 10 = 2.88J
V2 = 2.88 / 0.0144 = 200
V = √200 = 14.1 ms-1
Ek = 0.5 x 0.0288 x 14.12 = 2.9J (rounded
to 1 d.p.)
What do you notice? If there is no friction,
we see conservation of energy.
Crusty demons.
Bungee jumping.
Revision time!
• Complete the revision sheet using the 2
colour pen method.
• Next lesson we will compete in the
revision quiz show challenge!