Getting out of shape
Changing speed
Going round in circles

• They change an object’s
– speed
– shape
– Direction ( link )

• A force is a push or a pull exerted on an object
• So many sorts ...
– Pull of Gravity
– Push of ground upwards (Reaction)
– Electrostatic
– Magnetic
– Upthrust (on floating objetcs)
– Friction/air resistance/drag
– any more?

• Do things always move?
• So what’s happening?
• Forces can add together or oppose and cancel
• What else might happen?

– Cancel each other
– Have a zero resultant force
• What won’t happen?
• What else might happen?
• Balanced forces don’t affect overall motion

• As soon as something starts moving one force in particular appears out of nowhere and makes life awkward
• Friction: but is it always a nuisance?
– Rubber needs to grip the road or wheels spin and you get nowhere
– Brakes need to work so you can stop
• Understanding leads to control!
– Curving a football
– F1 cars

Force diagram Force A Force B
1 12 N to the left 7 N to the right
Size and direction of the resultant force
2 7N to the right 12 N to the right
3 7N to the left 7 N to the right
4 12 N to the left 12 N to the left

• Balanced forces?
• Stays still
• Unbalanced forces?
• Starts moving
• Already moving and unbalanced forces?
• Speed carries on changing.
• Already moving and balanced forces?
Imagine removing friction ...

• In a straight line, at rest or constant speed, unless
– an unbalanced force acts
• A resultant (balanced) force causes a change in speed or a change in direction or both.

Forces

• Once twisted, plasticine stays in the new shape or breaks - PLASTIC
• A rubber band regains its shape after being stretched and released
- ELASTIC
• An elastic material when released after being deformed returns to its original shape and size.
• Glass snaps - BRITTLE

• Set the apparatus up as shown in the diagram
• Place a 100g mass hanger on the end of the spring.
• Line up the top of the ruler, reading
0 cm, with the bottom of the mass hanger
• Now wear goggles!
• Add masses, 100g at a time and record the new length for each load, (so get a table ready!).
• Stop at a total of 1 kg
Label s needed!

• Provided the elastic limit is not reached:
– Straight line graph
– Passing through the origin
This is described as:
• a proportional relationship
• Doubling the load force doubles the extension.

• The way they stretch is so regular, they can be used to make a newtonmeter to measure forces.
• Mark the zero position
• Add 1 kg and mark the new position
• Divide the gap into 10 divisions
• Weigh something!
• This is called calibration

• Try stretching a rubber band gently then strongly.
• Try sketching the sort of graph you think you could draw for force against extension.
• Then hold it against your forehead and stretch it suddenly. What do you feel?
Resultant forces

• Apparatus and materials
•
For each student group:
• Retort stand, boss and clamp
• Mass hanger plus masses (100 g)
• Metre rule
• Selection of rubber bands, elastic cord
• Marker pen

• Measure the distance between the marks when the band is not stretched
• Add 50 or 100g and measure it again.
• Go on adding loads and recording the distance up to 600g
• Plot a graph of Load against Length

• Speed is a measure of how quickly something is moving speed  distance travelled time taken
• Actually, the above formula really tells you the average speed during the time interval
• As the time interval gets smaller, you get closer to calculating the instantaneous speed.

• This journey can be broken up into 5 stages.
• How far does he go in each stage?
• What is the total distance travelled?
• What is his average speed?

• Start
• Subjects
• Physics
• Force and Motion
• Displacement – time graphs

• In the “Forces and Motion” program sketch out this curve, as near as you can, and watch how the car moves.

• Design a journey for yourself. Be patient and learn how the program works.
• Note how far the car has gone at the end.
• Design a journey that gets you back to the starting point, but is different in how it gets you “there” and “back”.
• Print these off if you can!

• We can do this with a travel graph
– a plot of distance travelled vs time
• We must define a starting position and direction
– At starting position the distance=0
– One direction of travel is positive, the opposite direction is negative

• The velocity of an object gives its instantaneous speed and direction
– (This is called a VECTOR)
– Speed on its own is a scalar.
• As with displacement, the sign of the velocity indicates the direction
– Without direction, speed is a scalar
– a negative velocity means speed in the opposite direction

x
How would you represent something getting slower?
t
Note: distance can also become negative, if object travels in the opposite direction

• Going from A to B: + velocity
• Going from C to F: - velocity

• Copy the table
Time taken / s Distance run / m
0 0
1.65
2.76
3.71
4.63
10
20
30
40
5.52
6.38
7.23
8.09
8.96
9.83
50
60
70
80
90
100
• Draw a distance-time graph of the 100 m run.
• What is his average speed?
• What speed does he reach in the fastest 10m?
• What is his speed in the slowest 10 m?
• Describe from the graph how his speed changes during the race.

0 0
1.65
10
6.060606
2.76
20
9.009009
3.71
30
10.52632
4.63
40
10.86957
5.52
50
11.23596
6.38
60
11.62791
7.23
70
11.76471
8.09
80
11.62791
8.96
90
11.49425
9.83 100
11.49425
120
100
80
60
40
20
0
0 2 4 6 8 10 12
Series1

• The slope of the graph gives the speed
(strictly the velocity)
– slope  distance travelled time taken
(a) Slope = 60 / 10 = 6.0 m/s (b)
(b) Slope = 0 / 5 = 0.0 m/s
(c) Slope = 100 / 25 = -4.0 m/s
(a) (c)
(d)
(d) Slope = 40 / 15 = 2.7 m/s
• The steeper the line, the higher the speed

• Try to describe the motion shown in the graph
– What does the slope of the line represent?
– What does the slope of the dotted line tell you?

Constant speed backwards
Constant speed forward
Slope= average speed of return journey
Slope = speed
Speed=5/0.42=11.9 km/h stationary
After 160 minutes, we are back where we started

• Add up all the distances in each stage to get a total distance covered in the journey.
• What was his average speed over the whole journey?
• Can you work out his fastest speed forwards?
• Can you work out his fastest speed coming back?

• The weight of something is a measure of how strongly the Earth’s gravity pulls it down.
• The region where gravity acts is called a gravitational field .
• Gravity pulls more strongly on more massive objects, so we record the weight per kg as the field strength.

• Isaac Newton realised the significance of two simple ideas:
– It is the same force that holds moons in orbit as pulls things down on earth
– Every piece of matter attracts every other piece of matter

• “g” represents the strength of gravity, it varies from place to place.
• However far away you go, it just gets weaker and weaker but never vanishes completely.
 Earth: 10 N/kg
 Moon: 1.6 N/kg
 Mars: 3.8 N/kg
 Venus: 9.1 N/kg
 Mercury: 3.8 N/kg
 Pluto: 0.6 N/kg
 Sun: 271 N/kg
 100km asteroid: 0.04 N/kg

• Imagine a giant cannon firing a cannonball faster and faster
• The pull of gravity makes sure it lands ...
• Until it is going fast enough to fall round the earth!
• Gravity keeps the ball in orbit

• If we fire a satellite into orbit and don’t get the speed just right, it starts to overshoot before being pulled back...
• This distorts the orbit into an ellipse .
• Comets move in clear ellipses
• Most planetary orbits are almost circular.

• As they climb higher they lose KE and slow down.
• As they fall lower they lose GPE and gain
KE, speeding up
• Wherever they are gravity pulls them towards the centre of the Earth

• Moons go round planets
• Planets go round stars
• Stars revolve round galactic centres
• Hundreds of billions of stars in a galaxy
• Hundreds of billions of galaxies

• How fast is the Earth moving?
• Radius of orbit:
150,000,000 Km
• Time of orbit: ?
• Speed is: distance travelled time taken
• 2 πr / T
• 2x3.14x150,000,00
365x24
• km/hr