JLAL3 Squashing and Stretching- done.pptx
Items in this lesson
What do we know about squashing and stretching of objects?
What is happening here?
Changing Shape
Forces can change the shape of solid objects or stretch them
Which one is showing a solid?
Check
How many bonds can you see in this solid?
Bonds are elastic. What does this mean?
12
Changing Shape
What happens to the particles as this solid object is stretched?
What happens to the particles as this solid object is pushed back?
The bonds are stretched so the particles move further apart
The bonds are compressed so the particles move closer together
When the object changes shape we say it deforms.
Check
What is happening to this golf ball?
Changing Shape
The ball exerts a force on the man to the right
Interaction pair: the man exerts a force on the ball to the left
Changing Shape
What happens to the rope when you sit on a swing?
What is this force called?
It stretches tight
Tension
What happens to a stationary shape under multiple forces?
It may deform
It becomes heavier
It moves in a new direction
It remains unchanged
What can cause deformation in an object?
Multiple forces acting together
Heat exposure
Single force
Lack of forces
What is the term for shape change due to force?
Extension
Deformation
Tension
Compression
What happens to bonds during compression?
They are squashed together
They remain unchanged
They break
They are stretched apart
What force pushes back against weight?
Normal reaction force
Gravity
Tension
Friction
What is the effect of tension on bonds?
They are stretched
They are compressed
They lock together
They break
What is twisting in deformation called?
Torsion
Bending
Extension
Compression
Check
deforms
compress
stretch
Check
2
1
4
3
Try & Apply (p.17)
Try & Apply (p.17)
The bonds are compressed in the solid on the right so this solid has had a force acting on it.
When you stretch a rope bonds move further apart. They pull back together – this is tension.
When the football applies a force on the head the bonds in the head are compressed. They push back with a reaction force.
Changing Shape
Changing Shape
Hooke’s Law
Key Question: Explain what happens when you stretch a spring
*
Hooke’s Law
This spring is obeying Hooke’s Law.
This spring isn’t.
6 cm
12 cm
10 N
20 N
4 cm
8 cm
9 N
24 N
Hooke’s Law
This spring is obeying Hooke’s Law.
This spring isn’t.
4 cm
12 cm
3 N
9 N
2 cm
5 cm
7 N
14 N
What is Hooke’s Law?
If you double the force, you double the extension
Elastic Limit
This spring has reached its elastic limit.
Suggest what that means.
It has been permanently deformed and no longer obeys Hooke’s Law
Hooke’s Law
Hooke’s Law
Hooke’s Law
Hooke’s Law (p.16)
Check
10
At what force did this spring stop obeying Hooke’s Law?
Check
extension
Is this spring obeying Hooke’s Law?
Check
How could you find out if this spring is obeying Hooke’s Law?
Tasks (p.16)
1.1 cm
1.1 cm
when the force/mass is doubled, so is the extension
+
+
+
+
+
+
+
the graph is a straight line (passing through 0)
Try & Apply (Pg.21)
Try & Apply (p.17)
The extension is proportional to the force
/ if the force doubles so does the extension
It deforms and won’t return to its original length
the graph is not a straight line / the extension is not proportional to the force
Try & Apply (p.17)
If they are stretched past their elastic limit they will deform and change length. The jumper might hit the ground.
Investigating Hooke’s Law
How can this equipment be used to investigate Hooke’s Law?
Writing a method
A good method should include:
A list of the apparatus needed.
A clear step-by-step method for carrying out the experiment.
A diagram of how the apparatus will be set up.
Any safety precautions required to reduce risk.
Writing a method
What do you like about this method?
Equipment: stand, elastic band, mass holder, masses (10 g), ruler
Method:
Hang the elastic band on the arm of the retort stand.
Hang the empty mass holder onto the elastic.
Measure and record the original length of the elastic band.
Add four 10 g mass to the mass holder.
Measure and record the new length of the elastic band.
Repeat steps 4 and 5 for four additional 10 g masses.
Safety: Wear safety glasses at all times in case elastic pings off the stand or snaps.
Investigating Hooke’s Law
Hang the empty mass holder from the elastic band and measure the distance between dots
Add 40g and measure the new distance between the dots
Add another 40g and again measure the new distance
L1
L2
L0
Groups of 3
Investigating Hooke’s Law
Results table:
Conclusion?
More Practice
