1. 3.1 Introduction to forces
What holds the globe?
Some basic understanding about forces
Daily life examples of forces
Free-body diagrams
Check-point 1 1 2 3
Book 2 Section Introduction to forces

2. Book 2 Section 3.1 Introduction to forces
What holds the globe?
Which kind of force is used to hold the globe in each case?
Book 2 Section Introduction to forces

3. Book 2 Section 3.1 Introduction to forces
What holds the globe?
tension
magnetic force
normal force by hand
How does force affect motion?
Book 2 Section Introduction to forces

4. 1 Some basic understanding about forces
A force is…
1 a push or a pull on an object by another object (interaction)
2 a vector—represented by an arrow of a certain length (magnitude) in a specific direction.
Book 2 Section Introduction to forces

5. 1 Some basic understanding about forces
3 exist in pairs – action and reaction
4 can be measured by a spring balance in newtons (N)
Book 2 Section Introduction to forces

6. 2 Daily life examples of forces
contact force
non-contact force
E.g. tension, friction
E.g. weight, magnetic force
Book 2 Section Introduction to forces

7. 2 Daily examples of forces
a Tension in a string or a spring
Stretching a string or a spring tightly
produces tension
E.g. Tug of war
Players experience tension in string.
tension
Book 2 Section Introduction to forces

8. a Tension in a string or a spring
E.g.: Stretching a rubber band
tension
Your hands experience tension.
Book 2 Section Introduction to forces

9. 2 Daily examples of forces
b Normal force
Normal force is the force acting perpendicularly by the surface on an object in contact with it.
E.g. going down a slide
normal forces
normal force perpendicular to the surface
Book 2 Section Introduction to forces

10. 2 Daily examples of forces
c Friction
When an object slides or tends to slide over another object, friction is produced.
E.g. pulling a table
Friction always opposes the motion or the tendency of motion.
Book 2 Section Introduction to forces

11. Book 2 Section 3.1 Introduction to forces
c Friction
Friction is essential to our daily life.
Applications of friction:
Walking
Opening a bottle
Book 2 Section Introduction to forces

12. Book 2 Section 3.1 Introduction to forces
c Friction
Friction brings inconvenience sometimes.
Ways to reduce friction:
ball-bearings
lubrication oil
air cushion
Book 2 Section Introduction to forces

13. 2 Daily examples of forces
d Weight
Weight:
– pulling force acting on an object by the earth
– a non-contact force
– always pointing vertically downwards to the centre of the earth
– measured in newtons
Book 2 Section Introduction to forces

14. 2 Daily examples of forces
e Other forces
Electric force and magnetic force can act at a distance.
non-contact forces
Electric force exists between electric charges.
Magnetic force exists between magnets or between magnets and magnetic objects.
Video
3.1 Floating Globe
Book 2 Section Introduction to forces

15. Book 2 Section 3.1 Introduction to forces
3 Free-body diagrams
A free-body diagram can show all forces acting on an object.
E.g. a person standing
W : weight of the person
R : normal force acting on the person by the floor
Arrow: direction
Length: magnitude
Book 2 Section Introduction to forces

16. Book 2 Section 3.1 Introduction to forces
3 Free-body diagrams
Example 1
Drawing free-body diagrams
Book 2 Section Introduction to forces

17. Book 2 Section 3.1 Introduction to forces
Example 1
Drawing free-body diagrams
Draw the free-body diagrams of the lantern in each case.
(a)
(b) T W W R
Book 2 Section Introduction to forces

18. Book 2 Section 3.1 Introduction to forces
Check-point 1 – Q1
Which of the following free-body diagrams correctly shows all the forces acting on the volleyball at the moment before serving? A B C
Book 2 Section Introduction to forces

19. Book 2 Section 3.1 Introduction to forces
Check-point 1 – Q2
Which is the correct free-body diagram of a book resting on an inclined plane? A B C D
Book 2 Section Introduction to forces

20. Book 2 Section 3.1 Introduction to forces
Check-point 1 – Q3
Draw and label the forces acting on the fridge magnet.
friction
normal force
magnetic force
weight
Book 2 Section Introduction to forces

21. Book 2 Section 3.1 Introduction to forces
The End
Book 2 Section Introduction to forces