1. Learning outcomes
describe the effect of balanced and unbalanced forces on a body
Solve problems for a static-point mass
Identify forces acting on an object and draw free-body diagrams

2. No resultant force due to balanced forces
What are the effects of forces on motion?
Balanced and Unbalanced Forces
When forces acting on an object is balanced, there is no resultant force.
No resultant force due to balanced forces

3. What are the effects of forces on motion?
Balanced and Unbalanced forces
When forces acting on an object is unbalanced, there will be a resultant force
Resultant force due to unbalanced forces

4. How are forces acting in 2 dimensions added?
Resultant Force (of two perpendicular force)

5. How are forces acting in 2 dimensions added?

6. 3.3 How are forces acting in 2 dimensions added?
Resultant Force (of two perpendicular force)

7. How are forces acting in 2 dimensions added?
Resultant force (of 3 forces acting on an object in 2 dimensions)
Unbalanced forces
When 3 forces are applied to a point of an object , the resultant force (due to it being unbalanced) acting on the object can be obtained by adding the force vectors
Vectors A,B and C represent three forces

8. How are forces acting in 2 dimensions added?
Resultant force (of 3 forces acting on an object in 2 dimensions)
Unbalanced forces

9. How are forces acting in 2 dimensions added?
Resultant force (of 3 forces acting on an object in 2 dimensions)

10. How are forces acting in 2 dimensions added?
Resultant force (of 3 forces acting on an object in 2 dimensions)
Balanced forces
When 3 forces acting on an object is balanced , resultant force is 0
There is no acceleration
An object will either stay at rest or move with constant velocity

11. How are forces acting in 2 dimensions added?
Resultant force (of 3 forces acting on an object in 2 dimensions)

12. How are forces acting in 2 dimensions added?
Resultant force (of 3 forces acting on an object in 2 dimensions)

13. How are forces acting in 2 dimensions added?
Resultant force (of 3 forces acting on an object in 2 dimensions)
Example
Tom, Dick and Harry are pulling a metal ring. Tom pulls with a force of 100 N and Dick pulls with a force of 140 N at an angle of 70° to Tom. If the ring does not move, by scale drawing or other wise, determine what force Harry is exerting.

14. Free-body diagram
A diagram which shows all the forces acting on a body is called a free-body diagram
Consists primarily of a sketch of a body and arrows representing the forces acting on it
Some of these examples are weight, contact force and tension
We can use these to analyse the motion of the body

15. Some common forces used in free-body diagrams
Example : Weight
Some common forces used in free-body diagrams
Free-body diagrams
Weight of a body The weight of a body is the gravitational force exerted by the Earth on the body. The weight acts at the center of gravity of a body. It is always directed towards the center of the Earth
The free-body diagram of a body falling without air resistance is shown below.

16. Some common forces used in free-body diagrams
Example : Contact force
Some common forces used in free-body diagrams
Free-body diagrams
Contact force The contact force is the force exerted by the contact surface on the body. It acts perpendicularly to the contact surface.
Two forces act on the body: they are the contact force and the weight. The free-body diagram is shown below.

17. Some common forces used in free-body diagrams
Example : Tension
Some common forces used in free-body diagrams
Free-body diagrams
Tension The tension is directed along the length of the string and pulls equally on the support and the pendulum bob. The diagram below is not a free-body diagram as it shows forces acting on three bodies: the pendulum bob, the string and the support. It will not be useful in analysis.
To isolate the pendulum bob as a free-body, the only contact force acting on the bob is the tension. Hence, the free-body diagram of the pendulum bob consists of the tension and weight only. This is illustrated by the diagram below.

18. Free-body diagram Example :
The free-body diagram of the crate would consist of
The weight of the crate,
The tension force exerted on the crate,
The contact force of the floor of the crate.

19. Free-body diagram Example :
Weight and contact force are equal. No resultant force acting vertically
Acceleration of the crate is due entirely to the tension force

20. Free-body diagram Example :
Situation for a man near the Earth
Now consider the situation of a man who has jumped up and is not in contact with the earth

21. How do we analyse the effects ?
Free-body diagram
Example :
Situation for a man near the Earth
The pair of action and reaction forces is the gravitational force by the Earth on the man and the gravitational force by the man on the Earth
There are 2 free-body diagrams : One for the man and the other for the Earth

22. Free-body diagram Example :
Free-body diagram for a man near the Earth
The free-body diagram of the man shows the force acting on him only

23. Free-body diagram Example :
Free-body diagram for the Earth near a man
The free-body diagram for the Earth shows the force that acts on the Earth only

24. Free-body diagram Example :
Situation for a man standing on the Earth
Now consider the situation of a man standing on the Earth

25. Free-body diagram Example : Situation for a man standing on the Earth
The first pair of action and reaction forces is the gravitational force by the Earth on the man and the gravitational force by the man on the Earth
The second pair of action and reaction forces is the contact force by the man on the Earth and the contact force by the Earth on the man
There are two free-body diagrams : One for the man and one for the Earth

26. Free-body diagram Example :
Free-body diagram for a man standing on the Earth. The man is in equilibrium because the forces on him are equal and opposite.
The free-body diagram shows two forces : F1 and F2 acting on the man.
Since the man is at rest , the two forces are equal and opposite in direction

27. Free-body diagram Example :
Free-body diagram for the Earth near a man. The Earth is in equilibrium because the forces on it are equal and opposite.
The free-body diagram for the Earth shows two forces , F1 and F2 acting on the Earth
Since F1 = F2 , the Earth is also at rest

28. Free-body diagram

29. Free-body diagram

30. Free-body diagram

31. Free-body diagram
Free-body diagram of the gymnast

32. Free-body diagram
Free-body diagram of the beam

33. Free-body diagram
Free-body diagram of the Earth