In each question write down the value of component X

8N

X

80o

In each question write down the value of component X

5N

X20o

In each question write down the value of component X

7N X

18o

In each question write down the value of component X

4N

X

70o

In each question write down the value of component X

12N

X

15o

In each question write down the value of component X

30N

X50o

In each question write down the value of component X

8N

X

15o

In each question write down the value of component X

25N

X

40o

In each question write down the value of component X

12N

X

70o

In each question write down the value of component X

40NX

130o

In each question write down the value of component X

25N

X

150o

The Mechanics of Forces

Lesson Objective:

Understand Newton’s first law and be able to solve problems involving objects in equilibrium

Newton’s first law:Every body continues in a state of rest or in a state of uniform motion in a straight line unless acted on by a resultant force

A body that exhibits this type of behaviour is said to be in equilibrium

Key aspects for solving problems involving objects in equilibrium:1)Always draw a clear diagram of the objects2)Add all the forces acting on the objects to the diagram3)Write down equations for each separate object by resolving the forces on each object in two perpendicular directions4)Look at all the equations you have written down and solve them simultaneously to answer the question

Note: In essence we will do this same procedure for every type of question involving forces. As the situations become more complicated (not in equilibrium for example) the equations we write down will just be adjusted as necessary.

10 N

7 NA ?

B ?

What can you say about the size of forces A and B?

A particle resting in equilibrium with 4 forces acting on it:

30o

What can you say about the size of forces A and B now?

10 N

7 NA ?

B ?

A particle resting in equilibrium with 4 forces acting on it:

60o

What can you say about the size of forces A and B now?

10 N

7 N

A ?

B ?

A particle resting in equilibrium with 4 forces acting on it:

45o

60o

8 N

4 N

A ?

B ?

30o60o

B ?4 N

A ?

45o

40o

B ?A ?

15 N

30o

θo5 N

12 N

30o

60o

A particle is in equilibrium under the action of the forces shown. Find the magnitude of the forces P and S.

The forces in the diagram below are in equilibrium. Find R and θ.

The forces in the diagram below are in equilibrium. Find R and θ.

3tan436.9

3sin5 N

R

The forces below are in equilibrium. Find the magnitude of P and Q

The forces below are in equilibrium. Find the magnitude of P and Q