BB101 Engineering ScienceChapter 3 Force

FORCE BB101- ENGINEERING SCIENCE

UNIT SAINS JMSK PUO/DEC 2012 Page 24

3.0 : FORCE 3.1 The Concept Of Force Force and its units

Force can be defined as a push or a pull. Force is measured by N (Newton). A force is a vector quantity . A newton meter is used to measure force.

Example of force effects: (i) A change of movement ( external effect ) move, stop or accelerate

(ii) A change of shape (internal effect )

Weight and mass Mass and weight are different in physics. For example, your mass doesn't change when you go to the Moon, but your weight does.

Mass Weight

Mass shows the quantity weight shows the size of gravity, W = mg

m is mass in kg W is weight in Newton (N)



Newtons Second Law, F = ma

Newtons Second Law states that when the net force acting on an object is not zero, the object will accelerate at the direction of the exerted force. The acceleration is directly proportional to the net force and inversely proportional to the mass. It can be expressed in formula F = ma where:

F is the net force in N,

m is the mass of an object in kg and

a is its acceleration in m/s2.

Example:

Force in equilibrium

a) The resultant force, R acting on the object is zero.

b) Fx = 0 , Forces acting upward (F = forces acting downward(F c) Fy = 0 , Forces acting on the right (F = forces acting on the left(F



Simple Breakdown of Forces You can break down forces into several components easily. For example, the force F1 can be broken into two forces: Fx and Fy.

Magnitude of F1 =

Direction angle of F1 is

Example 1: The figure below shows three forces acting on a block. Calculate the resultant force.

)0cos1()0cos5()0cos4( 000 Fx )0sin1()0sin5()0sin4( 000 Fy

NFx )154( NFy 0

NFx 8

Resultant force =22 )0()8(( = 8 N to the right

Example 2: From Figure below, calculate FA and FB if the system in equilibrium 320

FA cos 320+ (- FB cos 0

0)+(2.5k cos 900)= 0 0.85 FA - FB + 0 = 0 0.85 FA = FB

FA sin 320+FB sin 0

0- 2.5k sin 900 0.53 FA+0- 2500=0 FA= (2500/0.53) FA = 4717 N

The following formulas are true: Fx=cosA*F1

Fy = sin A * F1

4 N

1 N 5 N

2.5 kN

FB

FA



0.85 FA = FB FB = 0.85(4717) =4009 N

3.2 The Concept Of Moment Of Forces

The moment of the force is the tendency of force to toist or rotate an object. Moments may cause objects to rotate clockwise or counter-clockwise.

MOMENT = FORCE X DISTANCE Or M = F x d . Unit : Nm Moments principal state that for equilibrium, the sum of the clockwise moments equals the sum of the counter-clockwise moments, and the sum of the forces up equals the sum of the forces down.

Centre of Gravitational for Equilibrium

Example:

Find the centre of gravitational ( x ).

Moments = Moments AND Force = Force



Resultant Moment

Resultant moment force = Total moment force acting

Example:

Tutorial 3.1 : the equilibrium of forces

1. The force A has magnitude of has a magnitude of 7.25 N. Find the component s for direction

angles of

a) = 5.00

b) = 125

c) = 245

d) = 340.0

2. Find the resultant force for the following:

(a) (b)

A

5N

12N

5N 12N

Calculate centere of gravity from point A.



12N

c)

3. Find the component of forces for the following:

4. Find the total forces that act on the R point

5. Find the resultant of forces for the following:

6.

Two horizontal forces of 300N each act on a 20kg

object as shown in Figure 3. If the angle between the

two forces is 60o, the object will move with an

acceleration of?

Figure 3



Tutorial 3.2 : moment of forces

1. Find the centre of gravitational (x) for the following:

Answer: Tutorial 3.1- Equlibrium of Forces

1. a) Ax = 7.22 N, Ay = 0.632 N b) Ax = -4.16 N, Ay = 5.94 N

c) Ax = -3.06 N, Ay = -6.57 N d) Ax = 6.81 N, Ay = -2.48 N

2. a) 17 N b) 7 N to the left (-7N)

3. a) Fx = 5N, Fy = 8.66 N b) Fx = 3N, Fy = 5.196 N

4. Resultant Force = 8.095 N 5.Resultant Force = 10.63 N

6. Acceleration = 25.98 ms-1

Answer: Tutorial 3.2- Moment of Forces

1. a) x = 4.0 m , b) x = 3.93 m c) x = 7.2 cm

b.

a.



Minimum requirement assessment task for this topic:

1 Theory Test & 1 End Of Chapter Specification of Theory Test : CLO1 - C1 & CLO3 (C2, A1) Specification of End Of Chapter: CLO3 - (C2,A1) *****************************************************************************************

COURSE LEARNING OUTCOME (CLO)

Upon completion of this topic, students should be able to:

1. Identify the basic concept of force, (C1)

2. Apply the concept of force in real basic engineering problems. (C2,A1)

Compliance to PLO : PLO1 , LD1 (Knowledge) Test 1 PLO4, LD4 (Critical Thinking and Problem Solving Skills) Test 1, End Of Chapter 1

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BB101 Engineering ScienceChapter 3 Force