Forces Notes Sp 11

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Name: ( ) Class:

SACSS - Sec 3E Science (Physics)

Dynamics (Forces)

Candidates should be able to:

(a) describe the effect of balanced and unbalanced forces on a body

(b) describe the ways in which a force may change the motion of a body

(c) identify forces acting on an object and draw free body diagram(s) representing the

forces

acting on the object (in at most 2 dimensions)

(d) recall and apply the relationship resultant force = mass x acceleration to new

situations or tosolve related problems

(e) explain the effects of friction on the motion of a body

(f) add two vectors to determine the resultant by a graphical method

What is a force

What is the !" unit of force ###############

1

What can a force do?


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Free Body Force Diagrams

A free body force diagram shows only the forces acting on a single body.

Types of

forces

What is it? How to draw it Example

Weight $ravitational

force acting on a

body%

&ertically

downwards from

the C$ of the

body%

'ormal

contactforce

force acting

perpendicular to2 surfaces in

contact with

each other%

erpendicular to

the surface*towards the side of

the body%

+riction force that

opposes the

motion or thetendency of

motion of a body

,etween the two

surfaces* in

opposite directionfrom the motion

(or tendency of

motion)%

ir

resistance

force exerted

by air on a body

to slow down its

motion%

-pposite to the

direction of motion

of the body%

.pthrust force that

pushes a body

upwards in a

fluid%

&ertically upwards

(for a fully

submerged body%)

/ension pulling force

exerted by a

long the string

and away from the

2


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string* spring*

chain etc%

object%

Can you draw a free body diagram of the following

Forces acting on a body Free Body Diagram

force of 20 ' is pushinga box to the right while aforce of 1 ' is pulling it tothe left%

pendulum bob weighing ' hanging from a string

Forces acting on a body Free Body Diagram

car moving with anengine force of 1000 'experiencing a frictionalforce of 300 '%

3 ' ball falling freely onearth and experiencing anair resistance of 2 '%

Resultant Force (F R) / Net Force

What is resultant force?

4esultant force is the force that acts on an object as a result of 2 or more

forces acting on the object%

56 What is the magnitude and direction of the resultant force acting on theobjects below

20' 30 '

(a) 23 ' (c)

63'

3


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(b) 7 ' (d) '

60 ' 8 ' 3' 8 '

When there is no resultant force acting on an object (F R = 0), we say that the forces acting

on the object is

When there is a resultant force acting on an object, we say that the object has

forces acting on it

!1 "#n object that has no resultant force acting on it $eans that there is no force acting

on it " %o you agree with the sentence& ' lain your answer

*ewton+s First aw of $otion states that when there is no resultant force acting on

an object, the object will continue in its state of or

*ewton+s -econ. aw of $otion states that the resultant force acting on an object is

the ro.uct of the object+s an.

an. the .irection of the resultant force is the sa$e as the object+s acceleration

/

Fig 1


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Summarising Newton s First and Second !aw"

When a resultant force acts on an ob#ect$ the ob#ect will

%%%%%%%% and mo&e in the direction of the

resultant force' /he acceleration is related to the resultant

force by the formula:

F R ma + =m =a =

When there is no resultant force acting on an ob#ect$

the ob#ect will either be stationary or mo&e with a

constant &elocity'

4esultant force = 0

ma = 0

since m cannot be 0 a = 0

the object will either move with a constant velocity or is at rest

F ma


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52: 60 ' of force is applied to an object with a mass of 9g and itstarts to move along a smooth floor% escribe the motion ofthe object ;ualitatively and ;uantitatively%

5 : What is the resultant force acting on a 300 g objectaccelerating at 3m


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accelerates at m


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difference in their magnitudes% /he direction of the resultant vectoris the direction of the vector with a bigger magnitude%

B%g%

(.) *dding &ectors when they are acting at an angle (otherthan 610 o)

(a) /riangle ethod

(b) arallelogram ethod

6

/*10 *


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1 What is the resultant force for the following set of forces&

2% Which diagram shows the resultant force of 8 ' and ' vectors

(A) (C)

(B) (D)

( )

10

F 1

F 2

F 1

F 2

F 1

F 2

F1

F 2

*3 N

/ *

*3 N

/ *

*3 N

/ *

*3 N

/ *


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8% /he diagram shows two of the forces (the 100 ' and 300 'forces) acting on a large stone which is being raised by a manusing a lever%

,y means of a scale diagram* determine the resultant force* F *of the two forces%

12

500 *

(force .ueto le:er)

500 N (weightof stone)

30

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Forces Notes Sp 11