2.5 FORCE
Example:
Balanced Force
When the forces acting on an object are balanced
, they cancel each other out. The net force
is zero
. Effect
: the object at is at rest
[ velocity = 0] or moves at constant
velocity
[ a = 0]
Weight, W = Lift, U Thrust, F = drag, G
When the forces acting on an object are not
balanced
, there must be a net force
acting on it. The net force is known as the unbalanced
force
or the resultant force.
Unbalanced Force/ Resultant Force
Effect : Can cause a body to
-
change it state at rest
(an object will accelerate
-
change it state of motion
(a moving object will decelerate or change its direction)
Force, Mass & Acceleration
The acceleration produced by a force on an object is directly proportional
to the magnitude of the net
force
applied and is inversely proportional to the mass of the object. The direction of the acceleration is the same as that of the net force
.
Newton’s Second Law of Motion
When a net force, F, acts on a mass, m it causes an acceleration, a.
Force = Mass x Acceleration F = ma
Relationship between a & F
a
F The acceleration, a, is directly proportional to the applied force, F.
Relationship between a and m
m
a
1
∝
The acceleration of an object is inversely proportional to the mass,
Experiment to Find The Relationship between Force, Mass & Acceleration
Relationship between
a & F a & m
Situation
Both men are pushing the same mass but man A puts greater effort. So he moves faster.
Both men exerted the same strength. But man B moves faster than man A.
Inference The acceleration produced by an object depends on the net force applied to it.
The acceleration produced by an object depends on the mass
Hypothesis The acceleration of the object increases when the force applied increases
The acceleration of the object decreases when the mass of the object increases
Variables:
Manipulated : Responding : Constant :
Force Acceleration Mass
Mass Acceleration Force
Apparatus and Material
Ticker tape and elastic cords, ticker timer, trolleys, power supply and friction compensated runway and meter ruler.
An elastic cord
is hooked over the trolley. The elastic cord is stretched until the end of the trolley. The trolley is pulled down the runway with the elastic cord being kept stretched by the same amount of force
An elastic cord is hooked over a trolley
. The elastic cord is stretched until the end of the trolley. The trolley is pulled down the runway with the elastic cord being kept stretched by the same amount of force
2.6 IMPULSE AND IMPULSIVE FORCE
Impulse The change of momentum mv - mu Unit : kgms-1 or Ns
Impulsive Force
The rate of change of momentum in a collision or explosion
Unit = N
m = mass u = initial velocity v = final velocity t = time
Longer period of time
Impulsive force decrease
Effect of time
Impulsive force is inversely proportional to time of contact
Shorter period of time
Impulsive force increase
Situations for Reducing Impulsive Force in Sports
Situations Explanation
Thick mattress with soft surfaces are used in events such as high jump so that the time interval of impact on landing is extended, thus reducing the impulsive force. This can prevent injuries to the participants.
Goal keepers will wear gloves to increase the collision time. This will reduce the impulsive force.
A high jumper will bend his legs upon landing. This is to increase the time of impact in order to reduce the impulsive force acting on his legs. This will reduce the chance of getting serious injury.
A baseball player must catch the ball in the direction of the motion of the ball. Moving his hand backwards when catching the ball prolongs the time for the momentum to change so as to reduce the impulsive force.
Situation of Increasing Impulsive Force
Situations Explanation
A karate expert can break a thick wooden slab with his bare hand that moves at a very fast speed. The short impact time results in a large impulsive force on the wooden slab.
A massive hammer head moving at a fast speed is brought to rest upon hitting the nail. The large change in momentum within a short time interval produces a large impulsive force which drives the nail into the wood.
A football must have enough air pressure in it so the contact time is short. The impulsive force acted on the ball will be bigger and the ball will move faster and further.
Pestle and mortar are made of stone. When a pestle is used to pound chilies the hard surfaces of both the pestle and mortar cause the pestle to be stopped in a very short time. A large impulsive force is resulted and thus causes these spices to be crushed easily.
Example 1 A 60 kg resident jumps from the first floor of a burning house. His velocity just before landing on the ground is 6 ms
-1
. (a) Calculate the impulse when his legs hit the ground. (b) What is the impulsive force on the resident’s legs if he bends upon landing and takes 0.5 s to stop? (c) What is the impulsive force on the resident’s legs if he does not bend and stops in 0.05 s? (d) What is the advantage of bending his legs upon landing?
Example 2 Rooney kicks a ball with a force of 1500 N. The time of contact of his boot with the ball is 0.01 s. What is the impulse delivered to the ball? If the mass of the ball is 0.5 kg, what is the velocity of the ball?
source:http://notaspmfizik.blogspot.com/2011/03/spm-f4-forces-motion.html
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