Tuesday, August 31, 2010

Questions that needs answers!!!

1. A ball is dropped from a height of 5.0m above the ground( take acceleration due to gravity to be 10 ms-2) Answer=10ms-1

2.A roller coaster of mass 500kg approaches the bottom of a loop at a speed 25ms-1. the loop can be approximated as a circle with radius 5.0m

(a) Calculate the kinetic energy of the roller coaster at the bottom of the loop
(b)Calculate the speed of the roller coaster when it reaches the top of the loop

3. A frictionless hydraulic press is used to lift a car. The area of piston X is 0.80m2 while the area of piston Y is 10m2. The total mass of the call in the piston Y is 1200 KG. Take g=10m s-2

(a) calculate the minimum force exerted by piston X to lift the car
(b) calculate the distace moved by piston X if the car is lifted by 0.40 m

Slide 28
Lastly i cant understand these formulas in ur blog, could you pls abbrivate the 2,3 and 4th formula. It will be helpful to me....
Pressure = Force/Area
=Weight/Area
=mg/A
=pVg/A
= pAhg/A
= hpg

Wednesday, July 21, 2010

Physics Chapter - Turning Effect Of Forces

    Pure Physics -Turning Effects of Forces (Moments)

    Definitions

    • Moment Force Perpendicular Distance -The moment of a force is the turning effect of a force, or the ability of the force to make something turn.
    • Moment of a force (M) about a point O is the product of the force (F) and the perpendicular distance (D) from the point to the line of action of the force.
    • SI unit: Newton (N)
    • Moments can be clockwise or anticlockwise.
    • The turning effect of a force depends on:
    • Location of applied force
    • Perpendicular distance between the point of application of the force and the pivot.

    Principle of Moments

    The principle of moments state that:
    When a body is in equilibrium, the sum of clockwise moments about the balanced point is equal to the sum of anticlockwise moments about the same point (pivot).Total clockwise moment = Total anticlockwise moment.

    • When the clockwise moment is not equal to the anticlockwise moment, there is a resultant moment. The object will rotate in the direction of resultant moment.
    • Therefore, if there is no resultant moment, the object is balanced!

    Center of Gravity (c.g.)

    • Definition: The center of gravity (CG) of a body is an imaginary point where the whole weight of the body seems to act in any orientation.
    • The CG of a regular object is at the center.
    • The CG of an irregular object is determined using a plumb line.
    • If a body is hanging freely at rest, its center of gravity is always vertically below the pivot, thus the plumb line method works. It can only be used for flat, irregular objects.

    Stability

    • Stability is a measure of the body’s ability to maintain its original position.
    • There are 3 types of stability:

    Stability Type Effect Explanation- Stable Equilibrium Object will return into original position after slight disturbance. Weight will generate an anticlockwise moment by bringing the cone back to its original position (done by the restoring moment).These types of objects usually have low CG and big/heavier bases.

    Unstable Equilibrium Object will topple/fall after slight disturbance .The weight of the cone will generate a clockwise moment outside the base area of the cone, thus there is a resultant moment and the object will fall.

    Neutral Equilibrium Object remains in new position after slight disturbance .The center of gravity neither rises nor falls, it remains at the same level. The lines of action of the 2 forces always coincide and there is no moment provided by weight to turn the cone.
    • Ways to improve stability of an object:
    • Lowering the CG (A lower CG will allow the line of action to act within the base area of an object)
    • Area of its base should be as wide as possible (allow line of action to act within base area)

Wednesday, July 14, 2010

I tried my best but coudnt find answers to these questions!! help pls!
Know the reason, tell me!!

1. A hollow ball is release from the bottom of a pool. After a short time it rises at a constant speed. What is the resultant force then acting on the ball

2. When a block of wood of mass 2kg was pushed along the horizontal flat surface of a bench, the friction force measured was 4n. when the block was pushed along the same bench with a force of 10 N, find the motion?

3. A block of mass 6 Kg is pulled across a rough surface by a 54 N force, against a friction force?
The acceleration of the block is 6m/s (squared). What is the value of F?

4.A person just supports a mass of 20 KG suspended from a rope.
What is the resultant force acting on the mass?

5. The total weight of a gas-filled ballon is 1500N. The ballon rises at a constant speed of 3m/s.
What is the resultant force acting on the balloon while it is rising?

6. An experiment is carried out by an astronaut on the surface of the moon. Acoin and a feather are dropped at the same instant from the same height?
Which item(s) will reach first and why?

And mainly how do you use the two methods such as tip-to-tail method and the other one? can you tell me a website where it is demonstrated clearly??

Saturday, July 10, 2010

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
Relationship between a & F
a α F
The acceleration, a, is directly proportional to the applied force, F.
Relationship between a and m ma1∝
The acceleration of an object is inversely proportional to the mass,

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

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
Determine the acceleration by analyzing the ticker tape.
Acceleration tuva−=
Determine the acceleration by analyzing the ticker tape.
Acceleration tuva−=
Procedure :
- Controlling manipulated variables.
- Controlling responding variables.
- Repeating experiment.

1. What force is required to move a 2 kg object with an acceleration of 3 m s-2, if
(a) the object is on a smooth surface?
(b) The object is on a surface where the average force of friction acting on the object is 2 N?
2. Ali applies a force of 50 N to move a 10 kg table at a constant velocity. What is the frictional force acting on the table?
3. A car of mass 1200 kg traveling at 20 m/s is brought to rest over a distance of 30 m. Find
(a) the average deceleration,
(b) the average braking force.
4. Which of the following systems will produce maximum acceleration?
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
19
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.
20
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?
21

2.7 SAFETY VEHICLE
Component
Function
Headrest
To reduce the inertia effect of the driver’s head.
Air bag
Absorbing impact by increasing the amount of time the driver’s head to come to the steering. So that the impulsive force can be reduce
Windscreen
The protect the driver
Crumple zone
Can be compressed during accident. So it can increase the amount of time the car takes to come to a complete stop. So it can reduce the impulsive force.
Front bumper
Absorb the shock from the accident. Made from steel, aluminium, plastic or rubber.
ABS
Enables drivers to quickly stop the car without causing the brakes to lock.
Side impact bar
Can be compressed during accident. So it can increase the amount of time the car takes to come to a complete stop. So it can reduce the impulsive force.
Seat belt
To reduce the inertia effect by avoiding the driver from thrown forward.
22

2.8 GRAVITY
Gravitational Force
Objects fall because they are pulled towards the Earth by the force of gravity.
This force is known as the pull of gravity or the earth’s gravitational force.
The earth’s gravitational force tends to pull everything towards its centre.
Free fall
�� An object is falling freely when it is falling under the force of gravity only.
�� A piece of paper does not fall freely because its fall is affected by air resistance.
�� An object falls freely only in vacuum. The absence of air means there is no air resistance to oppose the motion of the object.
�� In vacuum, both light and heavy objects fall freely. They fall with the same acceleration ie. The acceleration due to gravity, g.
Acceleration due to gravity, g
�� Objects dropped under the influence of the pull of gravity with constant acceleration.
�� This acceleration is known as the gravitational acceleration, g.
�� The standard value of the gravitational acceleration, g is 9.81 m s-2. The value of g is often taken to be 10 m s-2 for simplicity.
�� The magnitude of the acceleration due to gravity depends on the strength of the gravitational field.
Gravitational field
The gravitational field is the region around the earth in which an object experiences a force towards the centre of the earth. This force is the gravitational attraction between the object and the earth.
The gravitational field strength is defined as the gravitational force which acts on a mass of 1 kilogram. mFg= Its unit is N kg-1.
23
Gravitational field strength, g = 10 N kg-1
Acceleration due to gravity, g = 10 m s-2
The approximate value of g can therefore be written either as 10 m s-2 or as 10 N kg-1.
Weight
The gravitational force acting on the object.
Weight = mass x gravitational acceleration
W = mg SI unit : Newton, N and it is a vector quantity
Comparison between weight & mass
Mass
Weight
The mass of an object is the amount of matter in the object
The weight of an object is the force of gravity acting on the object.
Constant everywhere
Varies with the magnitude of gravitational field strength, g of the location
A scalar quantity
A vector quantity
A base quantity
A derived quantity
SI unit: kg
SI unit : Newton, N
The difference between a fall in air and a free fall in a vacuum of a coin and a feather.
Both the coin and the feather are released simulta-neously from the same height.
At vacuum state:
There is no air resistance.
The coin and the feather will fall freely.
Only gravitational force acted on the objects.
Both will fall at the same time.
At normal state:
Both coin and feather will fall because of gravitational force.
Air resistance effected by the surface area of a fallen object.
The feather that has large area will have more air resistance.
The coin will fall at first.
24
Two steel spheres are falling under gravity. The two spheres are dropped at the same time from the same height.
(a) The two sphere are falling with an acceleration.
The distance between two successive images of the sphere increases showing that the two spheres are falling with increasing velocity; falling with an acceleration.
(b) The two spheres are falling down with the same acceleration
The two spheres are at the same level at all times. Thus, a heavy object and a light object fall with the same gravitational acceleration.
Gravitational acceleration is independent of mass.
Motion graph for free fall object
Free fall object
Object thrown upward
Object thrown upward and fall
Example 1
A coconut takes 2.0 s to fall to the ground. What is
(a) its speed when it strikes the ground
(b) the height of the coconut tree.
25

2.9 FORCES IN EQUILIBRIUM
Forces in Equilibrium
When an object is in equilibrium, the resultant force acting on it is zero.
The object will either be
1. at rest
2. move with constant velocity.
Newton’s 3rd Law
Examples( Label the forces acted on the objects)
Resultant Force
A single force that represents the combined effect of two of more forces in magnitude and direction.
Addition of Forces
Resultant force, F = ____ + ____
Resultant force, F = ____ + ____
26
Two forces acting at a point at an angle [Parallelogram method]
STEP 1 : Using ruler and protractor, draw the two forces F1 and F2 from a point.
STEP 2
Complete the parallelogram
STEP 3
Draw the diagonal of the parallelogram. The diagonal represent the resultant force, F in magnitude and direction.
scale: 1 cm = ……
Resolution of Forces
A force F can be resolved into components which are perpendicular to each other:
(a) horizontal component , FX
(b) vertical component, FY
Fx = F cos θ
Fy = F sin θ
Inclined Plane
Component of weight parallel to the plane
= mg sin θ
Component of weight normal to the plane
= mg cos θ 27
find the resultant force
(d)
(e)
Lift

Stationary Lift
Lift accelerate upward
Lift accelerate downward
Resultant Force =
Resultant Force =
Resultant Force =
The reading of weighing scale =
The reading of weighing scale =
The reading of weighing scale =
28

Pulley
1. Find the resultant force, F
2. Find the moving mass,m
3. Find the acceleration,a
4. Find string tension, T
29

Monday, June 21, 2010

Forces

1. A force is a push or pull that one object exerts on another object. It produces or tends to produce motion, and stops or tends to stop motion.

Type of force

Nature of force

Contact force

The push experienced when two objects are pressed together.

Weight

The gravitational pull of the earth on an object.

Friction

This is a type of force that opposes motion. It exists between the surfaces of two objects in contact.

Tension

The pull at both ends of a stretched spring, string or rope.

resistance

This force is also known as viscous force. It is a type of force that is found in fluids such as oil or air.

Electric force

The push or pull between electric charges.

Magnetic force

The push or pull between magnets only or magnets or magnetic materials.

2. Friction and resistance are types of contact forces that oppose motion; they cause energy loss in the form of thermal energy.

3. The SI unit of force is Newton (N).

4. A force always acts in a particular direction.

5. Scalar quantities are physical quantities that have magnitude only,e.g. mass and distance

Vector quantities are physical quantities that possess both magnitude and direction, e.g. force and velocity.

6. Scalar quantities are simply adding or subtracting positive numbers together. For instance, a mass of 100g added to 200g gives a total mass of 300g.

7. When we add two vectors, we are trying to find a single vector that will produce the same effect as the two vectors added together. The single vector is called the resultant vector.

8. Different methods of adding vectors:

Addition of parallel vectors:

State of equilibrium

When two equal forces acting on the object is parallel, both has a force of 3N and acting in opposite directions. The resultant force is zero and the object is said to be in a state of equilibrium.

Addition of non- parallel vectors :

The two vector’s resultant force can be found using the:

Parallelogram method

Tip-to-tail method

9. What is the effect of a force on motion?

Observation

Effect of force

When a stationary soccer ball is kicked, it moves off in the direction it is kicked.

The ball accelerates from rest(i.e. zero velocity)

Acceleration is positive, and in the same direction as the force exerted by the kick.

A force can cause a stationary object to start moving.

A person rollerblading gives a push to move even faster.

The rollerblading accelerates and moves faster after the push.

Acceleration is positive, and velocity increases.

A force can cause a moving object to increase speed.

When cycling down a slope, the cyclist applies the brake to slow down the bicycle.

The bicycle decelerates

Acceleration is negative.

A force can cause a moving object to decrease speed.

During a game of badminton, a player intercepts and hits the shuttlecock to change its direction and motion.

The direction and magnitude of the velocity of the shuttlecock change.

Acceleration of the shuttlecock is non-zero.

A force can cause a moving object to change its direction of motion.

10. We can see that a force can cause:

1. A stationary object to start moving.

2. A moving object to increase speed.

3. A moving object to decrease speed.

4. A moving object to change its direction of motion.

11. For an object with zero acceleration, the different forces acting on it are balanced or add up to zero- i.e the resultant or net force is zero.

12. Every object will continue in its state of rest or uniform motion in a straight

Line unless a resultant force acts on it to change its state.

13. If the resultant force acting on an object is not zero, we say that the forces are

Unbalanced.

14. When a resultant force acts on a object of constant mass, the object will accelerate and move in the direction of the resultant force. The product of the mass and acceleration of the object is equal to the resultant force.

15.Formula

F=ma

F =resultant force

m= mass of object

a= acceleration of object

16. This equation tells us that:

1. A resultant force F on a object will produce an acceleration a.

2. Doubling the resultant force F on an object will double the acceleration a.

3. With the same resultant force F, doubling the mass will double halve the acceleration a.

17. For every action, there is an equal and opposite reaction, and these forces act

On mutually opposite bodies.

18. Four characteristics of forces:

1. Forces always occur in pairs. Each pair is made of an action force

And a reaction force.

2. Action and reaction forces are equal in magnitude.

3. Action and reaction forces act in opposite directions.

4. Action and reaction forces act on different bodies.

19. How does friction affect motion?

1. Friction always opposes motion between two surfaces in contact.

Negative effects of friction

Positve effects of friction

Reduces the efficiency of cars by upto 20%

Needed for walking or holding a pair of chopsticks

Causes wear and tear of moving parts in engines, motors and machines

Used in braking pads to slow down cars

20. Friction as a negative force:

- Wheels of a shopping trolley

- Ball bearings

- Lubricants and polishing surfaces

- Air cushion

21. Friction as a useful force:

-car tyres

- Parachutes

- Rock climbing