chapter 2, Lesson 5

 Friction

Friction is a contact force. It arises when an object is in contact with another object or material, and an external force is applied to the object in an attempt to move it.

If the force applied is sufficient to cause the object to be in motion, the friction is called dynamic friction. 

If the object does not move despite a force being applied to move it, the friction in action is static friction.

a static friction is greater than dynamic friction.

Friction and motion

Lets release a ball gently on the floor and let it roll on its own. Newton's first Law of motion suggest that if the net force on the ball is zero " Since  your hand is no longer in contact with the ball", the ball will not accelerate, and therefore it will move with constant velocity. will it?

It is a common observation that a ball eventually slows down and stops. it can therefore be inferred that the net force acting on the rolling ball is not zero, but negative. The negative force opposes the motion of the ball. The rolling ball experiences negative net force because of friction. 

Air resistance

When an objects move through air, the force of air pushes against the moving object to oppose its motion this is called air resistance.

Lets look at the motion of a parachutist. as he jumps off a helicopter, gravitational force pulls him downwards. Air resistance push him upwards.

If the Parachutist  opens his parachute, he will experience a much greater air resistance because of greater surface area of the open parachute.

When the parachutist is no longer accelerating but travelling at a uniform velocity, air resistance and gravitational force are said to be balanced. They are equal in magnitude but act on opposite direction.

A body travelling at high speeds in contract greater air resistance. hence a speeding car consumes more fuel, as extra energy is converted to do work against the greater air resistance to maintain a car at high speed.

Worked example

At a particular instant, Mr Mohammed car is travelling at 24 m/s on a level road when it runs out of petrol. it experiencing an average restiveness force of 500 N while in motion. The total mass of the car and Mr Mohammed is 1200 kg.

(a) state and explain the motion of the car at the moment the fuel tank is empty.

(b) calculate the intial acceleration of the car.

(c) assuming that the acceleration is constant, how long will it take the car to come to rest?

(a) The engine force becomes zero when the car runs out of petrol. This means that the net force acting on the car is now negative, and it will cause the car to slow down and finally come to rest.

(b) F net    =   ma

0- 500 = 1200 * a

a= -0. 42 m/s2

 Initial acceleration of the car is -0.42 m/s2. Note that a is negative in value. This implies that the car is slowing down.

(c) With an acceleration of -0.42 m/s2. the car slows down at a rate of 0.42 m/s every 1 s later. Hence it will take 57 s for the car come to rest.

Bird's Eye View for this chapter:

chapter 2, Lesson 4.

Newton's Laws of Motion

In the late 17th century, Sir Isaac Newton postulated that acceleration of an object is dependent upon two variables - the net force acting upon the object and the mass of the object.

It has been found later that the acceleration of an object, and inversely on the  mass of the object. As the net force increases so the will the object acceleration. However, as the mass of the object increases, it acceleration will decrease.

mathematically, they are related by the equation below:

F = m* a

Sir Issac Newton put forth a variety of laws which explain why objects move (or don't move) as the do. these laws have become popularly known as Newton's Laws of motion .

The relationship F= m*a describes Newton's second Law of motion

"For a constant mass object, the acceleration of an object as produced by a net force, is directly proportional to the magnitude to the net force, in the same direction as the net force, and inversely proportional to the mass of the object".

Worked example 3.2

A strong horse pulls a sledge from rest with a constant force of 650 N. the average friction acting on the sledge is 250 N when the sledge is in motion.

The sledge and its rider have a mass of 200 kg.

(a)  i. Determine the value of the net force acting on a sledge.

      ii. Hence Calculat the acceleration experienced by the sledge

(b) what is the speed of the sledge 5.0 s into its journey?

Solution:

(a) i. Fnet = 650 - 250 = 400 N

     ii. F net= m *a

          400      =   200 *a

a = 400 / 200 = 2 m/s2

acceleration experienced by the sledge is 2 m/s2

(b) a = v final - v initial / time taken

v = 2 * 5 = 10 m/s

speed of the sledge 5.0 s into its journey is 10 m/s

Body in equilibrium

Logically speaking a body not experiencing any net force, will therefore not speed up or slow down or change direction. Hence the body will not accelerate when it experiences zero net force.

This has two possible consequences:

1.If the body is not moving originally,it will stay at rest , unless acted upon by a net force. The figure below shows that Ala' stalled car will not move unless the net force is applied to move it. he needs his friend to help push his car.

 Ala' car will only move if his friends applied force is greater than the friction between the wheeles and the ground.

2. It will continue to move at the same speed, in the same direction, if it is originally  moving. for Example. a comet in outer space travels in a straight line at constant speed, unless a net force changes it motion.

we say that the body is in equilibrium if it is not acted upon by a net force.

Newton's first Law of motion relates to a body in equilibrium

*An object at rest tends to stay at rest and an object in motion tends to stay in motion at the same speed and on the same direction unless acted upon by a net force.

Inertia

All bodies resist being set into motion if they are originally at rest, and resist to stop once in motion, unless a net force is applied to change their state of rest or motion.

This property of a body to resist a change in its state of rest or motion is called inertia. if the body has a large mass, its inertia is large. This is easily observed, as a large amount of net force is required to change the state of rest or motion of a huge body. 

 Traffic accident usually happens when a big v speeds and can't stop. A very large force is required to stop its motion.

Chapter 2,Lesson 3

Addition and subtraction of Forces

Vectors acting on the same object can be replaced by a single vector with the same effect. This single vector is known as the resultant or net vector.

Unbalanced Forces 

Consider a box being pulled by a dog with a force of 120 N to the right. The box experiences a resistance of 20 N due to the friction between itself and the ground. In effect, the box is moving with a resultant force or net force of 100 N. We say that there is an unbalanced force of 100 N acting on the box.

This unbalanced force causes the box to accelerate towards the right. We can replace the two forces with a single-force arrow that represents a net force of 100 N actin towards the right.

Balanced Force

On the other hand, when two men push against a stationary car each with a force of 400 N and 500 N respectively the total force exert by th two acting on the car is 900 N. If the friction between the car and the ground 900 N, then the net force acting on the car will be zero N. This net Force of 0 N will not cause the stationary car to move. In this case, we say that the force acting on the car is balanced. when the force acting on the car are balance the car experienced no net force, or no resultant force is acting on the car.

Worked Example

Suppose two tug boats pull a stationary container ship at an angle of 30 to each other, with a horizontal force of 6000 N and 7000 N respectively. What is the resultant force acting on the container ship?

graphically

Effects of net Force

If there is net force acting on a body, the body will respond in any of the following ways

1. The shape and the size of the body will change. this effect of the force is clearly observed when you play with paste.

2. the direction of motion of the body will change when a force is applied to the volley ball, it bounce off in the direction different from its income one. 

3. the body will accelerate and its velocity will change over time. 

this means that : a. it will move faster if the net force is greater than zero. positive net force will speed up motion, suppose Mr. Ahmad is riding his snowmobile if the force developed by the engine is 1200 N, and the total force resisting its motion is 800 N, an unbalanced force or force (F net) of 400 N produce.

F net= F in the direction of motion - F in opposes motion = 

1200 - (300+500) = 400 N

hence, Mr. Ahmad snowmobile will accelerate and move faster and move faster.

b. It will slow down if the net force is less than zero. negative net force will oppose and slow down motion. suppose Mr. Ahmad snowmobile suddenly stills half way to its journey. this will reduce the forward thrust. so the net force acting on it while it is still in motion in the opposing force of 800 N.

 F net= F in the direction of motion - F opposes motion = 

                          0- 800 = -800 N

This negative net force will slow down the snowmobile, and will finally cause it to stop moving.

chapter 2, Lesson 2

Free body Diagram

As force is a vector quantity, it can be represented by a vector arrow. Representation of force vectors show the relative magnitudes and directions of all forces acting on a single object in a given situation. This is Known as free body diagram.

Below are some examples of free body diagrams that show all the forces acting on just a single body in each case.

Note the forces acting on the dumb-bell will move up if the lifting force is greater than its weight, which is the gravitational force acting on the dumb-bell.

Buoyant force or upthrust is an upward force exerted on an object immersed in a fluid, The following figure shows an example of a buoyant force acting on the duck. Another example of buoyant force in action can be observed in a balloon floating in the air.

Tension is the pulling force that is developed in a stretched material, like rope, string, spring, or chain. It is always directed along the length of the material doing the pulling. 

Normal force is a pushing force that exists between two objects in contact, acting perpendicularly to their surface of contact. Normal force opposes a force that presses the two objects together.
Clearly, each body on the earth experience gravitational force pulls the body towards the ground at all times, an opposing force of equal magnitude exists to maintain the body at rest in the example given.

        

Dynamics

While  Kinematics is that branch of physics which involves the description of motion, it does not examine the cause of motion.

Dynamics, on the other hand, involves an examination of both a description of motion and the forces that produce it.

3.1 Forces in Action

A force is either a pull or a push upon an object resulting from the object interaction with another object.

For example, the man is interacting with the car. He applies a force to push the car, causing it move. The Car in turn will exert force on the man's hand, called reaction force, which prevents the man's hands from sinking into the car's body. 

This interaction between objects does not end here. The car also interacts with the ground. As the car presses down on the ground, the ground pushes up on the car.

Unit of Force

The SI unit for force is newton, its symbol is N.

Was Sir Issac Newton the first person to see an apple falling from a tree? Probably not, but he would have likely been the first to actually question why it fell.
He was probably the first to decide that he wanted to know "Why?" and Persevered to find out the cause. This led to his famous works on the laws of universal Gravitation.


Force Measurement

To measure force in Science laboratory, we use the spring balance to pull the object of interest and measure the steady force registered.

measuring friction

We Can use spring balance to measure the friction between two surfaces. It Can also be used to measure the force required to pull (Open) the door.

To measure the Weight of an object, we hang the object of interest below the spring balance, and allow the pull of gravity to do its work.

In this way, the pull of gravity is measured as the weight of the object.

What will you be able to do at the end of this chapter "Dynamics"?

1. describe the effect of balanced and unbalanced forces on a body.

2. describe ways on which a force may change a motion of body.

3. Identify forces acting on the body and draw free body diagrams.

4. representing the forces acting on the body.

5. state that the mass of a body resist a change in the state of rest or motion of a body "Inertia".

6. recall and apply the relationship F=m*a to new situation to solve related problems. 

7. Explain the effect of friction on the motion of a body.

chapter "1", Lesson "2.c"

Acceleration due to Free Fall

When a feather and a coin are dropped from the same height simultaneously, both objects fall because of the pull of gravity. However, the feather will fall much more slowly when compared to the coin. This is because of the air resistance acting on the feather.

Pause & think

What if both objects are to be released in a Place where there is no air resistance?

In 1971, an astronaut David Scott, Commander for Apollo 15 performed a simple experiment on the moon. He released a hammer and a falcon feather While on the moon , which has no atmosphere. He managed to show the world that both objects fell with the same acceleration in the absence of air. Both objects landed on the moon's surface at the same time.

IT, Visit this website below to witness Commander David Scott dropping hammer and a falcon feather on the surface of the moon http://www.hq.nasa.gov/alsj/a15/a15v_1672206.mpg 

Pause & think

OK, if an object is free falling, it means:

(a) the effect of air resistance is negligible

(b) it experiences only pull of gravity 

(c) its acceleration is approximately 10 m/s2

(d) every one second later, it will be faster than before by approximately 10 m/s.

The feather and coin experiment 

Similar effects can be demonstrated on the earth when the objects are released in a glass tube where air can be removed.

When all the air has been removed from the glass tube, the feather and the coin which are released, will reach the bottom of the tube at the same time. This shows that without air resistance, objects of different weights fall with the same acceleration.

As the acceleration of the falling object in the absence of air is due to entirely to the effect of gravity, We call this acceleration due to gravity, or acceleration due to free fall, and is denoted by the symbol g.

A compact object with small surface area, which is dropped near to the surface of the earth, has an acceleration g of approximately 10 m/s2. 

Bird's Eye View for this chapter: