Chapter 3: Forces

Recap from ICT Day• A force is a push or a pull• Units: Newton (N)• Force is a vector quantity• Difference between component and resultant force• Difference between balanced and unbalanced forces• Possible effects of balanced forces on an object• Object remains at rest• Object continues with uniform motion in a straight line

• Possible effects of unbalanced forces on an object• Changes direction• Changes speed

Applying vs Receiving

A pushes B• A is applying a force on B.• B receives force from A.Force by A on B.

C pulls D• C is applying a force on D.• D receives a force from C.Force by C on D.

Types of component forces• Normal reaction force: Force by surface on object

N (by table on book)

N : Normal reaction force

Types of component forces• Friction: Force that opposes motion between surfaces in

contact

Force applied (by foot on floor)

Friction

Force

Friction

Friction• Positive and negative effects of friction.• TB Pg 61 to 63

Types of component forces• Drag force or resistance: Force that opposes motion for

objects moving through fluids such as air, liquid

Air resistance

Types of component forces• Tension: Pull exerted by a stretched spring, string on an object

attached to it

T

T : Tension

Types of component forces• Weight or gravitational force: Force by Earth on an object• W = m g• W = weight (N)• m = mass (kg)• g = 10

mg mg

Mass vs Weight• Mass is defined as the amount of substance in a body.• Weight of an object is the gravitational force, or gravity, acting

on it.

• On Earth,• Mass of person = 60 kg• Weight of person = 600 N

• On Moon,• Mass of person = 60 kg• Weight of person = 100 N

Types of component forces• Extra: Lift, thrust

Newton’s First Law of Motion• Newton’s First Law of Motion states that every object will

continue in its state of rest or uniform motion in a straight line unless a resultant force acts on it.

• Objects “keep on doing what they are doing”.• Resultant force is zero.

• Eg: An object resting on the table.• Eg: A soccer ball is rolling in a straight line at constant speed.

• Eg: sitting on a bus• What happens to you when the bus is moving off from rest?• What happens to you when the bus is braking suddenly?

Newton’s First Law of Motion• Video: sitting in a vehicle

• Video: moving vehicle and water

Newton’s Second Law of Motion

• Newton’s Second Law of Motion states that when a resultant force acts on an object of a constant mass, the object will accelerate in the direction of the resultant force. The product of mass and acceleration of the object gives the resultant force.

• F = ma• F = resultant force (N)• m = mass of object (kg)• a = acceleration of object ()

• One newton is defined as the force required to produce an acceleration of 1 in a body of mass 1 kg.

Newton’s Second Law of Motion

• Eg: Pushing 2 objects of different mass from rest with the same force. Which one would have a greater acceleration?

• Video: experiment on 2 objects

• Lighter object has greater acceleration, given the same force.

Newton’s Second Law of Motion

• Eg: Suppose 2 vehicles of different mass are travelling at the same velocity and the same force is applied to stop each vehicle. Which vehicle takes a longer time to stop?

• What about their stopping distance?• Inertia: the reluctance of the object to change its state of rest

or motion, due to its mass. (Chap 4.2 Pg 75)

• Demonstration of inertia • Interesting video

Apply F = ma to weight• F = ma• F = W (in N)• m is the mass of the object (in kg)• a is the acceleration due to gravity = gravitational field strength = g

Example 1:

Mr Wong pushes a car of mass 1000 kg with a force of 500 N to the right. Calculate the acceleration of the car, assuming there is no friction.

Example 2:

The frictional force between a box of mass 4.0 kg and the floor is 15 N. It is pushed across the floor with a constant force such that it accelerates at 0.80 . What is the force applied to the box?

Newton’s Third Law of Motion• Newton’s Third Law of Motion states that if body A exerts a

force on body B, then body B will exert an equal and opposite force on body A.

• and is an action-reaction pair.

• 4 characteristics of forces:• Forces always occur in pairs – an action and a reaction.• Action and reaction forces are equal in magnitude.• Action and reaction forces act in opposite directions.• Action and reaction forces act on mutually opposite bodies.

Newton’s Third Law of Motion• A book resting on the table.

Force by table on book

Force by book on table

Newton’s Third Law of Motion• A man of weight 600 N.

Gravitational force of man on Earth

Gravitational force of Earth on man

Free body diagram• A diagram used to show the relative magnitude and direction

of all forces acting on the object in a given situation.

• A force can be represented by an arrow.• Length of arrow determines its magnitude.• Direction of arrow determines direction of the force.

Free body diagram

N (by table on block)

mg (by Earth on block)

• A block resting on the table. Both forces must be equal length!

Free body diagram• A block is pulled across the table with a string with constant

velocity.Direction of motion

N (by table on block)

mg (by Earth on block)

T (by string on block)

Friction (by table on block)

Tension and friction must be equal length!

Free body diagram• A block is pulled across the table with a string with constant

acceleration.Direction of motion

N (by table on block)

mg (by Earth on block)

T (by string on block)

Friction (by table on block)

Tension is longer than friction!

Free body diagram• A parachutist falling with constant velocity, consider air

resistance.

mg (by Earth on parachutist)

Air resistance

Free body diagram• A car is moving on the road with constant acceleration,

consider air resistance.

Direction of motion

N

mg

Force by engine

Friction

Air resistance

Vector diagram• A block is pushed with a force of 10 N to the right and another

force of 15 N to the right.

• Resultant force = 10 N + 15 N= 25 N to the right

10 N 15 N

Vector diagram• A block is pushed with a force of 10 N to the right and another

force of 15 N to the left.

• Resultant force = 15 N – 10 N= 5 N to the left

10 N15 N

Example 3:• A block is being pushed with a force of 20 N to the east and

with a force of 15 N to the north. Find the resultant force acting on the block.

20 N

15 N

Example• A car is being pulled by 2 strings as shown below. The 2 forces

are at an angle of to the horizontal. Find the resultant force acting on the car.

500 N

500 N

Example500 N

500 N500 N

500 N

Where is the resultant force?

Where is the resultant force?

Where is the resultant force?

Vector diagram

Example 4:

A picture is hung on the wall with 2 strings from a nail.

(a) Draw a free body diagram of the picture.(b) Draw a vector diagram and determine the resultant force

acting on the picture.

Terminal velocity (Pg 66 of TB)

An object falling from a height with air resistance.• Initially, the object accelerates rapidly as the only force acting

on it is its weight.• As it continues falling, its velocity increases. As velocity

increases, air resistance increases. • Resultant force decreases and hence acceleration decreases. • Air resistance increases until its magnitude equals the weight

of the object. • Resultant force becomes zero and hence acceleration is zero.• Object falls with constant velocity.• This constant velocity is called terminal velocity.

Homework• Assignment Q1• TB Pg 68 Section B Q1• N2011 P2 Q9 [page (2)13 of TYS]

• By Friday (19/4)

What you have learnt(a) apply Newton's Laws to:

(i) describe the effect of balanced and unbalanced forces on a body(ii) describe the ways in which a force may change the motion of a body(iii) identify action-reaction pairs acting on two interacting bodies(stating of Newton's Laws is not required)

(b) identify forces acting on an object and draw free body diagram(s) representing the forces acting on the object (for cases involving forces acting in at most 2 dimensions)(c) solve problems for a static point mass under the action of 3 forces for 2-dimensional cases (a graphical method would suffice)(d) recall and apply the relationship resultant force = mass × acceleration to new situations or to solve related problems(e) explain the effects of friction on the motion of a body

References• Video on sitting in a vehicle http://

www.youtube.com/watch?v=8zsE3mpZ6Hw• Newton’s second law of motion: http://

www.youtube.com/watch?v=iwP4heWDhvw • Tablecloth trick: http://

www.youtube.com/watch?v=vfnt8Sdj7cs