PPMF102 – Lecture 1 PPMF102 – Lecture 1

Newton’s Laws of MotionNewton’s Laws of Motion

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NewtonNewton

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Newton’s First Law of MotionNewton’s First Law of Motion

Every object continues in its state of Every object continues in its state of rest or of uniform speed in a straight rest or of uniform speed in a straight line unless acted on by a nonzero line unless acted on by a nonzero forceforce

The tendency of a body to maintain The tendency of a body to maintain its state of rest or of uniform motion its state of rest or of uniform motion in a straight line is called in a straight line is called inertiainertia..

Newton’s first law is also called the Newton’s first law is also called the law of inertia.law of inertia.

Applications of Newton's first Applications of Newton's first law of motionlaw of motion

1.1. ““Wait until the bus stopped”Wait until the bus stopped”

2.2. ““Fasten your seat belt” in cars, Fasten your seat belt” in cars, aeroplanes, or roller coasters.aeroplanes, or roller coasters.

3.3. Headrests are placed in cars to Headrests are placed in cars to prevent whiplash injuries during prevent whiplash injuries during rear-end collisions. rear-end collisions.

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Car seat beltCar seat belt

Seatbelt for roller Seatbelt for roller coastercoaster

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Applications of Applications of Newton's first law of motionNewton's first law of motion4. 4. The head of a The head of a hammer can be hammer can be tightened onto the tightened onto the wooden handle wooden handle by by banging the banging the bottom of bottom of the the handle against a handle against a hard surface.hard surface.

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Applications of Newton's first law of Applications of Newton's first law of motionmotion5.5. To dislodge ketchup To dislodge ketchup

from the bottom of from the bottom of a a ketchup bottle, ketchup bottle, the bottle is the bottle is

often turned often turned upside upside down, thrust down, thrust downward at a high downward at a high speed and then speed and then abruptly halted.abruptly halted.

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Applications of Newton's first Applications of Newton's first law of motionlaw of motion

6.6. Blood rushes from your head to Blood rushes from your head to your feet when riding on a your feet when riding on a

descending elevator which descending elevator which suddenly stops. suddenly stops.

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Applications of Newton’s 1Applications of Newton’s 1stst law law7. 7. A tablecloth is A tablecloth is

whipped from whipped from beneath dishes beneath dishes placed on a placed on a

table top.table top.

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Applications of Newton’s 1Applications of Newton’s 1stst law law8. 8. Flip a coin in high-Flip a coin in high-speed car, bus, or speed car, bus, or plane and catch the plane and catch the vertically moving coin vertically moving coin as if the vehicle were as if the vehicle were at rest.at rest.

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Applications of Newton’s 1st lawApplications of Newton’s 1st law

Conceptual Question: Newton’s first law.Conceptual Question: Newton’s first law.

A school bus comes to a sudden A school bus comes to a sudden stop, and all of the backpacks on the stop, and all of the backpacks on the floor start to slide forward. What floor start to slide forward. What force causes them to do that?force causes them to do that?

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ForceForce

•A A forceforce is a is a pushpush or or pullpull. . •An object at rest An object at rest needs a force to needs a force to get it get it movingmoving..•A moving object A moving object needs a force to needs a force to change its change its velocityvelocity..

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ForceForce

•ForceForce is a is a vectorvector, having both , having both magnitudemagnitude and and directiondirection. . •The The magnitudemagnitude of a force can be of a force can be measured using a spring scale.measured using a spring scale.

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Various names of force Various names of force Force Symbol Unit

Weight W N

Friction f N

Normal force N N

Tension in a string

T N

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These are all forces with the same unit Newton (N).

Force causes velocity Force causes velocity of object to changeof object to change

•It doesn’t take a force to keep an It doesn’t take a force to keep an object moving in a straight line—it object moving in a straight line—it takes a force to change its motion or takes a force to change its motion or velocity.velocity.

•It takes a force to change either the It takes a force to change either the direction or the speed of an object.direction or the speed of an object.

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Newton’s Second Law of MotionNewton’s Second Law of Motion

The net force acting on an object The net force acting on an object equals the mass times the equals the mass times the acceleration of the object.acceleration of the object.

FF = m = maa

Newton’s Second Law of MotionNewton’s Second Law of MotionF F == mmaa

F F net force acting on an object net force acting on an object

[F [F net force net force

summation of all forcessummation of all forces

resultant forceresultant force

total force]total force]

m m mass of the object mass of the object

a a acceleration of the object acceleration of the object

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Newton’s Third LawNewton’s Third Law

Whenever one object exerts a force Whenever one object exerts a force on a second object, the second on a second object, the second exerts an equal and opposite force exerts an equal and opposite force on the firston the first

To every action there is an equal and To every action there is an equal and opposite reactionopposite reaction

“ “action” force and “reaction” force action” force and “reaction” force are acting on different objectsare acting on different objects

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Newton’s Third LawNewton’s Third Law

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Newton’s Third LawNewton’s Third Law

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Newton’s Third LawNewton’s Third Law

Newton’s Third Law of Motion

Conceptual question: What exerts the force to move a car?

Response: A common answer is that the engine makes the car move forward. But it is not so simple.

The engine makes the wheels go around. But if the tires are on slick ice or deep mud, they just spin. Friction is needed. On firm ground, the tires push backward against the ground because of friction. By Newton’s third law, the ground pushes on the tires in the opposite direction, accelerating the car forward.

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Force = mass x accelerationForce = mass x accelerationF = maF = ma

Unit: Newton (N)Unit: Newton (N)

1 N = 1 kgm/s1 N = 1 kgm/s22

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MassMass

A property of a bodyA property of a body

Quantity of matterQuantity of matter

A measure of the inertia of a bodyA measure of the inertia of a body

SI unit is kilogram (kg)SI unit is kilogram (kg)

Does not depend on locationDoes not depend on location

Weight—the Force of Gravity; and the Normal Force

Weight is the force exerted on an object by gravity.

Close to the surface of the Earth, where the gravitational force is nearly constant, the weight of an object of mass m is:

where

Weight—the Force of Gravity; and the Normal Force

An object at rest must have no net force on it. If it is sitting on a table, the force of gravity is still there; what other force is there?

The force exerted perpendicular to a surface is called the normal force. It is exactly as large as needed to balance the force from the object.

N

W

Weight—the Force of Gravity; and the Normal Force

Example 1: Weight, normal force, and a box.

A box of mass 10.0 kg is resting on a smooth (frictionless) horizontal surface of a table.

(a) Determine the weight of the box and the normal force exerted on it by the table.

(b) Now the box is pushed down on (b) Now the box is pushed down on with a force of 40.0 N. Again with a force of 40.0 N. Again determine the normal force exerted determine the normal force exerted on the box by the table.on the box by the table.

(c) If the box is pulled upward with a (c) If the box is pulled upward with a force of 40.0 N, what is the normal force of 40.0 N, what is the normal force exerted on the box by the table?force exerted on the box by the table?

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Weight—the Force of Gravity; and the Normal Force

Example 2: Accelerating the box.

What happens when a person pulls upward on the box in the previous example with a force greater than the box’s weight, say 100.0 N?

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WeightWeight

A force A force

A force of gravity acting on a body A force of gravity acting on a body

Weight = mass x gravity Weight = mass x gravity

SI unit is Newton (N)SI unit is Newton (N)

Depends on location i.e. strength of Depends on location i.e. strength of gravitygravity

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Friction – standard modelFriction – standard model

Force of resistance to the relative Force of resistance to the relative motion of two solid objects when the motion of two solid objects when the two surfaces are in contact.two surfaces are in contact.

Friction is Friction is directly proportional to directly proportional to normal forcenormal force..

Static friction Static friction – friction without motion.– friction without motion.

ffss = = ss N N

s s coefficient of static friction coefficient of static friction

Kinetic friction Kinetic friction – friction when there – friction when there is relative motion between surfaces.is relative motion between surfaces.

ffkk = = kk N N

k k coefficient of kinetic friction coefficient of kinetic friction

Friction is in fact a very complex Friction is in fact a very complex phenomenon phenomenon which cannot be which cannot be represented by a simple model.represented by a simple model.

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