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Chapter 4. Laws of Motion

Newton’s 1st Law

Newton’s 2nd, 3rd laws

Force

Net force

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So far, we learned about

Units, 1d motion, 2d motion, vectors, …

How to describe motions

Today,

Chapter 4: The Laws of Motion

Forces, Newton’s 1st, 2nd, and 3rd laws

Motivation

Then, what causes the motion ???

What causes an apple to fall?

What causes a ball to undergo projectile motion?

Answer

Chapter 4: The Laws of Motion

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ForcesThe measure of interaction between two objects (pull or push). Vector – force has magnitude (unit: N, Newton) and direction

Example

Gravitational Force on apple from earth

Force on apple from your hand

apple

Your hand

Earth

Force on one object from the other object

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Net Force netF

If forces from many objects are applied on a single object

Net force : Vector sum of forces on an object

Gravitational Force on apple from earth

Force on apple from your hand

apple

Your hand

Earth

net on apple hand gravityF F F

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iClicker Quiz

Two forces of magnitude 5 N are acting on an object.What is the magnitude of the net force?

A) 10 NB) 0 NC) Not enough information

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iClicker Quiz

Which is closest to the magnitude of the net force?

(a) Zero(b) 1.0 N(c) 4.0 N(d) 5.0 N(e) 7.0 N

3N and 4 N forces are acting on an object as shown.

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Example

2 N

3N

Find the net force and its magnitude.

Three forces are acting on an object.

4 N

90 deg120 deg

+x

+y

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Newton’s First Law of Motion (The Law of Inertia)

the object at rest remains at rest, the object moving continues to move with the same velocity

(same speed and same direction).

An additional force is NOT required to keep a cart moving with a constant velocity on a frictionless track!

If 0, netF

does not change.v

If net force on an object is zero.

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iClicker Quiz v = 2.5 m/s

A block is moving to the right at a constant velocity 2.5 m/son a frictionless horizontal surface.

Two forces F1 and F2 are exerted on the block in the opposite directionsas shown above.

While the block is moving at a constant velocity,Which force has a greater magnitude?

(a) F1

(b) F2(c) They have the same magnitude

1F

2F

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Newton’s Second Law of Motion

• Non-zero net force causes object to accelerate.

• An object of mass m has an acceleration a, equal to the net force F, divided by the mass of the object, m.

netFa

m

If 0, netF

changes, in other words, accelerates.v

m

netF

netFa

m

The acceleration is given by

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In this projectile motion, While the ball is going up, the direction of the net force is

(a)

(b)

(c)

(d)

(e)

iClicker

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Equal magnitude and opposite directionAction-reaction pair:

Action and Reaction forces are acting on different objects, NOT on a single object. Don’t add them to find the net force!!!

A B

A B

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Action-reaction versus net force• The 3rd law pair of forces are always applied to different

objects! Net force in Newton’s 1st and 2nd laws is the sum of forces on the

same object.

Examples of action & reaction

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Find reaction force for the gravity on the cantaloupe(a) Force on the cantaloupe from the table(b) Force on the earth from the cantaloupe(c) Force on the table from the cantaloupe(d) Force on the earth from the table(e) Force on the table from the earth

iClicker Quiz

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Next topic..

Various ForcesGravity forceNormal forceTension

Friction forces (static, kinetic)

Motivation

1 2 ...netF F F ma

To find the acceleration, we need to add all forces applied to an object of mass m.

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Various Forces

Gravitational Force

Normal Force

Tension Force

Friction Force

Spring Force

Other forces…

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Gravitational Force (Weight):

Magnitude:F = mg, where g = 9.8 m/s2

•Weight: The force that the Earth is pulling the object with near the surface of the Earth.

Direction: Pointing downward

Weight: From the Earth on objects near Earth Surface

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In this projectile motion, When the ball is at the point A,the direction of the gravity force is

(a)

(b)

(c)

(d)

(e)

iClicker

A

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m

A block of mass m is on an inclined surface as below. What is the direction of the gravity force on the block?

(a)

(b)

(c)

(d)(e)

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Direction:

Magnitude:Determined by analyzing forces

From surface on objects in contact

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m

A block of mass m is on an inclined surface as below. What is the direction of the normal force on the block?

(a)

(b)

(c)

(d)(e)

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Example: Person in an elevator

A person with a mass 50 Kg in an elevator is moving upward with an upward acceleration 21.1 /a m s

Identify all forces on the person and find magnitude and direction for each.

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A person is in a elevator and the elevator is moving downward but slowing down. The magnitude of the normal force from the floor on the person must be

A) greater than the weight of the personB) less than than the weight of the person.C) equal to the weight of the person.

iClicker Quiz

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A B

Direction: pulls along the ropeMagnitude : Constant along a single rope,

determined by analyzing forces

From the rope on objects at both ends

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M=10 kg Normal force = 50 N

Horizontal surface

Rope

Find the tension force from rope on block.

scale

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A bucket is being lowered by a very light rope with a constant downward velocity. The tension in the rope must beA) equal to the weight of the bucket.B) greater than the weight of the bucket.C) less than the weight of the bucket.

iClicker Quiz