FORCE AND MOTION Ms. Eman Alqurashi 1 Chapter-2. Ms. Eman Alqurashi 2 Objective 1: Explain the force and identify kind of forces. Objective 2: Analyze.

FORCE AND MOTION

Ms. Eman Alqurashi

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Chapter-2

Ms. Eman Alqurashi

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Objective 1: Explain the force and identify kind of forces.

Objective 2: Analyze and calculate the net force.

Objective 3: Explain the concept of Inertia.

Objective 4: Differentiate between mass and weight.

Objective 5: Differentiate between balanced and unbalanced forces.

Lecture Outlines

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2.1 Force

Force is a push or pull on an object to change its position.

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The force has both magnitude and direction so it is a vector quantity .

Like any vector quantity, force is represented by an arrow. The length of the arrow gives the magnitude of the force and the direction of the arrow gives the direction of the force.

The force is measured in Newton (N)

Magnitude of the forcedirection of the force.

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Forces between objects which are not in physical contact are called non-contact forces.

ExampleThe force between earth and the moon.

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2.2 Some Common Forces

2.2.1 Earth’s Gravitational Force (weight)

Credit: Fundamental of physics,” By Halliday, Resnick, and Walker,” 6th Ed. John Willey &Sons, 2001.

Is a measure of the amount of gravitational force acting on an object having mass (m).

It is directed downward toward the center of earth and represents the weight of the object w .

Fg = W= mg

Where ( g=9.8 m/s²)

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2.2.2 Tension Force

Tension force is a contact force (pull) provided by a string, rope, or wire on an object. The direction of the tension T in the direction of the rope.

Credit: “Physics for scientists and engineers a strategic approach,” by Randall D. Knight Pearson-Addison Wesley 2004.

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2.2.3 Normal Force

Normal force n is force provided by a surface (table or wall, for example) against an object that is pressing on the surface. Normal force is always points vertically upward to the surface.

Credit: “Physics for scientists and engineers a strategic approach,” by Randall D. Knight, Pearson- Addison Wesley 2004.

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2.2.4 Friction Force

Friction force f is a force provided by a surface (table or wall, for example) against any attempt to move an object along the surface in contact with.

Credit: “Physics for scientists and engineers a strategic approach,” by Randall D. Knight, Pearson-Addison Wesley 2004.

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Examples:

Identify the forces acting on the object

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Friction

a) Always acts parallel to the surface of contact and opposite to the direction of motion.

b) Acts in the direction of motion.

c) Is smaller when starting than moving.

d) In reality, there is no friction force.

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When a person begin to move forward, which force allows him to accelerate

a) Weight.

b) Normal force.

c) Air resistance.

d) Friction.

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2.2.5 Combining Forces: The Net (resultant) Force Fnet

The net force F is a single force resulted from combing all the forces such that it represents their net effect.

Find the net force?

The direction of the net force is always in the direction of the bigger force.

Force:Is the push or a pull on an object.

Net force: Is the combination of all forces acting on that object.

5N5N

5N5N

10N5N

10N

5N

0N

(5 + 5 = 10 N)

(5 _ 5 = 0 N)

(10 _ 5 = 5 N)

What is inertia?Is the tendency of an object to remain at rest if it’s already at rest , or to keep moving if it’s already moving.

Or

Is the property of matter that resists change in motion.

2.4 Mass and Inertia

An object with a large amount of inertia requires a large amount of force to start

or stop.

An object with a small amount of inertia requires a small

amount of force to start or stop.

Conclusion:Mass is a measure of inertia. Mass is measured in kilograms (kg)

Note: The more mass an object has the more inertia it will have.

The less mass an object has the less inertia it will have.

Mass Weight

The mass of an object is the amount of matter it contains.

Is a measure of the amount of gravitational force acting on an object having mass (m).(vector)

m = F a

w = mgWhere ( g=10m/s²)

Mass is measured in grams(g), kilograms(kg).

Weight is measured in pounds, Newton's(N).

mass and weight are directly proportional to one another.

Example: A cube of butter has a mass of about 0.1 kg . Find its weight? W = mg = 0.1 × 10 =1 N

a acceleration of the objet unit(m/s²).

g

acceleration due to the gravity or gravitational accelerationunit(m/s²).

F = ma Force of the objectunit (N).

W =mg Gravitational force or weightunit(N).

Remember:Gravitational force (weight) is proportional to mass double the mass and the gravitational force will be doubled which means the ratio of weight to mass is always the same . ( w / m = mg /m = g)

Is your weight in the moon the same as your weight in the earth? what about your mass?

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Example 2.3

How much mass is required to balance the object on the left side, which weighs 600N?

Equilibrium

Equilibrium for objects at rest (static equilibrium)

Equilibrium for moving objects (Dynamic equilibrium)

An object is in mechanical equilibrium

when the sum of all forces (the net force)

acting on it is zero∑F=0

Moving objects also can be in equilibrium when

the net force is zero ∑F=0

this is the case when an object moves with constant velocity

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Identify the kind of equilibrium?

Credit: “Physics for scientists and engineers a strategic approach” by Randall D. Knight Pearson-Addison Wesley 2004

At rest

Moving with

uniform motion

Moving with

constant velocity

Acceleration is zero

Net force is zero

The object can be in

equilibrium if it is

or

or

or

or

or

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If an object is moving with constant velocity along a straight line, then

a) No forces acting on the object.

b) Single constant force acting on the object in the direction of motion.

c) Single force acting on the object in the opposite direction.

d) A net force of zero acting on the object.

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If net force acting on an object is not zero, then the object will accelerate.

a) Trueb) False

a=0 is the condition for mechanical equilibrium.

a) Trueb) False

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Balanced and unbalanced forces

F1

F2 F3

balanced forces have 0netF

Unbalanced forces have 0netF

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Example:The sketch shows a painter’s staging in mechanical equilibrium . The person in the middle weights 250N, and the tension in both ropes are 200N. What is the weight of the staging?

∑F=0

∑F = 200 + 200 – 250 – W =0

W = 200 + 200 – 250 =150 N