Unit3 force friction

MOTION AND FORCES

BLOCK 2

MOTION AND FORCES

■ AREA OF INTERACTION: HUMAN INGENUITY

■ GUIDING QUESTIONS: HOW DO FORCES INFLUENCE THE WAY MATTER BEHAVES?

DEFINITIONS: P.52

■ FORCE

■ NET FORCE

■ UNBALANCED FORCE

■ BALANCED FORCE

■ NEWTON (UNIT):

■ INERTIA:

■ FIRST LAW OF NEWTON

DEFINITIONS: P.52

■ FORCE: is a push or a pull

■ NET FORCE two or more forces act on an object at the same time that are combined

■ UNBALANCED FORCE unequal forces in opposite directions

■ BALANCED FORCE forces on an object that are equal in size and opposite in direction

■ NEWTON (UNIT): is the unit to measure force

■ INERTIA: is the tendency of an object to resist any change in its motion.

■ FIRST LAW OF NEWTON an object moving at a constant velocity keeps moving at the same velocity unless an unbalanced net force act on it.

Regarding with the following video:

Write all the aswer in your notebook

■ Observe 2 objects to Compare their velocity and calcule the difference of velocity (suppose distance and time).

■ Describe an example of a possitive, negative and a zero acceleration.

■ Describe concept of friction, force, first law of newton with an specificexample of this chapter.

■ Find an example of unbalanced and balanced force and write it down.

■ Do the same execises with the movie that you have chosen

º

■ Movie

SECOND LAW OF NEWTON

■ It state that the acceleration of an object, is in the same direction as thenet force on the object.

Formula: acceleration (m/s2)= net force (newtons)

mass (kilograms)

a=F net

m

Solve equations:

a=F net

m

Friction

■ Is the force that opposes the motion of two surfaces that are touching each other.

Find definitions:(70-74)

■ Friction

■ Types of friction: Static, sliding, rolling, air resistance, Fluid

■ Write and draw an example of each type.

SELF CHECK P56.

■ PAG.1,2,3,4,6

Third law of Newton

■ The third law of newton describes action-reaction pair in this way: when one objectexerts a force on a second object, the second one exerts a force on the first that isequal in strengh and opposite in direction.

Relationship between force, mass and acceleration

If an object has:

Less mass then,it need less F to accelerate it

More mass then it need more force to accelerate it

When we study forces we use arrows to indicate the directionof the force

Force exercises

1. If the mass of an helicopter is 4500 kg and the net force on it is 18 00O N what’sthe helicopter acceleration?

Data Unknown Formula Substitution Result

■ 2. What is the net force on a dragster with a mass of 900 g. If its acceleration is 32 m/s2?

■ 3.A car is being pulled by a tow truck. What’s the car’s mass if the net force on the car is 3,000 N and its acceleration is 2 m/s2?

■ 4. A 8495 g mass has an acceleration of 5.5 m/s2. What is the net force applied?

■ 5. A 3200-N force is applied to a 160 Kg mass. What is the acceleration of the mass?

■ 4. A 85 kg mass has an acceleration of 5.5 m/s2. What is the net force applied?

■ 5. A 3200-N force is applied to a 160 kg mass. What is the acceleration of the mass?

■ 6. If you are pushing on a box with a force of 20 N and there is a force of 7 N on the box due to sliding friction, what is the net force on the box.?

Exercises

1. A baseball has a mass of 0.52 kg. When hit by a bat, the ball accelerates at a rate of 35 m/s2. What’s the force of the bat on the ball.

Data unknown Formula SubstitutionResult

2. A mechanical shovel is used to throe some soil with a force of 320 N. While on the shover, thesoil accelerates at a rate of 2 m/s2. What’s the mass of the soil.


3. What’s the acceleration of a 200 g ball that is kicked with a force of 110 N?


p.834-835

■ 25. A 85 kg mass has an acceleration of 5.5 m/s2. What is the net force applied?

467.5 N

■ 26. A 3200-N force is applied to a 160 kg mass. What is the acceleration of the mass?

20 m/s2

■ 27. If you are pushing on a box with a force of 20 N and there is a force of 7 N on thebox due to sliding friction, what is the net force on the box.?

140 N

Gravity

■ Gravity: Is an attractive force between two objects that depends on themasses of the objects and the distance between them.

■ Is the natural force that causes things to fall toward the earth

Difference between mass vs. weight

1. Weight is a force, the gravitational force exerted on an object.

2. Mass is the amount of matter something has.

They are related, as mass weight

■ Formulas

F= m· a w=m · g

F= m · g

F= force (w=weightm= massg=gravity (m/s2)

Weight:Gravitacional force exerted on an object

■ They are 4 basic forces:

■ 1) Gravity

■ 2) Electromagnetic

■ 3) Strong nuclear

■ 4) Weak nuclear

Anything that has mass is attracted by theforce of gravity.

■ FORCE OF GRAVITY IN OTHER PLACES:

MOON: 1.6 m/s2 VENUS: 8.9 m/s2

MARS: 3.6 m/s2 JUPITER 25 m/s2

SATURN: 11 m/s2 EARTH 9.8 m/s2

Exercises of weightMOON: 1.6 m/s2 VENUS: 8.9 m/s2

MARS: 3.6 m/s2 JUPITER 25 m/s2

SATURN: 11 m/s2 EARTH 9.8 m/s2

1. Find your weight on Earth

Data unknown Formula Substitution Result

2. Find your weight on the moon


3. Find your weight on Jupiter


Friction:

The force that opposes the motion of two surfaces that are touching eachother.

F=ma F=mg acceleration (m/s2)

w=mg

w=weight (weight is a force)

m=mass

g=gravity gravity: g=9.8 m/s2

Types of friction:

a) Static: The frictional force that prevents 2 surfaces from slidingpast each other.

b) Sliding friction: The force that opposes the motion of two surfacessliding past each other.

c) Rolling: is the frictional force between a rolling object and thesurfaces it rolls on.

d) Fluid: opposes the motion of objects that are moving throughfluids (liquids and gases)

e) Air-resistance: Opposes the motion of objects that moves throughair.

Law of universal Gravitation

The gravitational force between two objects depends on their masses and the distance between them

F=G m1 . m2 G is the gravitational force constant

d2

If the mass of either of the objects increases, the gravitational force between them increases.

If the objects are closer together, the gravitational force between them increases

FREE FALL

■ Any motion of a body where gravity is the any force acting upon it

■ 2 characteristics of free falling objects:

– A) They don’t encounter air resistance

– B) They accelerate downwards at a rate of 9.8 m/s2 (on Earth)

Falling objects

Terminal velocity is the highest velocity reached by a falling object.

v= a · t

v= velocity

a=acceleration

t= time

Example:

An acorn fall from the top of an oaktree to the ground in 15 s. What’s the velocity of the acornwhen it hits the ground?

Data Unknown Formula Substitution Result

Falling objects exercises1. A billard ball is dropped from the roof of the building. It takes 35 seconds to strike theground. What’s the velocity of the ball the instant it strikes the ground?Data Unknown Formula Substitution Result

2. What’s the downward velocity of an object dropped from the top of a tower at the endof 4 minutes.Data Unknown Formula Substitution Result

3. A can falls from a 3-story bulding, it reaches a velocity of 18 m/s. How much time did ittake to reach this velocity?Data Unknown Formula Substitution Result

4. An object hits the ground after 0.0039 h. Find its velocity.

5.A piece of cement falls from a construction site and reaches a velocity of 1.2 km/h. Find the time ittook to reach the velocity.

Review1. A rope is used to pull a car. The rope exerts a force of 4680 N, giving it an acceleration of 9.1 m/s2. What

is the mass of the car?

2. What’s the acceleration of a 79 kg ball that is kicked with a force of 88 N?

3. What is your mass if you weigh is 1019.2 N.

4. If your mass is 67 kg on Earth but your weight is 1675 N, in which planet are you?

5. If you are in Mars and you weigh 151 N, What is your mass?

6. What is the acceleration that changes its speed from 45 m/s to 34 m/s in 4.5 minutes?

7. What is the net force on a 130 kg bike that changes its speed from 13 m/s to 52 m/s in 8.2 minutes?

1. A rope is used to pull a car. The rope exerts a force of 4680 N, giving it an acceleration of 9.1 m/s2. What is the mass of the car?

2. What’s the acceleration of a 79 kg ball that is kicked with a force of 88 N?

3. What is your mass if you weigh 1019.2 N.

4. If your mass is 67 kg on Earth but your weight is 1675 N, in which planet are you?

5. If you are in Mars and you weigh 151 N, What is your mass?

6. What is the acceleration that changes its speed from 45 m/s to 34 m/s in 4.5 minutes?

7. What is the net force on a 130 kg bike that changes its speed from 13 m/s to 52 m/s in 8.2 minutes?

■ FIRST LAW OF NEWTON an object moving at a constant velocity keeps moving orat rest at the same velocity unless an unbalanced net force act on it.

■ SECOND LAW OF NEWTON: It state that the acceleration of an object, is in thesame direction as the net force on the object.

■ THIRD LAW OF NEWTON describes action-reaction pair in this way: when oneobject exerts a force on a second object, the second one exerts a force on the firstthat is equal in strengh and opposite in direction

Identifying the Laws of Newton

1. A magician pull a tablecloth, from under some dishes and glasses that are on a table without disturbing them. _________________

2. Rockets are launched into space using jet propulsion, the rocket accelerates out to space._________________

3. Pushing a child on a swing is easier that pushing an adult on the same swing. ________________________

4. A student leaves a pencil on a desk, It remains there until another student comes along and picks it up. _________________________

5. A soccer player kicks a ball with his foot and his toe is pushed backward _____________

Study guide

■ Definitions

■ 1st, 2nd and 3rd law of Newton and examples

■ Exercises (Net force, a=F/m, F= m·a, m=F/a)

■ Friction (Theory)

■ Weight vs mass (vocabulary and exercises: W=m·g, m=W/g, g=W/m)

■ SI units

■ Law of Universal Gravitation

■ Free Fall

Unit3 force friction

Science

Transcript of Unit3 force friction