Unit II ForcesUnit II Forces

Force = a push or pull on an objectForce = a push or pull on an objectmeasured in Newtons (N)measured in Newtons (N)

4 Ways Force Effects an Object4 Ways Force Effects an Object

1. Increase v

2. Decrease v

3. Change Direction

4. Change Shape

What are the forces action on box What are the forces action on box being pushed?being pushed?

Which box is moving?Which box is moving?

A B

Net Force = sum of forces acting Net Force = sum of forces acting on an objecton an object

Forces are vectorsForces are vectors Same direction add Same direction add Opposite direction Opposite direction

subtractsubtract

Balanced vs. Unbalanced ForcesBalanced vs. Unbalanced Forces

Balanced Forces: the forces on an object are equal and opposite.

-no change to the motion

-object does not accelerate

Unbalanced Forces: the forces are not equal and opposite

-the object will change

-the object will accelerate

Friction: force that opposes motion Friction: force that opposes motion of an objectof an object

1. Static Friction

2. Rolling Friction

3. Sliding Friction

4. Fluid Friction

Laws of MotionLaws of Motion

12.2-12.312.2-12.3

11stst Law of Motion Law of Motion

An Object in An Object in motion will remain motion will remain in motion, or an in motion, or an object at rest will object at rest will remain at rest remain at rest unless acted upon unless acted upon by an unbalanced by an unbalanced forceforce

teachertech.rice.edu/.../Newton/law1.html

InertiaInertia

The tendency of an The tendency of an object to keep object to keep doing what it is doing what it is already doingalready doing

22ndnd Law of Motion Law of Motion

Force, mass and Force, mass and acceleration are all acceleration are all relatedrelated

22ndnd Law Law

Force = mass x accelerationForce = mass x acceleration

F = maF = ma

33rdrd Law of Motion Law of Motion

Action and Action and ReactionReaction

For every For every action there action there is an equal is an equal and opposite and opposite reactionreaction

http://upload.wikimedia.org/wikipedia/commons/f/f6/Skaters_showing_newtons_third_law.png

Weight Weight pull of gravity acting on an pull of gravity acting on an objectobject

Weight = mass x acceleration due to gravityWeight = mass x acceleration due to gravity

Mass Mass amount of matter in an amount of matter in an objectobject

How is weight different than mass?How is weight different than mass?

Sample ProblemsSample Problems

1. A boy pushes a cart of groceries 1. A boy pushes a cart of groceries forward with a total mass of 40 kg. forward with a total mass of 40 kg. What is the acceleration of the car if What is the acceleration of the car if the net force on the cart is 60 N?the net force on the cart is 60 N?

1.5 m/s²

2. What is the upward acceleration of a 2. What is the upward acceleration of a helicopter with a mass of 5000 kg if helicopter with a mass of 5000 kg if a force of 10000 N act on it in an a force of 10000 N act on it in an upward direction?upward direction?

2 m/s²

3. An automobile with a mass of 1200 3. An automobile with a mass of 1200 kg accelerates at a rate of 3 m/s/s kg accelerates at a rate of 3 m/s/s in the forward direction. What is in the forward direction. What is the net force acting on the the net force acting on the automobile?automobile?

3600 N

4. A 25 N force accelerates a boy in a 4. A 25 N force accelerates a boy in a wheelchair at 0.5 m/s/s. What is wheelchair at 0.5 m/s/s. What is the mass of the boy and the the mass of the boy and the wheelchair?wheelchair?

50 kg

Sample ProblemsSample Problems

5. A girl pushes a box of books 5. A girl pushes a box of books forward with a total mass of 80 kg. forward with a total mass of 80 kg. What is the acceleration of the box What is the acceleration of the box if the net force on the box is 40 N?if the net force on the box is 40 N?

0.5 m/s²

6. An truck with a mass of 3600 kg 6. An truck with a mass of 3600 kg accelerates at a rate of 2 m/s/s in accelerates at a rate of 2 m/s/s in the forward direction. What is the the forward direction. What is the net force acting on the automobile?net force acting on the automobile?

7200 N

7. A 30 N force accelerates a boy on a 7. A 30 N force accelerates a boy on a tricycle at 0.5 m/s/s. What is the tricycle at 0.5 m/s/s. What is the mass of the boy and the tricycle?mass of the boy and the tricycle?

60 kg

14.1 Work 14.1 Work Definition: the product of distance and Definition: the product of distance and

the force in the direction an object the force in the direction an object movesmoves

W = F x dW = F x d

SI unit for work: newton-meter (N·m) SI unit for work: newton-meter (N·m) or joule (J)or joule (J)

If there is no movement, no work is If there is no movement, no work is donedone

PowerPower

What is power?What is power?• The rate of doing workThe rate of doing work• Doing work faster requires more powerDoing work faster requires more power

Power = WorkPower = Work TimeTime

SI Unit – Watt (W)SI Unit – Watt (W)

MomentumMomentum

Momentum = mass x velocity of an Momentum = mass x velocity of an objectobject

Momentum is large if product of Momentum is large if product of mass and velocity is largemass and velocity is large

Objects with large momentum’s are Objects with large momentum’s are hard to stophard to stop

MomentumMomentum

Momentum of an object at rest = 0Momentum of an object at rest = 0

SI Units = kg*mSI Units = kg*m

ss

MomentumMomentum

Law of Conservation of MomentumLaw of Conservation of Momentum

If no net force acts on the system, the If no net force acts on the system, the total momentum of the system does total momentum of the system does not change.not change.

In a closed system, momentum is In a closed system, momentum is conserved.conserved.

MomentumMomentum

ProblemProblem

Which has more momentum, a Which has more momentum, a 0.046 kg golf ball with a velocity of 0.046 kg golf ball with a velocity of 60 m/s or a 7.0 bowling ball with a 60 m/s or a 7.0 bowling ball with a velocity of 6 m/s?velocity of 6 m/s?

Remember: Momentum = mass x velocityRemember: Momentum = mass x velocity

12.4 Universal Forces12.4 Universal Forces

What are the 4 universal forces?What are the 4 universal forces?• ElectromagneticElectromagnetic• Nuclear (Strong and Weak)Nuclear (Strong and Weak)• GravityGravity

How are these different from forces How are these different from forces talked about in the 12.1-12.3?talked about in the 12.1-12.3?• All act over a distance between particlesAll act over a distance between particles

Describe each universal force Describe each universal force and what makes them unique.and what makes them unique.

1.1. Electromagnetic ForceElectromagnetic Force• only force that can attract and only force that can attract and

repelrepel

2.2. Gravitational forceGravitational force

• An attractive force between 2 An attractive force between 2 massesmasses

Describe each universal force Describe each universal force and what makes them unique.and what makes them unique.

Nuclear forceNuclear force: act within nucleus : act within nucleus to hold it togetherto hold it together

3.3. Strong – acts only on neutrons Strong – acts only on neutrons and protons in the nucleus (100x and protons in the nucleus (100x stronger than charges)stronger than charges)

4.4. Weak – attractive force that acts Weak – attractive force that acts over a short rangeover a short range

Measured in units of

Can be represented by

Can be

Causes no change to the object

Causes the object to

Force

unbalanced

Measured in units of

Can be represented by

Can be

Causes no change to the object

Causes the object to

Force

Accelerate

unbalanced

balancedNewtons

arrows

State of Motion