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Forces and Motions. Chapter 13 – The Nature of Forces. Forces. 13.1 What is a Force? Anything that changes the state of rest or motion of an object It’s what causes ACCELERATION Has magnitude and direction Therefore force is a vector. 13.1 Force.

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forces and motions

Forces and Motions

Chapter 13 – The Nature of Forces


13.1 What is a Force?

  • Anything that changesthe state of restor motionof an object
  • It’s what causes


  • Has magnitude and direction
    • Therefore force is a vector
13 1 force
13.1 Force

A force can cause a resting object to move,

or it can accelerate a moving object by changing the object’s speed or direction.

13 1 combining forces
13.1 Combining Forces

1) Balanced Forces:

5 N 5 N


Equal forces in opposite directions cancel each other to produce no net force and therefore no movement.

0 N

13 1 combining forces unbalanced
13.1 Combining Forces - Unbalanced

Unbalanced Forces

Result when the net force acting on an object is NOTequaltozero

When an unbalanced force acts on an object, the object acceleratesand moves in the direction of the net force

13 1 combining forces1
13.1 Combining Forces

2) Adding Forces:

5 N 5 N 10 N


Two forces, working in the same direction will add together to produce a net force larger than either original force.

13 1 combining forces2
13.1 Combining Forces

3) SubtractingForces:

10 N 5 N 5 N


Two forces working in opposite directions will subtract to produce a net force in the direction of the larger force.

13 2 friction
13.2 Friction

Friction is a force that opposes motion between two surfaces that are in contact.

There are two main types of friction:

Static - keeps things “static” (i.e. stationary)

Force resisting motion of an object at rest.

Ex- the force that is keeping this block from

sliding downhill

Kinetic - friction that occurs with

objects in motion.

Three types of kinetic friction: Sliding, Rolling and Fluid

13 2 friction1
13.2 Friction

Rolling Friction: happens when an object rolls over a surface. Works opposite to the wheels’ motion.

Static Friction: works in the opposite direction to the intended motion. Object is at rest.

Static friction

Direction of motion

Direction of motion



seating plan
Seating Plan




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Anne Marie



Michel B



  • Physics (25 mins, April 4th)
    • Acceleration
    • Chapter 13.1
    • Chapter 13.2
  • Chemistry (25 mins, April 7th)
    • Acids and Bases
      • (yes, that’s really it, so be really prepared)
13 2 friction2
13.2 Friction

Fluid Friction: happens when an object pushes fluid aside. The surfboard overcomes the fluid friction of the water.

Sliding Friction: acts between the sled and the snow, in the opposite direction to the sled.



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The friction between surfaces that are stationary

force that exists when objects slide past each other

force that exists when a round object rolls over a flat surface (usually less than sliding friction)

force that exists when an object moves through a fluid (air, water)

  • a book sitting on a table
  • rocks laying on the side of a mountain

-a hockey puck

on ice

-a child going down a slide

-a roller blade on a sidewalk

-a bowling ball going down a bowling alley

-a plane flying through the air

-a swimmer swimming in a pool

13 2 friction3
13.2 Friction

Reducing Friction

  • A smooth surface does not have as much friction as a rough surface.
  • A lubricant, ball bearings or even air can be used to reduce friction.
13 3 gravity
13.3 Gravity

Gravity is a natural phenomenon in which objects that havemassareattractedto one another.

It is an attractive force thatpulls objects together.

13 3 gravity1
13.3 Gravity

Earth’s gravity acts downward toward the center of the Earth.

There is an upward force that balances gravity.

Falling Objects

Terminal velocity: the constantvelocity of a falling object when the force of airresistanceis equal in magnitude& opposite in direction to the force of gravity.

Question: What other force is not present in a vacuum that would affect acceleration?

Answer = air resistance

In a vacuum, two objects would

accelerate at the same rate, regardless of their shape, because both are in free fall.

Free Fall: the motion of a body when

only the force of gravity is acting onthe body.

newton s 1st law of motion
Newton’s 1st Law of Motion

According to Newton’s 1st Law, the state of motion of an object does not change as long as the netforce is zero.

a) An object at rest will stay at rest unless a force acts on it.

b) An object in motion tends to stay in motion unless a force acts on it.

Ex: A soccer ball will remain at reston the grass unless a force is acted on it. It will then stay in motion until another force acts on it.

This law is also called the

“Law of Inertia”

Inertia: the tendency of an object to resist being moved or,

if the object is moving,

to resist a change in speed or direction

until an outside force acts on the object.

Ex. In a car crash:

You continue forward because of inertia


Newton’s 2nd Law of Motion

Force equals mass times acceleration.

F = ma

newton s 2 nd law of motion
Newton’s 2nd Law of Motion
  • Force = mass x acceleration
    • Units: Newtons (kg·m/s2)
  • Acceleration of gravity on Earth = 9.8 m/s2
  • Acceleration of gravity on the Moon =1.6 m/s2
  • Weight = mass x acceleration due to gravity
    • On Earth, weight ≈ mass x 10m/s2
    • On the Moon, weight ≈ 1/6 weight on Earth
newton s 2 nd law of motion1
Newton’s 2nd Law of Motion
  • Question 1:
    • If you weigh 540N on Earth, how much would you weigh on the Moon?
  • Question 2:
    • A) If your mass is 42.5 kg on Earth, what is your mass on the Moon?
    • B) What would your weight on Earth be?
    • C) What would your weight on the Moon be?

90 N

540N x 1/6 =

42.5 kg!!!

425 N

70.8 N



This will help you think about everything you have learned

Worksheet Newton’s 2nd Law

Questions 1-8

(see, it could be worse!)

newton s 3rd law of motion
Newton’s 3rd Law of Motion

For every action there is an equal and opposite reaction.


Action and Reaction on Different Masses

Consider you and the Earth

Action: Earth pulls on you

Reaction: you pull on Earth


Action: tire pushes on road

Reaction: road pushes on tire


Reaction: gases push on rocket

Action: rocket pushes on gases