 Taking ownership or control of the information learned in class and taking responsibility for your work

This means:
• Practicing (reviewing, restating, preparing)
• Applying (creating, thinking, using)
• Planning (studying, completing projects and homework, being prepared-pencils, books)

Chapter 2

 Force
 a push or pull that one body exerts on another

What forces are being exerted on the football?
F kick
F grav

 Forces are measured in Newtons (kg .
m/s 2 ).
 Forces are measured using a spring scale or force meter.

 Balanced Forces
 forces acting on an object that are opposite in direction and equal in size
 no change in velocity

Balanced Forces
(Balanced Forces = No motion)
5 N Object
5 N
Notice that all the forces are equal and pointed in the opposite direction. Hence they balance each other – or cancel each other.
Equal Pushing
Forces may cancel each other and produce No Motion !

5 N
Balanced Forces
(Balanced Forces = No motion)
5 N
Equal Pushing
Equal Pulling
5 N 5 N
Forces may cancel each other and produce No motion!

 Net or unbalanced Force
 unbalanced forces that are not opposite and equal
 velocity changes (object accelerates)
F net
F friction
N
W
N
F pull

Unbalanced Forces
Causes Motion
Adding Forces Subtracting Forces

Unbalanced Forces
Causes motion
Adding Forces
5 N
Object
5 N
Notice that all the forces are pointed in the same direction. Hence they add together.

10 N = 5 N 5 N
• Two forces can subtract to produce a net force in the direction of the larger force.
– Negative motion occurs (Slow Down)

Unbalanced Forces
Causes Motion
Subtracting Forces
5 N
Object
5 N
5 N
Notice that all the forces are unequal and pointed in the opposite direction. Hence they are unbalanced and in opposition to each other
– or one partially cancels the other.

10N
10N
10N
Balanced
Push i.e. Pushing a
Car
No motion
10N
Balanced
Pull i.e. Tug-of-war
No motion

10N
10N
Additive
Un-Balanced
Same
Direction i.e. sled
10N
10N
Subtractive Un-Balanced
10N
Opposite
Direction i.e. football tackle
Faster motion
Slower motion

Friction
 Friction
 force that opposes motion between 2 surfaces
 depends on the:
• types of surfaces
• force between the surfaces

Friction
 Friction is greater...

 between rough surfaces when there’s a greater force between the surfaces
(e.g. more weight)

 Mass and gravity have a directly proportional relationship.
 The distance between the centers of masses of the two objects and gravity have an inversely proportional relationship.

Gravity
 Gravity
 force of attraction between any two objects in the universe
 increases as...
• mass increases
• distance decreases

Gravity
 Who experiences more gravity - the astronaut or the politician?
 Which exerts more gravity the Earth or the moon?
less distance more mass

Gravity
 Would you weigh more on Earth or
Jupiter?

Jupiter because...
greater mass greater gravity greater weight

Balanced Forces
(Balanced Forces = No Motion)
Ground pushes up
Gravity pulls down
Gravity pulls down on you…
The ground pushes back up…
THIS KEEPS YOU WHERE YOU ARE!
If these football players push on each other equally as hard, will either one move?

Newton’s First Law
 Newton’s First Law of Motion

An object at rest will remain at rest and an object in motion will continue moving at a constant velocity unless acted upon by a net or unbalanced force .

Two Key Points

Newton’s First Law
 Newton’s First Law of Motion

“Law of Inertia”
 Inertia

 tendency of an object to resist any change in its motion increases as mass increases

Can you explain how this trick works, using newton’s first law of motion?

Newton’s Second Law
 Newton’s Second Law of Motion

The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass.
F = ma

Newton’s Second Law a
 F m m
F a
F = ma
F: force (N)
m: mass (kg)
a: accel (m/s 2 )
1 N = 1 kg ·m/s 2


Calculations
What force would be required to accelerate a 40 kg mass by 4 m/s 2 ?
GIVEN:
F = ?
m = 40 kg a = 4 m/s 2 m
F a
WORK :
F = ma
F = (40 kg)(4 m/s 2 )
F = 160 N


Calculations
A 4.0 kg shotput is thrown with 30 N of force.
What is its acceleration?
GIVEN: m = 4.0 kg
F = 30 N a = ?
F m a
WORK : a = F ÷ m a = (30 N) ÷ (4.0 kg) a = 7.5 m/s 2

ConcepTest
 Is the following statement true or false?

An astronaut has less mass on the moon since the moon exerts a weaker gravitational force.

False! Mass does not depend on gravity, weight does. The astronaut has less weight on the moon.

Newton’s Third Law
 Newton’s Third Law of Motion

When one object exerts a force on a second object, the second object exerts an equal but opposite force on the first.

Newton’s Third Law
 Action-Reaction Pairs
 The hammer exerts a force on the nail to the right.
 The nail exerts an equal but opposite force on the hammer to the left.

Newton’s Third Law
 Action-Reaction Pairs
 The rocket exerts a downward force on the exhaust gases.
 The gases exert an equal but opposite upward force on the rocket.
F
G
F
R

Newton’s Third Law
 Action-Reaction Pairs
 Both objects accelerate.
 The amount of acceleration depends on the mass of the object and the force applied.
 Force = mass x acceleration F=ma
 Small mass
 more acceleration
 Large mass
 less acceleration