Forces
&
Motion
Motion
• A change in the position of an object
• Caused by force (a push or pull)
Force
•
•
•
•
A push or pull on an object
Measured in units called newtons (N)
Measured with a spring scale
Forces act in pairs
Types of Force:
–
–
–
–
–
gravity
electric
magnetic
friction
centripetal
Inertia
• An object at rest will remain at rest until acted
upon by an unbalanced force.
• An object in motion will remain in motion until
acted upon by an unbalanced force.
• When all forces acting on an object are equal
(balanced)
Balanced Forces
• When all the forces acting on an object are equal
(net force is zero)
• Balanced forces do not cause a change in motion.
• Balanced forces can change the physical
properties of an object without changing its
motion.
• In your notes, describe an example of a balanced
force.
How Can Balanced Forces
Affect Objects?
• Cause the shape of an object to change without
changing its motion
• Cause an object at rest to stay at rest or an object
in motion to stay in motion (inertia)
• Cause an object moving at a constant speed to
continue at a constant speed
• In your notes, describe an example of a balanced
force affecting an object.
Unbalanced Forces
• When all the forces acting on an object are not
equal
• The forces can be in the same direction or in
opposite directions.
• Unbalanced forces cause a change in motion.
• In your notes, describe an example of an
unbalanced force.
How Can Unbalanced Forces
Affect Objects?
• Acceleration is caused by unbalanced forces:
–
–
–
–
–
–
slow down
speed up
stop
start
change direction
change shape
• In your notes, describe an example of an unbalanced
force affecting an object.
Net Force
• The total of all forces acting on an object:
– Forces in the same direction are added.
• The object will move in the direction of the force.
– Forces in opposite directions are subtracted.
• An object will move in the direction of the greater
force.
Calculating Net Force
10 N
30 N
http://commons.wikimedia.org/wiki/File:Jack-in-the-box.jpg
A child was playing with a jack in the box. The lid pushed
down with 10 N of force, while the spring pushed up with 30
N of force. What is the net force applied by the spring?
In your notebook, calculate the net force and show all of your
work.
Check Your Answer
10 N
30 N
http://commons.wikimedia.org/wiki/File:Jack-in-the-box.jpg
A child was playing with a jack in the box. The lid pushed
down with 10 N of force, while the spring pushed up with 30
N of force. What is the net force applied by the spring?
30 N up - 10 N down= 20 N up
Calculating Net Force
30 N
Tug of War
Game
50 N
Two competitors are playing tug of war.
What is the net force? Which direction will the rope move?
In your notebook, calculate the net force and show all of your
work.
Check Your Answer
Tug of War
Game
30 N
50 N
Two competitors are playing tug of war.
What is the net force? Which direction will the rope move?
50 N to the right – 30 N to the left = 20 N to the right
Mass and Weight
• Mass is the amount of stuff (matter) in an object.
– The mass of an object will remain the same anywhere in the
universe.
– Mass is measured in grams or kilograms.
• Weight is the mass (kg) of an object multiplied by the
acceleration of gravity (9.8 m/s2).
– The weight of an object depends on the gravitational pull of the
location in the universe.
– The gravitational pull on the Moon is only 1/6 of Earth’s
gravitational pull.
– Weight is measured in newtons (N).
Mass and Weight
Example: F=ma
A person with a mass of 25 kg x 9.8 m/s2 gravitational
acceleration is equal to 245 N of weight force.
F= 25 kg x 9.8 m/s2
F = 245 N
The person exerts 245 N of force on the ground.
If the person went to the Moon, they would only weigh 1/6
of 245 N.
Force
Force (N) = mass (kg) x acceleration (m/s2)
F
÷
m
x
a
Force
Acceleration (m/s2) = force (N) ÷ mass (kg)
F
÷
m
x
a
Force
Mass (kg) = force (N) ÷ acceleration (m/s2)
F
÷
m
x
a
Practice
The acceleration of a sprinter is 10 m/s2. The force exerted
on the starting blocks is 650 N. What is the mass of the
sprinter?
Force =
Mass =
Acceleration =
Check Your Answer
The acceleration of a sprinter is 10 m/s2. The force exerted
on the starting blocks is 650 N. What is the mass of the
sprinter?
Force = 650 N
Mass = 65 kg
Acceleration = 10 m/s2
Mass = force ÷ acceleration
Mass = 650 ÷ 10
Mass = 65
Practice
A 56 kg cart is accelerating at 15 m/s2. Calculate the force
exerted on the cart?
Force =
Mass =
Acceleration =
Check Your Answer
A 56 kg cart is accelerating at 15 m/s2. Calculate the force
exerted on the cart?
Force = 840 N
Mass = 56 kg
Acceleration = 15 m/s2
Force (N) = mass (kg) x acceleration (m/s2)
Force = 56 x 15
Force = 840
Practice
A boat is pulling a 54 kg wake boarder. She accelerates at
2 m/s2. Calculate the force that is causing the acceleration.
Force =
Mass =
Acceleration =
Check Your Answer
A boat is pulling a 54 kg wake boarder. The force the boat
is exerting on her is 108 N. Calculate her acceleration.
Force = 108 N
Mass = 54 kg
Acceleration = 2 m/s2
Acceleration = force ÷ mass
Acceleration = 108 ÷ 54
Acceleration = 2