Forces

1
Forces Motion
2
Motion

• A change in the position of an object
• Caused by force (a push or pull)

3
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

4
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)

5
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.

6
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.

7
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.

8
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.

9
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.

10
Calculating Net Force
10 N
30 N
http//commons.wikimedia.org/wiki/FileJack-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.
11
Check Your Answer
10 N
30 N
http//commons.wikimedia.org/wiki/FileJack-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
12
Calculating Net Force
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? In your notebook, calculate the net force
and show all of your work.
13
Check Your Answer
Tug of War Game
50 N
30 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
14
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
  Earths gravitational pull.
• Weight is measured in newtons (N).

15
Mass and Weight

• Example Fma
• 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.

16
Force

• Force (N) mass (kg) x acceleration
  (m/s2)
• F
• m x a

17
Force

• Acceleration (m/s2) force (N) mass (kg)
• F
• m x a

18
Force

• Mass (kg) force (N) acceleration (m/s2)
• F
• m x a

19
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

20
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

21
Practice

• A 56 kg cart is accelerating at 15 m/s2.
  Calculate the force
• exerted on the cart?
• Force
• Mass
• Acceleration

22
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

23
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

24
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