Force

• Force: (F) push or pull one object exerts on another.
• Measured in Newtons ( 1 N = 1 kg*m/s²)
• Forces can cause a change in an object’s motion.
• More than one force can act on an object at one time.
• Net Force: the combining of forces when two or more forces act on an object.

F = 2 N

F = 5 N

Net Force = 3 N to the left

Force

• Balanced Forces: forces that are equal in size and opposite in direction.
• Does not cause a change in motion because the net force on the object is 0.

F = 4 N

F = 4 N

Net Force = 0 N

Forces on an object can be balanced or unbalanced.

Force

• Unbalanced Forces: forces that are not equal in size and opposite in direction.
• Net force causes the object to move in the direction of the force.

F = 2 N

F = 3 N

Net Force = 6 N to the right

Forces on an object can be balanced or unbalanced.

F = 7 N

Newton’s 1^st Law of Motion

• Newton’s 1^st Law: an object will move at a constant velocity until a net force acts on it.
• An object in motion will stay in motion unless a force acts on it.
• An object at rest will stay at rest unless a force acts on it.

Sir Isaac Newton came up with 3 laws in 1687 that describe how forces affect an object’s motion.

Newton’s 1^st Law of Motion

• Newton’s 1^st Law is also known as the Law of Inertia

When you slam on your brakes, your body still flies forward because it wants to stay in motion like it was before you braked. This is why you wear a seatbelt!!

• Inertia: The tendency of an object to resist a change in motion.
• The more mass an object has, the more inertia it has.

What kinds of forces change an object’s motion?

• Friction: resistance to motion when two objects are in contact with one another.
• Air resistance: resistance an object feels when travelling through the air.
• Gravity: the attraction two objects have on one another
• The force of gravity pulls us towards Earth.

Friction

Depends on 3 factors:

1. The rougher the surface, the greater the friction.
• Rougher surfaces have greater microwelds (microscopic bumps on surfaces that cause friction) which increases the resistance between the objects.
2. The greater the force pushing the two objects together, the greater the friction.
• Increases the opposition of the microwelds between the objects.
3. The greater the surface area of the two objects touching, the greater the friction.
• Increases the contact between the objects.

Friction

Types of Friction:

• Static Friction: force between two surfaces that are NOT moving past each other
• Sliding Friction: force between two surfaces that are sliding past each other
• Rolling Friction: force between a rolling object and the surface it rolls on.

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Air Resistance (drag)

Determined by:

1. Speed
• The greater the speed the greater the resistance
2. Size
• The larger the object the greater the resistance
3. Shape
• The flatter the object the greater the resistance

“Friction in the air”

Law of Universal Gravitation

• Any two masses exert an attractive force on each other.
• Depends on:
• Mass: the greater the mass, the greater the gravitational attraction
• Distance: the lower the distance, the greater the gravitational attraction

Gravity

• This force causes all falling objects to have an acceleration due to gravity of 9.8 m/s².
• Regardless of mass and ignoring air resistance.

Attraction of you to Earth pulls objects in the air downward.

Terminal Velocity

• Maximum velocity a falling object will reach
• Occurs when the force of gravity and air resistance become balanced
• Net force = 0 therefore no acceleration

Force of gravity pulls you down.

Force of air resistance opposes the opposite direction.

= 55 m/s or 118.06 mi/hr

in Humans

Newton’s 2^nd Law of Motion

• Newton’s 2^nd Law: A net force acting on an object causes the object to accelerate in the direction of the net force.
• The greater the mass, the greater the force needed to accelerate it.
• The greater the net force, the greater the acceleration.

How do motion and force relate?

The ball will accelerate in the direction of the net force the mallet applies.

The greater the force applied, the faster the ball will move.

Newton’s 2^nd Law of Motion

F = Force

Measured in N

F = ma

Can be summarized as an equation.

m = mass

Measured in kg

a = acceleration

Measured in m/s²

Example #1: You are pushing a friend on a sled. You push with a force of 40N. Your friend and the sled together have a mass of 80 kg. Ignoring friction, what is the acceleration of your friend on the sled?

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F = 40 N

m = 80 kg

a = ?

F = ma

m

m

a = F

m

a = 40 N

80 kg

a = 0.5 m/s²

Weight

• Weight: The force of gravity on an object
• Because it is a force, you can always calculate weight using F = ma
• When finding weight, acceleration due to gravity is always used = 9.8 m/s²

m = 42 kg

a = 9.8 m/s²

F = ?

​

F = ma

F = ma

F = (42)(9.8)

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F = 411.6 N

Example #2: Find the weight of a suitcase that has a mass of 42 kg.

Weight vs. Mass

• Mass = the amount of matter in an object.
• Weight = the force of gravity on an object
• Meaning it can change based on location!

• The further from Earth you are, the lower your weight
• On the moon your weight would be 1/6^th what it is now.
• If you could be on another planet your weight would change based on the gravity of that planet
• On Jupiter your weight would be over twice as much as it is now.

Newton’s 3^rd Law of Motion

• Newton’s 3^rd Law: every action has an equal and opposite reaction.
• When one object exerts a force on a second object, the second one exerts a force on the first that is equal in size and opposite in direction.
• Ex. When you jump on a trampoline, the trampoline exerts the same force on you but pushes you the opposite direction.

Law of Conservation of Momentum

• Momentum: (p) mass in motion.

• All moving objects have momentum.
• Momentum is transferred between objects in a collision.

p = momentum

Measured in kg*m/s

p = mv

m = mass

Measured in kg

v = velocity

Measured in m/s

Law of Conservation of Momentum

• In a collision, because the forces acting on the two objects are equal and opposite (Newton’s 3^rd Law), the transfer of momentum must be the same.

🡪 Law of Conservation of Momentum: momentum is never created or destroyed in a collision, it only is transferred.

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Law of Conservation of Momentum

Example #3: What is the momentum of a car with a mass of 1300 kg traveling at a speed of 28 m/s?

p = ?

m = 1300 kg

v = 28 m/s

p = mv

p = (1300 kg)(28 m/s)

p = 36,400 kg*m/s

Law of Conservation of Momentum

Example #4: Ball #1 is rolling 11 m/s directly toward a 0.17 kg ball #2 at rest. During the collision, ball #1 stops and ball #2 is launched forward at 9 m/s. What is the mass of ball #1? (think about the Law of Conservation of Momentum)

p₁ = ?

m₁ = ?

v₁ = 11 m/s

p₂ = ?

m₂ = 0.17 kg

v₂ = 9 m/s

p = mv

Due to Law of conservation of momentum…

p₁ = p₂

p₂ = (0.17)(9)

p₂ = 1.53 kg*m/s

p₁ = m₁v₁

v₁

v₁

m₁ = p₁

v₁

m₁ = 1.53

11

m₁ = 0.14 kg

m₁ = ?

m₂ =

0.17 kg

v₁ = 11 m/s

v₂ = 9 m/s

m₁ = p₁

v₁