Newton's Laws

Newton's Laws

Newton's 1st Law

(Law of Inertia)

• "An object in motion stays in motion and an object at rest stays at rest UNLESS acted upon by an UNBALANCED force."

• Mass = Inertia (more mass means more inertia)

Multiple Choice

Inertia depends on ______

• Velocity

• Speed

• Distance

• Mass

Multiple Choice

A skateboard rolls down a hill, reaches the level surface and eventually comes to a stop. The fact that the skateboard ultimately stops is best explained by ____.

• an unbalanced force would be required to keep the sled moving forward at constant speed

• the presence of an unbalanced force (e.g., friction) can cause a moving object to stop

• the presence of inertia causes it to stop

• the natural tendency of any object is to ultimately stop

Multiple Choice

An astronaut kicks a soccer ball in outer space at a location far from significant influences of gravity and air resistance. The soccer ball will _________.

• continue in motion with the same speed and direction

• eventually stop as its inertia slowly becomes used up

• eventually stop since all objects ultimately "lose their steam"

• either a, b, or c -- depending on whether the astronaut continues to kick it

Multiple Choice

List the objects in order from least to greatest inertia.

• CDBA

• ABCD

• BCDA

• CBAD

Newton's 2nd Law

(Law of Acceleration)

• An object's acceleration is PROPORTIONAL to the Force it expereinces (more force, more acceleration)

• An object's acceleration is INVERSELY PROPORTIONAL to its mass (more mass, less acceleration)

Newton's 2nd Law

•  $F_{net}\ =\ ma$  

•  $a\ =\ \frac{F_{net}}{m}$  

•  $m\ =\ \frac{F_{net}}{a}$  

• The man applies 400 N of force to each ball. What is the acceleration of each ball?

Newton's 2nd Law

• Weight is a force calculated using Newton's 2nd Law (measured in Newtons)

•  $F_g=mg$  

•  $F_g$  = weight or force of gravity

• m = mass

• g = acceleration due to gravity (10 m/s/s on Earth)

Multiple Choice

True or False. Acceleration of an object is directly proportional to the net force acting on it

• True

• False

Multiple Choice

Your car has a mass of 2500 kg and a force of 5000 N. How fast will it accelerate?

• 20 m/s²

• 0.5 m/s²

• 2 m/s²

• 120 N

Multiple Choice

A force of 90N is applied to each cart below, which one will accelerate the fastest?

• Cart 1
• Cart 2
• Cart 3 
• Cart 4

Multiple Choice

A 25 kg bike accelerates at 10 m/s². What was the force?

• 250 N

• 2.5 N

• 25 N

• 15 N

Multiple Choice

If the man pushes with an equal amount of force, which object will accelerate more?

• Car because it has more mass

• Lawnmower because it has less mass

• Car because it has less mass

• Lawnmower because it has more mass

Multiple Choice

What is the weight of a 25 kg object on Earth?

• 2.5 N

• 25 N

• 250 N

• 2500 N

Newton's 3rd Law

(Law of Action-Reaction)

"For every force there is an equal and opposite force."

Newton's 3rd Law

All forces come in pairs.

If you push down on the Earth, it pushes up on you with exactly the same amount of force.

Poll

A bug hits the schoolbus windshield on your ride. What is true of the forces experienced by the bug and the bus windshield?

• The bug experiences more force

• The bus windshield experiences more force

• The bug and the bus windshield experience the exact same amount of force

They experience the same force!

Don't confuse the force with the EFFECT of the force...

We know the bug is probably going to get smushed but the bus is fine...that goes back to Newton's 2nd Law!

Newton's 3rd Law

Example:
Action force: Your foot pushes down on the earth with 400 N of force.
Reaction force: The earth pushes up on your foot with 400 N of force.

RESULT - you get accelerated forward a lot because you have less mass than the Earth!

 $a=\frac{F_{net}}{m}$  

Action - Reaction Pairs

To determine the action-reaction pair and direction, just switch the objects and think about the opposite direction.

Example:

Action: I push forward on a box.

Reaction: The box pushes backwards on me.

Multiple Choice

According to Newton's Third law the strength or magnitude of forces will be...

• smaller
• equal
• larger

Multiple Choice

When a person is skateboarding, their leg is pushing BACKWARD against the ground. In turn, the ground pushes __________ against their foot allowing them to skateboard.

• DOWN

• UP

• FORWARD

• BACKWARD

Multiple Choice

When a bird is flying, their wings are pushing DOWN against the air. In turn, the air pushes __________ against their wings allowing them to fly.

• DOWN

• UP

• FORWARD

• BACKWARD

Multiple Choice

When Ronda Rousey got kicked in the face, What is the interaction pair?

• The foot and the face
• Ronda Rousey and media criticism 
• The foot and the air flow
• Ronda Rousey and the floor

Multiple Choice

When a heavy football player and a light one run into each other, who exerts more force

• The light one

• Their force are equal

• The heavy one

Multiple Choice

What is the reaction force to a foot pushing down on the floor?

• The foot going through the floor
• The floor pushing back against the foot
• The floor breaking to the foot

Multiple Choice

A baseball player bats a ball with a force of 1,000 N. The ball exerts a reaction force (without any additional forces action on it) against the bat of...

• Less than 1, 000 N
• 1,000 N
• More than 1,000 N

Multiple Choice

A player hits a ball with a bat. The action force is the impact of the bat against the ball. The reaction force is...

• The grip of the player's hands on the ball
• the air resistance of the ball
• the force of the ball against the bat

Multiple Choice

An archer shoots an arrow. The action force is the bowstring against the arrow, the reaction force is...

• Air resistance against the bow
• the arrow's push against the bowstring
• the grip of the archer's hand on the bow