Newton's Third Law of Motion

Question 1

what does a wall do when you push onto it

Answer 1

• it exerts an equal and opposite force back on you

- by pushing back on you

Question 2

student a and b push against each other. what happens when they do this

Answer 2

• they exert a force on the other one
• so they exert equal and opposite forces on each other
• causing them to move away from each other

Question 3

what does newtons third law of motion always apply to

Answer 3

a pair of objects

Question 4

how does newtons third law apply to the relationship between the earth and the moon

Answer 4

• the earth exerts a gravitational force on the moon

- while the moon also exerts a gravitational force on the earth

Question 5

what are the effects of both of these gravitational forces acting on both objets

Answer 5

• the gravitational force exerted on the moon by the earth is what keeps it in orbit
• while the gravitational force of the moon is what gives rise to tides

Question 6

what are the main rules that must be covered in order for newtons third law to work

Answer 6

• they forces must act on two separate bodies
• the forces must be of the same type
• they must act along the same line
• the forces must be equal in magnitude
• and they must act in opposite direction

Question 7

can there only be one type of force acting on the two bodies

Answer 7

no

Question 8

what is an example of two pairs of forces acting between two bodies

Answer 8

someone standing on earth

Question 9

what are the two forces acting in this system and how

Answer 9

• the gravitational force of the person pulls the earth up while the gravitational force of the earth pulls the person down
• as the person is standing on the earth, there is also a contact force of the person pushing against the earth
• and the earth pushing on the woman
• this is the normal reaction force

Question 10

what term could be used to simplify newtons third law

Answer 10

every action has an equal and opposite reaction

Question 11

why shouldnt this definition be used however

Answer 11

• because it doesnt define the difference between an action and a reaction or which is which in a system
• action refers to the force acting on one body while reaction refers to the second body’s resultant force on the first body
• this is too ambiguous and cant even be applied in every case

Question 12

what is an example of how that phrase cant be applied to this law

Answer 12

• with the gravitational forces between objects
• they more or less simultaneously exert their forces on each other
• one doesnt wait for a gravitational force to be exerted on it before it begins exerting its gravitational force as a reaction

Question 13

using the example with the person standing on the earth, why are they considered to be in equilibrium

Answer 13

• the weight on the person acting downwards and the reaction force from the surface of the earth on them upwards are equal and opposite
• this means that there is no resultant force acting on the person
• meaning they are at rest and therefore in equilibrium

Question 14

would the person still be in equilibrium if she jumped

Answer 14

no

Question 15

why wouldnt the person be in equilibrium

Answer 15

• there will still be the gravitational force of the earth acting on the person as well as their gravitational force acting on the earths
• these forces are equal and opposite
• however, because shes not touching the ground there is no normal reaction force
• as there is only one force acting on them when they exerts two, they are not in equilibrium