Internal vs. External Forces
This type of force is created inside a rope when it is pulled but not when it is pushed.
What is tension?
This internal force pushes different parts of an object in opposite directions, often causing bending or breaking.
What is shear?
This law explains why the ground pushes you upward when you jump.
What is Newton’s Third Law?
This property allows a trampoline to safely support jumpers.
What is elasticity?
This happens to internal forces when a greater external force is applied.
What is “they increase”?
When a person pushes on a wall, this type of force acts on the wall first and causes internal forces to form.
What is an applied force?
A student compresses a spring but does not let go. This force pushes back on the student’s hand.
What is elastic (spring) force?
In an interaction, these forces are equal in size but opposite in direction.
What are action and reaction forces?
A bent paper clip that stays bent shows this property.
What is plasticity?
When friction increases between two surfaces, this happens to the object’s motion.
What is “motion slows or stops”?
This external force acts even when an object is not moving but surfaces are in contact.
What is friction?
A wrench twisting a metal bolt creates this internal force inside the bolt.
What is torsion?
When a swimmer pushes water backward, this force pushes the swimmer forward.
What is the reaction force?
This property helps prevent permanent damage in bridges and playground equipment.
What is elasticity?
A beam fails because internal forces exceed the material’s ability to resist them. This depends on the beam’s:
What are its properties?
A steel beam bends slightly under a heavy load because these forces develop inside the beam.
What are internal forces?
When a heavy truck drives over a bridge, this internal force increases in the bridge pillars.
What is compression?
When a hammer strikes a nail, this object experiences the reaction force.
What is the hammer?
This explains why two objects experiencing the same force may change shape in different ways.
What is “changes in shape depend on an object’s properties”?
This explains why bridges are designed to bend slightly instead of staying rigid.
What is “to reduce internal forces and prevent breaking”?
This explains why pushing harder on an object increases internal forces even if the object does not move.
What is “External forces cause internal forces within an object”?
When external forces act unevenly across an object, this type of internal force is most likely to cause cracking.
What is shear?
If an object does not move when pushed, this must still be true according to Newton’s Third Law.
What is “the object pushes back with an equal force”?
This property is more desirable for car crumple zones to absorb force permanently.
What is plasticity?
A rope snapping during a lift shows that this internal force became too great.
What is tension?