ST. ROSE: Science Inelastic and Elastic Collisions with Newton's 2nd Law

What is a Collision

A collision occurs when momentum or kinetic energy is transferred between objects.

What is Kinetic Energy

Energy of motion

What is Momentum

The quantity of that motion (kinetic)

What are the types of Collisions

Inelastic and Elastic

What is an Inelastic Collision

1. In a collision where objects do not bounce apart

2. momentum is conserved, but kinetic energy is not conserved.

3. Kinetic energy may convert into heat, sound, or deformation

What is a Elastic Collision

1. Objects bounce apart after a collision.

2. No energy is lost through heat, sound, or deformation

3. In elastic collisions, momentum and kinetic energy are conserved.

What happens in a perfectly Inelastic Collision

Objects stick together, so their final velocities are the same

What is another name for Newton’s 3rd law

Action-Reaction Force Pairs

For an Action-Reaction Pair there must be a…

two forces and two objects

What are some common misconceptions about Newton’s 3rd law

1. “For every action FORCE there is an equal and opposite reaction FORCE,” applies only to forces

2. Actions do not cancel out because cancellations of forces only happen to one object

3. Forces occur simultaneously, and the reaction force is not delayed

Example of Newton’s 3rd law only applying to FORCES

A toddler grabbing a sibling’s water glass and the sibling screaming is an “action-reaction” in everyday terms, but not related to Newton’s 3rd Law because it’s not a force interaction in the physics sense.

The relation between Newton’s 3rd law and Newton’s 2nd Law Example

• When you jump off a dock, you apply a force on the dock (action force).

• The dock simultaneously applies an equal and opposite force on you (reaction force).

• These forces act on different objects and do not cancel each other out because cancellation of forces only occurs when forces act on the same object

• Newton’s 2nd Law F=ma

• The dock’s mass is very large, so it accelerates very little when pushed.

• Your mass is smaller, so you accelerate significantly when pushed back by the dock’s force.

An example of Newton’s 3rd Law happens at the SAME TIME

• The magnitude of the force exerted by the second object is only as much as needed to exactly balance the forces applied by the first object, regardless of the object’s potential to exert a greater force.

• If you push a wall with 50 newtons, the wall pushes back with exactly 50 newtons, despite being capable of exerting much more force.

• If you push a smaller person with 50 newtons, they push back with 50 newtons but accelerate faster due to lower mass.