Transcript Reaction

Reaction
Forces Between Objects

Try to kick a log.
• The log may move (a force on the log from your foot)
• Your toe may break (a force on your foot from the log)

There are forces on each object at contact.
Force Both Ways

Fsb

Fbs
force of the
ball on the
surface
force of the
surface on the
ball

A falling ball on a paper
surface can break through.

The ball exerts a force on
the paper.

A falling ball on a hard
surface rebounds.

This is a change in velocity,
so the surface exerts a force
on the ball.
Law of Reaction
3
For every action there is an equal and opposite
reaction.

Forces between two objects act in pairs.
F12 = -F21 (Newton’s third law)

reaction force acts
on the rocket
rocket
hot gas is
forced out
acceleration
increases or
changes velocity
Force Pairs

Forces between objects come in pairs.
• A force from object 1 to object 2
• A force from object 2 to object 1

These are forces exactly equal and opposite.
Force is Not Acceleration

Fsb

Fbs
force of the
ball on the
surface
force of the
surface on the
ball

The ball breaking paper
doesn’t stop.

There is a small force on the
ball that slows it down.

The acceleration is small
compared to the breaking
paper.
Not Velocity

Fsb
On a rebound the ground
doesn’t move.

There is a small acceleration
of the earth.
Mass of the Earth: 6 x 1024 kg
Mass of the ball: 0.06 kg
Earth’s acceleration is about
10-26 times smaller

Fbs
force of the
ball on the
surface

force of the
surface on the
ball
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