Figure 1. These objects are at rest1. They are not moving.
"An object at rest remains at rest unless acted on by an external, unbalanced force.
An object in motion will stay at rest unless acted on by an unbalanced force."
"An object at rest1"
This part is pretty self explanatory. Rest just means stationary,
no movement. A book on a table, your fridge, they aren't
moving, so they are at rest.
Figure 1. These objects are at rest1. They are not moving.
"remains at rest1 unless acted on by an external, unbalanced force."
This just means that they will stay stationary until an outside force is applied on it.
If you're still having a bit of trouble under standing that, just think of an object not moving (at rest).
Think of Figure 1. How weird would it be if the book just randomly started moving?
That's all that this part is saying; object at rest stay at rest until something/someone tries to move it (applying external,
unbalenced force), which is what the second part of it means.
Figure 2. Figure 2a on the left shows the ball at
rest. Figure 2b shows an external, unbalanced
force applied to the ball, making it move.
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1: At Rest -- Stationary; not moving.