5.8 Newton's Laws

Question 1

What does Newton’s first law state?

Answer 1

An object at rest stays at rest, and an object in motion stays in motion at the same speed and direction unless acted on by an outside force

Question 2

What is inertia?

Answer 2

A body’s natural resistance to any change in motion

Question 3

What are two other names for Newton’s first law?

Answer 3

The law of inertia and the law of equilibrium

Question 4

Example of inertia in everyday life?

Answer 4

Pushing a heavy cabinet: more effort to start moving than to keep moving

Question 5

Example of inertia in the body?

Answer 5

A patient lying in bed struggles at first to sit up but finds it easier to keep moving once started

Question 6

Why is understanding inertia important in PT?

Answer 6

It explains why early mobilization is challenging and helps plan safer interventions

Question 7

What does Newton’s second law state?

Answer 7

Acceleration of a body depends on the force applied and the body’s mass

Question 8

What is the equation for Newton’s second law?

Answer 8

Force = mass × acceleration

Question 9

How does more force affect acceleration?

Answer 9

It increases acceleration

Question 10

How does more mass affect acceleration?

Answer 10

It decreases acceleration

Question 11

Example of Newton’s second law in objects?

Answer 11

A four-drawer cabinet requires more force to move than a two-drawer cabinet

Question 12

Example of Newton’s second law in the body?

Answer 12

Two patients with equal muscle strength but different body mass have different force requirements for the same movement

Question 13

Why must PTs consider mass as well as strength in exercise prescription?

Answer 13

Because heavier patients require more force to perform the same task, even if strength grades are equal

Question 14

What does Newton’s third law state?

Answer 14

For every action, there is an equal and opposite reaction

Question 15

Example of Newton’s third law with an object?

Answer 15

A notebook is held still because gravity pulls down while muscles apply an equal upward force

Question 16

Example of Newton’s third law in PT?

Answer 16

During sit-to-stand, pushing down on a chair produces an equal upward reaction force that helps lift the body

Question 17

Why might a patient struggle with sit-to-stand despite using the chair?

Answer 17

Weak upper body force leads to insufficient reaction force from the chair

Question 18

Why is Newton’s third law important in PT?

Answer 18

It applies to transfers, walking, balance recovery, and exercise design