1. MOTION & ITS APPLICATIONS  Newton’s First Law of Motion (P .35-37)
SPH4C COLLEGE PHYSICS
MOTION & ITS APPLICATIONS
 Newton’s First Law of Motion (P )

2. Galileo’s View of Force & Motion
Prior to the 1600s, early scientists held a simplistic view of motion. They noticed that moving objects would spontaneously slow down for no apparent reason. Since they did not know about friction they concluded that a constant net force was needed to keep an object moving. They thought – based upon the teachings of Aristotle – that larger net forces made things move faster and smaller net forces made things move slower.

3. Galileo’s View of Force & Motion
Galileo questioned this view. He performed real experiments with a ball rolling down and up sloped ramps. He also performed virtual experiments to try to explain his ideas.

4. Galileo’s View of Force & Motion
He reasoned that a ball speeds up as it rolls down a slope, then moves with constant velocity along the horizontal surface, and finally rolls up the far slope to the same level it started from.

5. Galileo’s View of Force & Motion
And if the ramp is made less steep, the ball still rises to the same height.

6. Galileo’s View of Force & Motion
He further reasoned, logically, that if the slope of the second plane was zero, the ball, once rolling, would continue forever with no loss of speed, in an attempt to reach its original height.

7. Galileo’s View of Force & Motion
Of course, when he performed the real experiment he observed that the ball rolling along the horizontal surface eventually stopped. However, he was able to assume correctly that the ball slowed down to zero velocity because of friction.

8. Inertia
Galileo concluded that once an object starts moving, it will continue moving at a constant velocity if there is no friction present. Galileo used the concept of inertia to help explain his conclusion. Inertia is the property of matter that causes it to resist changes in motion. The inertia of an object depends on the mass of the object.
INERTIA
property of matter that causes it to resist changes in motion
is directly proportional to the mass of the object

9. Examples of Inertia NOTE!
Since an object’s mass is a measure of its inertia, and vice versa, there are many everyday examples of objects resisting a change in their motion, including the following:

10. Examples of Inertia
When a car stops suddenly, a passenger in the front seat not wearing a seat belt continues to move forward (due to the passenger’s inertia) and collides with the car’s windshield.

11. Examples of Inertia
A magician pulls a smooth tablecloth quickly out from under a place- setting of expensive china. Due to the inertia, the dishes remain at rest where they were, and are not broken.

12. Examples of Inertia
If a coin is balanced on a horizontal card on top of a glass, and the card is then flipped away, the coin, because of its inertia, will drop into the glass.

13. Examples of Inertia
It is more difficult to get a stalled car moving by pushing it than it is to keep it moving. Also, it is difficult to stop a stalled car that is coasting by pushing backwards on it. The car’s inertia makes it difficult to move when stopped, and difficult to stop, when moving.

14. Newton’s First Law of Motion: The Law of Inertia
Although Galileo discovered the notion of “inertia” it was Newton who coined the term when he summarized Galileo’s work in his book Principia Mathematica. Because it was included with Newton’s other laws of motion, it is often referred to as Newton’s first law of motion.
NEWTON’S FIRST LAW OF MOTION
also known as law of inertia
if the net external force on an object is zero, the object will remain
at rest or continue to move at a constant velocity

15. Newton’s First Law of Motion – Implications
This law has several significant implications, which must be clearly understood:
Objects at rest remain at rest unless acted upon by an external unbalanced force. For example, a ball on a horizontal floor will remain at rest forever, unless someone gives it a push.

16. Newton’s First Law of Motion – Implications
This law has several significant implications, which must be clearly understood:
Moving objects continue to move in a straight line at a constant speed, unless acted upon by an external unbalanced force. For example, a car moving into a flat icy curve will tend to continue in a straight line, off the side of the road.

17. Newton’s First Law of Motion – Implications
This law has several significant implications, which must be clearly understood:
3. An external force is required to change the velocity of an object. Internal forces have no effect on an object’s motion. For example, a passenger pushing on the dashboard of the car does not cause the car’s velocity to change.

18. Newton’s First Law of Motion – Implications
This law has several significant implications, which must be clearly understood:
4. The external force must be unbalanced; that is, two equal opposing forces acting on an object will not change its velocity. For the object’s velocity to change, the vector sum of the applied forces on the object must be different than zero.

19. Newton’s First Law of Motion
IMPLICATIONS OF ...
objects at rest tend to remain at rest
objects in motion tend to remain in motion
if the velocity is constant, the net force acting on it must be zero
if the velocity is changing (in direction and/or magnitude) the change must be caused by a net external force acting on the object

20. Newton’s First Law of Motion
PRACTICE
Older cars did not have headrests, but all new cars do. How do headrests help prevent injuries during a rear-end collision? Use Newton’s first law to explain your answer.
During a rear-end collision, the car will suddenly accelerate forward and so will your body because the seat exerts a force directed forward on your torso. In a vintage car with no headrest, there is no force applied to the head.
As a result your head will continue to remain at rest (law of inertia). Your head will initially appear to snap backwards relative to your body as your body accelerates forward, possibly resulting in a neck injury known as whiplash.

21. Newton’s First Law of Motion
The headrest in a modern car helps push the head forward with the rest of the body. This helps to prevent whiplash since your neck does not bend backwards as far during a rear- end collision.

22. Newton’s First Law of Motion
PRACTICE
2. How does a seat belt work?
A seat belt uses the sudden decrease in velocity of the car to activate a gear mechanism. In this design, the seat belt strap is attached to a spool which in turn is attached to a gear. Beneath this gear is a pendulum that is free to swing back and forth.
When the car comes to a sudden stop, the pendulum swings forward due to inertia. This causes the pendulum to move a metal stop into the teeth of the gear, locking the seat belt in place.

23. Newton’s First Law of Motion
APPLICATIONS OF ...
Newton’s first law is observed and applied in many situations and
technological applications including the restraint systems in
automobiles (i.e. headrests, seatbelts, airbags, ...).

24. INTERACTIONS OF SCIENCE & TECHNOLOGY
problem identified (i.e. automobile deaths)
research and a solution invented (i.e.airbags)
solution causes other problems( i.e.air bag deaths)
research begins again

25. Check your understanding
TEXTBOOK P.37 Q. 1,5,6