1. Lesson 4.2 Newton’s First Law
Essential Question: What is Newton’s First Law?

2. If you set a book on your desk, what happens to it?
What happens to the book if you give it a nudge?

3. What does force do?
It takes force to make an object start moving or change direction.
The more massive an object is, the larger the force that is required for a given change.

4. What happens to an object with no force acting on it?
The object could be at rest.
The object could also be moving with a constant velocity.

5. What is Newton’s First Law of Motion?
An object at rest remains at rest, and an object in motion continues in motion with constant velocity (that is, constant speed in a straight line) unless the object experiences a net external force.

6. What is inertia?
The tendency of an object to resist being moved or, if the object is moving, to resist a change in speed or direction.
It’s the tendency of an object not to accelerate.
Mass is a measure of inertia
More mass produces more resistance to a change in velocity

7. Which object in each pair has more inertia?
A baseball at rest or a tennis ball at rest
A tennis ball moving at 125 mi/h or a baseball at rest

8. Newton’s First Law is often referred to as the law of inertia.
Newton’s First Law says that when the net external force on an object is zero, the object’s acceleration (or the change in the object’s velocity) is zero.

9. What is an external force?
A single force that acts on an object as a result of the interaction between the object and its environment

10. What is a net force? The vector sum of all forces acting on an object.
a single force whose external effects on a rigid body are the same as the effects of several actual forces acting on the body

11. The net force can be found by using the methods for finding resultant vectors.
The net force is equivalent to the one force that would produce the same effect on the object that all of the external forces combined would.

13. What is mass?
Mass is a measure of the amount of matter and it is also a measure of inertia.
The inertia of an object is proportional to the object’s mass.

14. Imagine a basketball and a bowling ball at rest side by side on the ground. You supply a net force on each ball. If the two are pushed with equal force, the basketball will accelerate much more than the bowling ball does. Why?

15. The greater the mass of the body, the less the body accelerates under an applied force.
A light object undergoes a larger acceleration than does a heavy object under the same force.

16. How do you find the net force on an object?
Define the problem and identify the variables.
Select a coordinate system, and apply it to the free-body diagram.
Find the x and y components of all vectors.
Find the net force in both the x and y directions.
Find the net force.

17. A crate is pulled to the right with a force of 82
A crate is pulled to the right with a force of 82.0 N, to the left with a force of 115 N, upward with a force of 565 N, and downward with a force of 236 N. Find the magnitude and direction of the net force on the crate.

18. Derek leaves his physics book on top of a drafting table that is inclined at a 35° angle. The free-body diagram shows the forces acting on the book. Find the net force acting on the book.

19. A flying, stationary kite is acted on by a force of 9. 8 N downward
A flying, stationary kite is acted on by a force of 9.8 N downward. The wind exerts a force of 45 N at an angle of 50.0° above the horizontal. Find the force that the string exerts on the kite.

20. A man is pulling on his dog with a force of 70
A man is pulling on his dog with a force of 70.0 N directed at an angle of 30.0° to the horizontal. Find the x and y components of this force.

21. A gust of wind blows an apple from a tree
A gust of wind blows an apple from a tree. As the apple falls, the gravitational force on the apple is 2.25 N downward, and the force of the wind on the apple is 1.05 N to the right. Find the magnitude and direction of the net force on the apple.

22. The wind exerts a force of 452 N north on a sailboat, while the water exerts a force of 325 N west on the sailboat. Find the magnitude and direction of the net force on the sailboat.

23. What is equilibrium?
The state in which the net force on an object is zero.
Objects are either at rest or moving with constant velocity.

24. To determine whether a body is in equilibrium, find the net force.
If the net force is zero, the body is in equilibrium.

25. What if an object has a net force not equal to zero?
To achieve equilibrium, a second force equal and opposite to the net force must be applied.

27. A light hangs from two cables. One cable has a tension of 5
A light hangs from two cables. One cable has a tension of lb and is at an angle of 17.2° with respect to the ceiling. What is the weight of the lamp if the other cable makes an angle of 78.8° with respect to the ceiling?

28. A light hangs from two cables. If one cable has a tension of 18
A light hangs from two cables. If one cable has a tension of N and is at an angle of 33.9° with respect to the ceiling and the other cable has a tension of 15.2 N, what is the angle the second cable makes with respect to the ceiling?

29. A light hangs from two cables. One cable has a tension of 25
A light hangs from two cables. One cable has a tension of N and is at an angle of 7.5° with respect to the ceiling. What is the tension in the other cable if it makes an angle of 20.2° with respect to the ceiling?

30. What is Newton’s First Law?