1. Lesson 4.3 Newton’s 2nd and 3rd Laws
Essential Question: What are Newton’s 2nd and 3rd laws?

2. Derek Jeter is hitting baseballs and softballs just for fun
Derek Jeter is hitting baseballs and softballs just for fun. He hits the baseball and the softball with the same exact force, which one has the greater acceleration?

3. What did Newton’s 1st Law tell us?
When there is no net force, the object is in equilibrium
When there is a net force the object undergoes a change in its velocity
But how much does a known force affect the motion of an object?

4. Imagine pushing a stalled car through a level intersection
Imagine pushing a stalled car through a level intersection. Which situation would allow you to push the car in a shorter time? One person pushing the car? A group of people pushing the car?

6. How do force and acceleration relate?
The acceleration of an object is directly proportional to the net force acting on the object.
The mass of an object affect the object’s acceleration.
It requires less force to accelerate a low-mass object than it does to accelerate a high-mass object at the same rate.

7. Newton’s 2nd Law…
Details the relationship among net force, the mass and the acceleration.

8. What is Newton’s 2nd Law?
If equal forces are applied to two objects of different masses, the object with greater mass will experience a smaller acceleration, and the object with less mass will experience a greater acceleration.

9. Newton’s 2nd Law states…
The acceleration of an object is directly proportional to the net force acting on the object and inversely proportional to the object’s mass.
net force = mass x acceleration

10. The acceleration of an object is in the direction of the net force
The acceleration of an object is in the direction of the net force. If you push or pull an object in a particular direction, it accelerates in that direction. The acceleration has a magnitude proportional to the magnitude of the net force. If you push twice as hard, the acceleration is twice as big.

11. The magnitude of the acceleration is inversely proportional to the mass of the object. The larger the mass, the smaller the acceleration for a given force.

12. Increasing the force will increase the acceleration.
Increasing the mass will decrease the acceleration.

13. Problem Solving…
If more than one force is acting on an object, you must find the net force.
The acceleration will be in the direction of the net force.

14. Roberto and Laura are studying across from each other at a wide table
Roberto and Laura are studying across from each other at a wide table. Laura slides a 2.2 kg book toward Roberto. If the net force acting on the book is 1.6 N to the right, what is the book’s acceleration?

15. Space-shuttle astronauts experience accelerations of about 35 m s 2 during takeoff. What force does a 75 kg astronaut experience during an acceleration of this magnitude?

16. An 8.5 kg bowling ball initially at rest is dropped from the top of 11 m building. The ball hits the ground 1.5 s later. What is the net force on the falling ball?

17. The net force on the propeller of a 3. 2 kg model airplane is 7
The net force on the propeller of a 3.2 kg model airplane is 7.0 N forward. What is the acceleration of the airplane?

18. The net force on a golf cart is 390 N north
The net force on a golf cart is 390 N north. If the cart has a total mass of 270 kg, what are the magnitude and direction of the cart’s acceleration?

19. A car has a mass of 1. 5× 10 3 kg. If the force acting on the car is 6
A car has a mass of 1.5× kg. If the force acting on the car is 6.75× N to the east, what is the car’s acceleration?

20. A soccer ball kicked with a force of 13. 5 N accelerates at 6
A soccer ball kicked with a force of 13.5 N accelerates at 6.5 m s 2 to the right. What is the mass of the ball?

21. A 2.0 kg otter starts from rest at the top of a muddy incline 85 cm long and slides down to the bottom in 0.50 s. What net force acts on the otter along the incline?

22. What do we know about forces?
A force is exerted on an object when that object interacts with another object in its environment.
Forces always exist in pairs.

23. Consider a moving car colliding with a concrete barrier
Consider a moving car colliding with a concrete barrier. The car exerts a force on the barrier at the moment of collision. Furthermore, the barrier exerts a force on the car so that the car rapidly slows down after coming into contact with the barrier.

24. Newton’s 3rd Law states…
If two objects interact, the magnitude of the force exerted on object 1 by object 2 is equal to the magnitude of the force simultaneously exerted on object 2 by object 1, and these two forces are opposite in direction.
For every action, there is an equal and opposite reaction.

25. What is an action-reaction pair?
The forces that the objects exert on each other when two objects interact with one another.
The force that object 1 exerts on object 2 is called the action force.
The force that object 2 exerts on object 1 is called the reaction force.

26. What do we know about action and reaction forces?
The action force is equal in magnitude and opposite in direction to the reaction force.
The reaction force occurs at exactly the same time as the action force.
The action and reaction forces coexist.

27. Action and reaction forces each act on different objects
Action-reaction pairs do not imply that the net force on either object is zero.
The action-reaction pairs are equal and opposite, but either object may still have a net force acting on it.

28. Consider driving a nail into wood with a hammer
Consider driving a nail into wood with a hammer. The force that the nail exerts on the hammer is equal and opposite to the force that the hammer exerts on the nail. But there is a net force acting on the nail, which drives the nail into the wood.

29. What about field forces?
Newton’s 3rd law also applies to field forces. These will also exist in pairs.

30. What are Newton’s 2nd and 3rd laws?