1. Stuff Ya Gotta Know-Newton's 1st & 2nd Laws
Chapter 4
Newton’s 1st and 2nd Laws of
Motion

2. Stuff Ya Gotta Know-Newton's 1st & 2nd Laws
What do you think?
If a net force acts on an object, what type of motion will be observed?
Why?
How would this motion be affected by the amount of force?
Are there any other factors that might affect this motion?
When asking students to express their ideas, you might try one of the following methods.
(1) You could ask them to write their answers in their notebook and then discuss them.
(2) You could ask them to first write their ideas and then share them with a small group of 3 or 4 students. At that time you can have each group present their consensus idea. This can be facilitated with the use of whiteboards for the groups.
The most important aspect of eliciting student’s ideas is the acceptance of all ideas as valid. Do not correct or judge them. You might want to ask questions to help clarify their answers. You do not want to discourage students from thinking about these questions and just waiting for the correct answer from the teacher. Thank them for sharing their ideas. Misconceptions are common and can be dealt with if they are first expressed in writing and orally.
Hopefully, students will follow Newton’s first law (balanced forces produce no acceleration) with the idea that unbalanced forces produce accelerations. They may posit that a greater force will produce a greater acceleration. They may also use the idea of inertia from the previous section to realize that a greater mass corresponds to a smaller acceleration. On the other hand, it may be difficult for some students to let go of the idea that a force is necessary to maintain constant motion. Revisit this misconception with examples throughout this presentation.

3. Newton’s First Law
An object moves with a velocity that is constant in magnitude and direction, unless acted on by a nonzero net force
“A body in motion tends to remain in motion; a body at rest tends to remain at rest.”
The net force is defined as the vector sum of all the external forces exerted on the object

4. External and Internal Forces
External force
Any force that results from the interaction between the object and its environment
Internal forces
Forces that originate within the object itself
They cannot change the object’s velocity

5. Inertia
Is the tendency of an object to continue in its original state of motion
Either moving, or at rest

6. Mass
A measure of the resistance of an object to changes in its motion due to a force
Scalar quantity
SI units are kg

7. Weight
The magnitude of the gravitational force acting on an object of mass m near the Earth’s surface is called the weight w of the object
w = m g is a special case of Newton’s Second Law
g is the acceleration due to gravity
g can also be found from the Law of Universal Gravitation

8. Gravitational Force Mutual force of attraction between any two objects
Expressed by Newton’s Law of Universal Gravitation:

9. Newton’s Second Law
The acceleration of an object is directly proportional to the net force acting on it, and inversely proportional to the object’s mass

10. Stuff Ya Gotta Know-Newton's 1st & 2nd Laws
Newton’s Second Law
Increasing the force will increase the acceleration.
Which produces a greater acceleration on a 3-kg model airplane, a force of 5 N or a force of 7 N?
Answer: the 7 N force
Increasing the mass will decrease the acceleration.
A force of 5 N is exerted on two model airplanes, one with a mass of 3 kg and one with a mass of 4 kg. Which has a greater acceleration?
Answer: the 3 kg airplane
Be sure students understand what is meant by the terms “directly proportional” and “inversely proportional.”
A simulation from the Phet web site is available to help students visualize the force and the acceleration. The web address is:
Choose the “Motion” simulations, then select “motion in 2D.” You can turn off the vectors and just allow students to observe the motion. Then ask the students to predict the acceleration vector. Which way will it point? Will it have a constant size? After predicting, show the acceleration vector. Next, have them predict the force vector’s direction and size. After predicting, show the force vector and both vectors. Then you can try the other motions described on the screen and ask them to observe the motion, describe the acceleration, and describe the forces.
This exercise allows students to see that accelerations are caused by forces. We see the accelerations, but often do not see the forces.

11. Newton’s Second Law (Equation Form)
Stuff Ya Gotta Know-Newton's 1st & 2nd Laws
Newton’s Second Law (Equation Form)
F represents the vector sum of all forces acting on an object.
F = Fnet
Units for force: mass units (kg)  acceleration units (m/s2) = kg·m/s2
The units kg•m/s2 are also called newtons (N).
It is often useful to write the equation as a = F/m to show students the relationship between force and acceleration and between mass and acceleration. It is easier to see that forces cause accelerations when the equation is written in this form.
Even though students saw these units in section 1, they may not recall the fact that newtons are simply a short name for the SI units of kg•m/s2. When solving problems, they will need to know this equivalence in order to cancel units.
Remind students of the other units for force, such as dynes (g•cm/s2) and pounds (slug•ft/s2).
= m·a

12. Multiple Objects – Example
When you have more than one object, the problem-solving strategy is applied to each object
Draw free body diagrams for each object
Apply Newton’s Laws to each object
Solve the equations

13. Classroom Practice Problem
Stuff Ya Gotta Know-Newton's 1st & 2nd Laws
Classroom Practice Problem
Space-shuttle astronauts experience accelerations of about 35 m/s2 during takeoff. What force does a 75 kg astronaut experience during an acceleration of this magnitude?
Answer: 2600 kg•m/s2 or 2600 N