Physics: Force, Acceleration, and FBD's

Physics: Force, Acceleration, and FBD's

Connecting Force, Mass, and Acceleration

Review from the Text

•  $F=ma$  

• Force is the ability to cause a change in state of motion (yes, a push or pull...but more!) - measured in Newtons (N)

• Mass is ability to resist a change in state of motion - measured in kg

• Acceleration is a change in velocity per unit time - measured in  $\frac{m}{s^2}$  

Linear Relationship

•  $a=\frac{F}{m}$  

• If we keep force (F) constant (for example, 100N in the graph), then we can analyze the following relationship:

• If mass goes up, acceleration decreases.  Think about pushing a matchbox car vs pushing a full size car.  Your force can still move the full size car, just much more slowly!  Similarly, as mass goes down, acceleration increases.  This illustrates what we call an INVERSE relationship.

Or think about it this way...

• The 3 different slopes pertain to three different situations with different masses

• Clearly, we can see the greatest mass of 3 carts (1/3m) results in the smallest acceleration values

More Force = More Acceleration!

• For a constant mass value, if we "push harder", we accelerate faster. This is a direct, linear relationship

• Examine the green point: 0.4N force creates a 0.5 m/s/s acceleration

• Examine the red point: a 0.8N force creates a 1 m/s/s acceleration

Fill in the Blanks

Multiple Choice

If you increase the force on an object, its acceleration? 

• decreases
• stays the same
• also increases
• stops

Multiple Choice

If a 2kg bird is pushed by the wind with a force of 2N, how fast does the bird accelerate?

• 1 m

• 2 m/s²

• 4m/s²

• 1 m/s²

Multiple Choice

What is the net force?

• 24N west

• 24N east

• 130N east

• 130N west

43 is supposed to be a 42...the answer is correct!

Multiple Choice

What is the unit for FORCE? 

• N (Newtons)
• lbs (pounds)
• g (grams)
• m/s (meters per second)

Multiple Choice

What is the magnitude (size) and direction of the net force?

• 14 N Down

• 14 N Up

• 6 N Up

• 6 N Down

Multiple Choice

What is the net force?

• 5 N

• 45 N to the left

• 5 N to the right

• 45 N to the right

Free Body Diagrams!

• Simple representations of the situation

• Draw and label forces as VECTORS to indicate magnitude and direction using arrows

Add the Force Vectors to find the NET force

• Only a net force can cause acceleration (unbalanced)

• When forces balance each other out, there is no net force and thus, no acceleration caused

• Net force here is 40N to the left; THIS is what we would use in conjunction with the F = ma formula to determine acceleration

Fill in the Blanks

Multiple Choice

Is there a net force on this satellite?

• no

• yes, 0.5 N to the right

• yes, 14N up and down

Fill in the Blanks

Multiple Choice

The parachuter is FALLING. Which value for the force due to drag (D) would cause the parachuter to "fall faster"?

• 600N

• 700N

• 800N

Multiple Choice

The parachuter is FALLING. Which value for the force due to drag (D) would cause the parachuter to "fall slower"?

• 600N

• 700N

• 800N