Download Newton`s Three Laws of Motion

Forces and Newton’s Three Laws of Motion
Forces act on objects all the time. The combination of all the forces acting on an object will determine
if the object is moving, how it is moving, or how it’s motion changes. What is a Force?
Newton’s First Law: Principle of Inertia
An object at rest remains at rest, or an object in motion remains in it's state of motion unless acted on
by an unbalanced force. Therefore, if the forces acting an on object are balanced, the acceleration of
the object is zero (a = 0 m/s2). What does this mean?
What is inertia?
Newton’s Second Law: Law of Unbalanced Forces
Unbalanced forces (the net force) cause objects to accelerate. Fnet = ma
The greater the net force on a mass, the greater the acceleration.
If the same size force is applied to two different objects, the object with the greater mass
experiences a smaller acceleration.
An unbalanced force is also called a net force. The net force is the sum of the forces acting on an
object.
Newton's 3rd Law: Law of Interacting Force Pairs
For every force, there is an equal and opposite force. Forces occur when objects interact with each
other. Both objects interacting experience the same size force acting in opposite direction.
Using your notes, complete the table below.
Quantity
distance
time
velocity
acceleration
mass
force
(include different
kinds of forces)
Variable
Definition
Units
Forces...
are a push or a pull of one object on another
are the reason why an object moves the way it is moving or it’s motion changes (Fnet)
are vector quantities
are measured in Newtons (N = kgm)
s2
Equations:
Fg = mg
Fnet = ma
Fnet = sum of the forces ("Add" forces in the horizontal (x) direction and the vertical (y) direction
independently)
There are many different kinds of forces. All forces are a push or a pull of one object on another.
Some are contact forces (when the two objects touch) some are non contact forces (when two
objects don't touch).
Different kinds of forces acting on objects:
Weight (Fg)
Normal Force (FN)
Applied Force (Force of Tension)
Friction (Ff)
Air Resistance (Fair)
Net Force (Fnet)
Representing Forces On Objects: Free Body Diagrams (FBD)
Free body diagrams show how an object is moving and why it is moving that way.

Draw arrows on an object identifying all forces acting on the object.

Also include velocity and acceleration arrow (if applicable).
1. A pumpkin sitting on a desk (at rest)
2. A pumpkin is pushed from rest
3. A pumpkin is pushed at constant speed
4. A pumpkin slides to a stop
(after I stop pushing it)
5. A pumpkin falls.
6. A pumpkin is suspended from a string
1. What is the difference between mass and weight?
2. What is the weight of a 165 kg pumpkin on Earth? (g = 9.8 m/s2)
What is the weight of the pumpkin on the moon? (g = 1.62 m/s2)
3. A donkey attached to a very heavy cart is lifted into the air. Once the donkey is in the air, are the
forces acting on the donkey balanced?
4. A 6 kg cat is pulled across the floor at constant velocity.
Are the forces balanced?
Draw all the forces acting on the cat.
What is the weight of the cat?
5. Todd and his box accelerate down a hill.
Are the forces balanced?
Todd has a mass of 32 kg. If the net force
acing on Todd is 38 N, what is his acceleration?
6. Vicki pushes a 100 kg wooden crate across the stage. What force must she overcome to move
the box from rest? Draw all the forces acting on the box.
7. To get the box to move, Vicki pushes with a force of 420 N. The force of friction is 411 N.
Find the net force acting on the box. What is the acceleration of the box?
8. Once Vicki gets the crate moving, the force of friction drops to 300 N. What size force must
Vicki apply to keep the box moving at constant speed?
9. Liz and Ben are moving a wagon of pumpkins at constant speed. Draw the forces acting on
the wagon. How do the applied forces of Ben and Liz compare to the force of friction?
10. State if the forces acting on the object are balanced or unbalanced. Find the Fnet acting on
each of the objects. Add velocity and acceleration arrows to indicate if the object is at rest,
moving at constant speed or accelerating (speeding up or slowing down).
FN = 70 N
FN = 60 N
Ff
Fpush
30 N
Fg = 70 N
Fg = 50 N
FN = 20 N
70 N
Fg = 60 N
Ff
Fpull
12 N
12 N
Fg = 20 N
11. Abagail and Billy are standing on skateboards. Abagail pushes Billy. What will happen?
12. Sara's car was parked in a lot when her car was rear ended by Angela. Explain why Angela's car
needs repair after the collision.
13. Helga and Olga (two witches on broomsticks) have an argument while hovering in the air. Helga
pushes Olga with a force of 25N. What happens to Helga?