AIM: What are Newton’s three laws, and how do they describe how an object behaves?

Do Now:- Draw a Free Body Diagram for the block below if

1. it is at rest.2. If it is moving to the right at a constant velocity.3. If it is accelerating to the left.

Calculating the Force of Gravity• The force of gravity acting on an object is also known

as the objects weight

–weight is not mass!–Mass NEVER changes

• The force of gravity (Fg) in Newtons, acting on an object depends on – the objects mass (m) in kilograms– the acceleration due to gravity (g) in m/s2 (changes if you

are not on Earth)

• The formula is Fg = mg

The Newton• The Newton is the unit of any force.• It is a derived unit, which means it is a combination

of other fundamental units.• To determine the fundamental units a Newton is

made up of1. Find a formula for force 2. Plug in the unit for each variable3. Combine the units

2

2

s

kgm N

s

m(kg) N

mgFg

Calculating Weight

• What is the weight of a 60Kg physics student?

• A car weighs 20,000N. What is the car’s mass?

Newton’s 3 laws• 1st law: The Law of Inertia

– An object at rest will stay at rest, and an object in constant velocity motion will stay in constant velocity motion unless acted on by a net (unbalanced) force.

– INERTIA is an object’s “want” to resist a change in motion– INERTIA depends ONLY on the objects mass

• The more massive, the more inertia regardless of the speed of the object.

• 2nd Law: The law of acceleration– Fnet = ma

• Ex: what is the acceleration of a 3kg mass being acted on by a unbalanced force of 6N?

• 3rd Law: The law of action-reaction forces– ANY force acting on an object, has an equal and opposite reaction force

– Force of “A” on “B” is equal and opposite to the force of “B” on “A”

Newton’s 1st law- objects in equilibrium• When an object is in equilibrium, the sum of all the

forces acting on the object is zero.• We can make this statement more specific by saying– The sum of all the y-components of the forces are equal

to zero– The sum of all the x-components of the forces are equal

to zero.

1st law: The Law of Inertia

Using Newton’s first law, explain why the table settings do not go flying

Newton’s 2nd LawFnet =ma

• When solving force problems, ALWAYS follow these stepsStep 1: find the motion statement in the problem.AT REST or CONSTANT VELOCITY or ACCELERATINGStep 2: draw a Free Body Diagram of the objectStep 3: Make a force statement based on your free body

diagram and motion statement.IN EQUILIBRIUM – opposite forces are equalACCELERATING – the vector sum of the forces =ma

Step 4: solve

1. A 10kg crate is at rest on a horizontal surface. What is the Normal force acting on the crate?

1. Determine the object’s state of motion (Is it in equilibrium or not?)

2. Draw a free body diagram

3. Make a force statement based on the object’s state of motion

4. Solve for the missing forces

2. A 5kg crate is moving at a constant velocity on a horizontal surface while being pulled by a 20N force.

A. What is the Normal force acting on the crate?B. What is the friction force acting on the crate?

1. Determine the object’s state of motion (Is it in equilibrium or not?)

2. Draw a free body diagram

3. Make a force statement based on the object’s state of motion

4. Solve for the missing forces

3. A 30kg crate is accelerating to the right at a rate of 2m/s2 while being pulled by a 40N force.

A. What is the Normal force acting on the crate?B. What is the friction force acting on the crate?

1. Determine the object’s state of motion (Is it in equilibrium or not?)

2. Draw a free body diagram

3. Make a force statement based on the object’s state of motion

4. Solve for the missing forces

Hanging Signs and Tension in a string

• Draw a Free Body Diagram for all the signs seen below.

mA=5kg mB=5kg mC=5kg

40o 40o

Finding the Tension Force

1. Draw a free body diagram2. Break down any angled forces

if necessary.3. Examine all the x-components and

set their sum equal to zero4. Examine all the y-components and

set their sum equal to zero5. Resolve your resultant vector

A=5kg

Finding the Tension Force

1. Draw a free body diagram2. Break down any angled forces

if necessary.3. Examine all the x-components and

set their sum equal to zero4. Examine all the y-components and

set their sum equal to zero5. Resolve your resultant vector

A=5kg

Finding the Tension Force

1. Draw a free body diagram2. Break down any angled forces

if necessary.3. Examine all the x-components and

set their sum equal to zero4. Examine all the y-components and

set their sum equal to zero5. Resolve your resultant vector

mC=5kg

40o 40o

1. A 2000kg car is moving at a constant speed down a high way. If the engine provides a force of 300N, a. what is the frictional force acting on the car?b. What is the normal force acting on the car?

2. A 6N force to the right acts concurrently with a 12N force to the left on a 3kg object. c. What is the magnitude and direction of the net force

acting on the object?d. What is the magnitude and direction of the object’s

acceleration?e. What is the normal force acting on the object?

3. A 2Kg crate is accelerating to the right at 4m/s2 when acted on by a 14N force. a. What is the friction force acting on the object?b. What is the normal force acting on the object?

4. A 30Kg child sits on top of a 10kg cratec. What is the normal force acting on the child?d. What is the normal force acting on the crate

5. A 23Kg girl is sitting at rest in a tire swing. What is the tension in the rope?

6. A 30Kg child decides to ride in an elevator while standing on a scale that can read her weight.a. What does the scale read while the elevator is at rest on the

bottom floor?b. What does the scale read as the elevator accelerates upward at

2m/s2 c. What does the scale read as she moves upwards at a constant

speed?d. What does the scale read as she slows down at a rate of 1.5m/s2

when reaching the top floor?e. What does the scale read as she accelerates downward at 3m/s2

on her way back to the loby?f. What does the scale read as she moved downwards at a

constant speed?g. What does the scale read as she slows down at a rate of 2.5m/s2

when returning back to the ground floor?

7. Two children are fighting over a 2kg toy. If one child pulls to the right with a force of 20N and the other child pulls to the left with a force of 14N, what is the acceleration of the toy?

8. While pulling a 30Kg sled across the ice, one person uses a 40N force to the North while the other person uses a 60N force to the East? What is the magnitude of the resultant force?

9. A worker pushes a 20Kg crate across a horizontal surface at a constant speed. If the force the worker applies is 55N and an angel of 60o to the horizontal.a. What is the vertical component of the push force?b. What is the horizontal component of the push force?c. What is the magnitude of the friction acting on the

crate?d. What is the normal force acting on the crate?

10. A mother pulls her 20Kg child in a wagon at a constant speed by applying a 100N force at an angle of 55o above the horizontal.a. What is the vertical component of the pull force?b. What is the horizontal component of the pull force?c. What is the magnitude of the friction acting on the

wagon?d. What is the normal force acting on the wagon?

11. A 7Kg sign is hung from a horizontal ceiling with two ropes. One rope makes a 45o angle with the ceiling and the other makes a 60o angle. What is the tension in each rope?

12. A 3kg sign is hung from a building with two chains. One chain is perfectly horizontal and attached to a side wall. The other chain makes a 25o angle to the vertical. What is the tension in each chain?

13. The following picture is hanging on a wall. determine the weight of the picture.

14. The following sign can be found in Glenview. The sign has a mass of 50 kg. Determine the tension in the cables.

15. After its most recent delivery, the infamous stork announces the good news. If the sign has a mass of 10 kg, then what is the tensional force in each cable?

16. Find T1 and T2

40o

12kg

T1

T2

17. Find T1 , T2 and T3

40o

12kg

T1

T2

T3

5kg