Newton's Laws-

Newton's 3rd Law

N3 Example

**FRICTION**

Friction Intro - Air Resistance

**Unit 3: Newton's Laws of Motion**

**What is a Force?**

What is a force?

Newton's Laws-

Newton's 2nd Law

Mass and Weight

**Sections 4.1-4.3 & 5.1-5.3**

Ex 1. - Free Body Diagrams

What forces are acting on you?

Ex. 2 - Net Force

A. Draw a free body diagram for the apple in each picture to the right. Label the forces that are acting on the apple in each case.

B. Compare the magnitude of the forces acting on the apple in each picture, how can the relationships between the forces be used to predict the motion of the apple?

What is the net force acting on each object?

4 N

4 N

4 N

2 N

3 N

4 N

1 N

2 N

2 N

If each object shown, were free to move which ones would and which way?

An object that has a net force of zero is said to be in equilibrium, why? Which objects above are in equilibrium?

Ex 3 - N1

While rushing through the grocery store, you place two soup cans on the their side in the bottom of your cart. In your haste you do not notice some spilled juice on the floor in front of you. At the last moment you notice the spill and quickly come to a stop. After stopping, you notice your soup cans rolling to the front of your cart. Then when you get started again, you notice the cans roll to the back of the cart.

Ex 4 - Inertia & Mass

What is the definition of inertia?

What is the definition mass?

What is the relationship between inertia and mass?

Are they the same thing?

Does this observation obey Newton's laws? Which law? Can you think of another similar situation?

**What is a force**

**Representing Forces**

Ex 5 - N2 with one Force

A. A 7000kg car slows to a stop with an acceleration of 12.5m/s^2, what is the braking force exerted on the car? How much is exerted per brake?

B. A 1.5kg baseball experiences a 5N force, what is the baseballs acceleration?

C. A 80kg student jumps off a desk what is the force felt by the student?

D. What is the acceleration of a 15kg mass when a 8N force is applied to it?

EX 6 - N2 with Multiple Forces

A. A 3kg book lies motionless on a desk, what is the normal force exerted on the book?

B. A 10kg falling object has an acceleration of 8m/s^2 what is the force of air resistance acting on the object.

C. A 5kg water pail is being lowered to the bottom of a well with a downward speed of 5m/s and downward acceleration at 3m/s^2, what is the tension in the rope?

D. If the full pail from the previous problem has a mass of 8kg, what is the tension in the rope if it is accelerated, from rest, upward at 1m/s^2?

Mythbusters, tablecloth chaos

Paul Hewitt - table cloth trick

Ex 7 - Elevator problems

A ) If the student above is standing on a bathroom scale inside the elevator, when would the scale have the highest reading? (ABC or D) Draw a FBD to Help.

B) When would it be the most accurate? Explain

Ex-7 Cont'd

Determine the reading on the scale in N for B and C shown to the left. (m=45kg)

Would the result change if a spring scale were used instead? Explain

Ex 8 - Stacked Books

A Chemistry book is laying motionless on a desk, when a Physics book is placed on top of it. Draw a FBD for the table and each book. Indicate which forces are a 3rd law pair.

Ex 8 Cont'd

A. If the Physics book has a mass of 2.5kg and the Chemistry book has a mass of 1.8kg, determine the normal force exerted on each object.

B. If the table has a mass of 8kg, what is the normal force exerted by the ground?

N2

Elevator Problems

Ex 11 -N3 Examples

A. While decorating a Christmas Tree a student hangs a .35kg train ornament from a branch. Then hangs a .15kg bulb from the smoke stack on the front of the train. Will the ornaments remain on the tree if the hooks that hold each ornament can exert an upward force of 5N each before bending? (You need to find the force exerted by each hook.)

B. While shopping for thanksgiving dinner a overworked teacher mistakenly places a 5kg turkey on the bottom of the bag, a 3kg sack of potatoes on top of the turkey and 1.5kg bag of green beans on top of the potatoes.

i. What is the upward force exerted by the turkey on the potatoes?

ii. The grocery bag can exert a maximum upward force of 80N before breaking, will the bag hold when the teacher pulls up on the handles?

Ex. 10 - Identifying N3 Force Pairs

A. Draw a free body diagram for the student sitting in a chair and the chair. Identify all 3rd law pairs.

B. Draw a free body diagram for a student hanging from monkey bars. Draw a FBD for the student and the monkey bars, identify all 3rd law pairs.

C. Draw a free body diagram for each mass if a 3kg mass is hung from a 2 kg mass, which is hung from a 1kg mass, which is hung from a ring stand. Identify all 3rd law pairs; which string would have the highest tension.?

ANSWER TO B:

F = 9.8*2.5+9.8*1.8+9.8*8

=12.3*9.8=120.54 N

A.

Student Chair

Student Monkey Bars

B.

Fn

Fg

Fn

Fg

Fstudent

Fn

Fg

Fstudent

F T

Fg

ANSWERS

Ex 11 Continued

C. After noticing how much weight the bag was going to have to support the Physics teacher removed the turkey from the bag and placed it in its own identical bag.

i. If the bag is accelerated at 3m/s^2 what is the normal force exerted by the bag on the turkey?

ii.What is the largest acceleration the bag containing the potatoes and green beans can experience without breaking?

iii.What is the largest acceleration the bag containing the turkey can experience without breaking?

Ex. 12- Air Resistance

A basketball and a bowling ball with approximately the same radius, are dropped from the roof of a very tall building.

A. Which ball will experience the largest force of air resistance at terminal velocity?

B.Which object will reach terminal velocity first?

C. Which object will have the larger terminal velocity?

D. If a shot put, that has the same mass as the bowling ball is dropped at the same time as the bowling ball, which will reach terminal velocity first? Which will have the higher terminal velocity?

Ex - 13 Air Resistance Again....

An 95kg adventurous student decides to go sky diving.

A. Draw a free body diagram for the student immediately after they leave the plane and after they reach terminal velocity.

B. What is the force of air resistance acting on the student when they reach terminal velocity?

C. At the moment the student opens their parachute they feel a large force and acceleration upward, why do they feel this force?

Ex. 14 - Friction Example

A 35kg girl is trying to move her newly constructed 20kg snowman out of the sun and into some shade. So she moves the snow man onto a 3kg sled. The coefficient of static friction is 0.14 and kinetic friction is 0.1.

A. If the little girl pulls on the sled's rope with a force of 28N will the sled move?

B. What is the minimum force required for the little girl to move the sled snowman combo?

C. If the little girl's 20kg dog decides to help by grabbing the rope and applying a 15N force along with the girl's 28N force. Will the sled move? If so what is the sled acceleration?

Ex. 15 Another Friction Example

While conducting a lab a student connects a spring scale to 5kg mass resting on the carpeted floor.

A. When the spring scale reads 15N the 5kg mass has an acceleration of 1m/s^2, what is the coefficient of kinetic friction?

B. You try to measure the coefficient of static friction by applying a 6 N force and the 5kg mass does not move, what is the force of friction acting on the 5kg mass?

C. Another student states that they only needed to apply 5N of force to get the 5kg mass moving, does this agree with the observation in part A &B?

D. Still trying to get the block to move you keep applying a force using the spring scale, and at the moment the block begins to move the spring scale reads 17N, what is the coefficient of static friction for the carpeted floor?

Friction -Kinetic & Static