Lesson 3-6 Forces and Motion

PHS 110 Fundamentals of Physical Science

Thursday 3/7/2013

Announcements

• No class or lab next week (3/10-16/2013) due to Spring Break

• Lab #7 Force, mass, acceleration due during your lab session on Monday 3/18/2013.

• Almost done with Unit 3 on Vectors & Forces.• Unit 3 Test on Thursday 3/21/2013.

Today’s Agenda:

• Check Unit 3 HW #5• Forces and Motion “Clicker” Questions: 15

items.• These questions will also help prepare for

Vectors and Forces Test.

Part I: Force Conceptual Questions

• Complete these questions on a blank piece of paper.

• Your answers will be submitted at the end of the session for credit.

• Check with your neighbor/ask questions as we proceed.

1. If the total force acts in the same direction as the crate is sliding, the crate

A. slows downB. speeds up C. remains at same speed D. slows down, changes direction and then

speeds up going the other way E. remains at same speed, but changes

direction

Crate was moving to the right

Then, the guy pushed the crate

2. If the total force acts in the opposite direction as

the cabinet is sliding, the cabinet would A. slow downB. speed up C. remain at same speed D. slow down, change direction and then

speed up going the other way E. remain at same speed, but change

direction

Cabinet was moving to the left

Then, the guy pushed the cabinet

3. If there is zero total force acting on on the

refrigerator, the refrigerator would A. slow downB. speed up C. remain at same speed D. slow down, change direction and then

speed up going the other way E. remain at same speed, but change direction

Refrigerator was moving to the right

Then, the guy pushed the refrigerator

4. A cabinet is speeding up as it slides to the right across the room. Which of the following is a possible free body diagram?

A B C

5. A car is moving towards the right. Then a force is applied and the free body diagram looks like the one shown at right.

Draw what you think the position-time graph would look like. Assume to the right is the positive direction.

Force diagram

5. (cont) Which position-time graph best matches your idea?

6. A lady bug walks in a circle at a constant rate. Is the lady bug accelerating?

A. No, because she is not speeding up or slowing down.

B. Yes, because her speed is changing.

C. No, because she is only changing direction.

D. Yes, because her change in direction means her velocity is changing.

7. A lady bug walks in a circle at a constant speed. What could the acceleration and velocity vectors look like at the instant shown in the picture?

A.

B.

C.

D.

PART II: QUANTITATIVE FORCE PROBLEMS

Your instructor will first solve the examples on the following two slides.

Re-Cap of Newton’s 2nd Law:The acceleration of an object is …• Directly proportional to the net force

applied to the object• Inversely proportional to the mass of the

object𝑎∝

∑ 𝐹𝑚

∑ 𝐹=¿𝑚𝑎¿

Using Newton’s 2nd Law:• Draw a force diagram showing ALL forces

acting on the object. Label any force sizes that you know on the diagram.

• Decide whether the object accelerates and if this occurs in the x- or y- direction.

• Plug in all the parts to the N’s 2nd equation (either horizontally x, or vertically y) and solve for what you need.

∑ 𝐹 𝑥=¿𝑚𝑎𝑥 ¿ ∑ 𝐹 𝑦=¿𝑚𝑎𝑦 ¿

An elevator is moving up at a constant velocity of 2.5 m/s, as illustrated in the diagram. The man has a mass of 85 kg. Construct a force diagram for the man. How large is the force that the floor exerts on the man?

Example A:

The elevator now accelerates upward at 2.0 m/s2. Construct a new force diagram for the 85 kg man. Calculate how much force the floor must now exert on the man.

Example B:

Part II: Quantitative Force Questions

• Add these questions to your other answers for today.

• Your answers will be submitted at the end of the session for Credit.

• Check with your neighbor/ask questions as we proceed.

8.A man pushes his child in a grocery cart (his push is perfectly horizontal, not at an angle). The total mass of the cart and child is 30 kg. The force of friction on the cart is 15 N. Suppose the cart travels at a constant speed. Draw a force diagram for this situation. How hard must the man be pushing?

9. Suppose the man pushes harder so that the cart now cart accelerates at 1.50 m/s/s. Draw a new force diagram. How hard is the man pushing now? (The frictional force is still 15.N. The mass of cart and child together is 30 kg).

PART III: FREE FALL PROBLEMSYour instructor will first solve the examples on the following two slides.

Jaen drops a 2.5 kg brick off the top of a building.a) Draw a force diagram for the brick as it falls and calculate its acceleration. Why is this acceleration not a surprise?b) The brick hits the ground after falling 2.5 seconds. How fast was it travelling the instant before it hit?c) How tall was the building? (Hint: Use a velocity vs. time graph.)

Example C:

A 5 kg cat jumps off a four story building and falls to the ground. The cat spreads out its body so that an air resistance force of 10 N acts on it as it falls.

10.Draw a force diagram and find the cat’s acceleration.

11.Find the cat’s speed in at t = 3s.12.Find the time required for it to reach a

speed of 50 m/s.

Use the acceleration you calculated on the previous slide. 13. Sketch the velocity vs. time graph for the cat if

it falls for 10 seconds. How fast will it be going at the 10 second mark?

14. Recall that the AREA under a velocity vs. time graph gives how far an object goes (displacement). Use this fact and your velocity graph to find a the cat’s displacement after falling for two seconds.

15. Now calculate the cat’s displacement after falling for four seconds. Is it twice the value you calculated in part d? Why or why not?

Homework:

• Finish the Lab Report for Lab #7 forces and acceleration.

• During Lab session #8 you will complete problems on forces and motion to help review for the exam.

• Complete Unit 3 HW #6 for Tuesday’s class (3/19/2013) after Spring Break.