Physics 11 Unit 4 FORCES

Name:______Date:______

Physics 11 – Unit 4 FORCES

4.2 – Newton’s Laws of Motion

Recall: Newton’s 1st Law – an object with no net force acting on it remains at rest or moves with a constant velocity in a straight line.

Newton’s 2nd Law – the acceleration of a body is directly proportional to the net force on it and inversely proportional to its mass. nd v Recall: Newton’s 2 Law: F= mav

Example 1: the space shuttle has a mass of 2.03 x 106 kg. If each engine is capable of exerting a force of 12.5 MN.

(a) What is the maximum acceleration of the space shuttle? Note: there are 2 engines.

(b) Why does the maximum acceleration change as the engines burn?

Example 2: A spacecraft has a mass of 3.25 x 104 kg. Its engine can exert a thrust force of 7.25 x 104 N. a) At what rate will the spacecraft accelerate?

b) If the spacecraft maintains this constant acceleration for 24 s before turning off the engine, and its initial velocity was 5.6 x 103 m/s, what will the final velocity of the spacecraft be?

c) How far will the spacecraft travel during the 24 s? d) What will the velocity of the spacecraft be after 48 s? 60 s? 1000 s?

Newton’s 3rd Law – when one object exerts a force on a second object, the second . . . ______

Free Body Diagrams: (FBDs)

A free body diagram is a picture that represents the object that you are analyzing with a small dot. Any forces acting on the object are represented with arrows (roughly to scale, if possible). Newton’s Laws are used to figure out what forces are acting on the object.

Ex. 1) Draw a FBD for the following masses hanging from a rope: Some types of forces:

One of the forces that you are most acquainted with is the force of gravity. This is the force that the earth’s mass exerts upon you. The force of gravity is calculated as follows:

Another word for the force of gravity is ______.

(Weight is a unit of force, and is measured in ______, not ______.)

Ex) Find the weight of a 35 kg object (a) on the Earth (b) on the moon (where g=1.60 m/s2)

To solve force equations, we will use a tug-of-war analogy. We will draw a FBD, carefully label all of the forces, and then use our knowledge of physics to decide which force is the winning (strongest) force. We will then use the following equation:

______- ______= ______

Where the net force is equal to ______.

Ex. 2) Find the tension in the rope:

Ex. 3) Find the tension in the rope. Ex. 4) Find the tension in the rope if the mass is moving up at a constant speed of 3.8 m/s.

Ex. 5) Mr. Marzouk (m= 78 kg) jumps out of a plane. Once he pulls his parachute, air drag has a force of 728 N. What is his net acceleration?

Ex. 6) Reagan jumps from a 4.5 m high cliff into a lake. Assuming his mass is 65 kg,

a) Find how long it takes until he hits the water.

b) Find his velocity as he hits the water.

c) If Reagan’s feet just barely touch the bottom of the pool (which has a depth of 2.5 m), what was his average acceleration while he was in the water?

d) What was the net force that the water exerted on him?

Ex. 7) Draw a FBD for the forces acting on a book sitting on a table. Name: ______Date: Dec. 1 ST . 2010 Lesson 2 Assignment #1

1. A rope is attached to a 2.0 kg rock and pulled. Write a net force equation if the mass is a) accelerating up ( T - mg = ma)

T b) accelerating down (mg - T = ma)

c) Find the cord tension if the mass is accelerating at 2 kg i) 0.0 m/s2 (20. N) Fg ii) 1.0 m/s2 UP ( 22 N)

iii) 2.0 m/s2 DOWN (16 N)

iv) 9.8 m/s2 DOWN (0.0 N)

2. Find the acceleration of the mass if the cord tension is

a) 30. N b) 19.6 N c) 9.6 N

string

2 kg (5.2 m/s2 up; 0.0m/s2; 5.0 m/s2 down)

3. In the diagrams below, assume the force F is the only horizontal force on the block. If the first block accelerates at rate a, then find an expression for the unknown acceleration. (a/2,2a)

acceleration = ? acceleration = a

F F 2M M

frictionless surface

acceleration = a acceleration = ? 2F F M M

frictionless surface 4. A parachutist free-falls for a while and then opens their parachute. If the force of air resistance from the parachute is equal to the weight of the person, then which of the following is true? a) the person slows down b) the person speeds up c) the person moves at a steady speed (explain your answer using principles of physics)

5. A cyclist is moving to the right steadily at 8.2 m/s. If the force of air drag is 130 N, then what is the force of propulsion from the road?

(130 N right)

6. A mass is being pulled up by a cable. If the mass is moving up and slowing down, compare the forces that act on the mass and write a net force equation

(mg>T ; mg- T = ma)

7. Find the weight of a 80.0 kg person a) on the Earth b) on the Moon c) 2000. km above the surface of the Earth, where g= 5.70 N/kg

(784 N; 128 N; 456 N)

8. An object in free-fall experiences only the force of gravity. for such an object a) draw a force diagram

b) write a net force equation (mg = ma)

c) solve for the acceleration (a= g)