1 motion – newton’s laws 1. every body continues in its state of rest or uniform motion unless...

1

Motion – Newton’s Laws

1. Every body continues in its state of rest or uniform motion unless it is acted upon by a net external force.

2. The acceleration of a body is equal to the net force acting on the body divided by the mass of the body.

a = FNET / m

FNET = ma

2

Part 2, B: MOTION 2

Newton's Laws

F = m a

Net force is the vector sum of the individualforces

7 N 2 N 5 N

net

3

Part 2, B: MOTION 2

Newton's Laws

F = m a

The direction of the acceleration is the sameas that of the net force.

5 N

net

If the mass is 10 Kg,the acceleration is0.5 m/sec² to the left.

4

Part 2, B: MOTION 2

Newton's Laws

Weight: The force of gravity on an object.

How would you measure this?

5

Part 2, B: MOTION 2

Newton's Laws

Weight: The force of gravity on an object.

How would you measure this?

Put the object in free fall and measure theacceleration. Then

W = mg.

6

Part 2, B: MOTION 2

Observation: Motion in an arc possible onearth.

7

Part 2, B: MOTION 2

Newton's Laws

Example: Air resistance

W

Without air resistance,

W mg a = = = g

m m

8

Part 2, B: MOTION 2

Newton's Laws

Example: Air resistance

W

With air resistance,

W-R a = < g

m

R

9

Part 2, B: MOTION 2

Newton's Laws

If R = W, what will happen?

(a) The object will begin to moveupward.(b) The object will stop immediately.(c) The object will slowly come to a stop.(d) The object will continue to fall, butat a constant velocity.(e) None of the above.W

R

10

Part 2, B: MOTION 2

Newton's Laws

If R = W, what will happen?

(a) The object will begin to moveupward.(b) The object will stop immediately.(c) The object will slowly come to a stop.(d) The object will continue to fall, butat a constant velocity.(e) None of the above.W

R

11


Special Case: Friction

* Friction resists the motion of an object

12




* Frictional force is in the opposite direction of the motion of the object

13





* Frictional force is proportional to the weight of the object

14





* Frictional force is proportional to the weight of the object

FFRICTION α mg

FFRICTION = μmg

The constant of proportionality μ is called the coefficient of friction

15



There are two types of friction:

Static Friction – frictional force that holds an object stationary.

μs → Coefficient of Static Friction

16




Kinetic Friction – frictional force associated with a sliding object.

μk → Coefficient of Kinetic Friction

17




FACT: It requires more force to set an object into motion than it does to keep an object moving.

μs > μk

18



Example: A force is applied to an object, causing the object to slide on a table (with friction) at a constant velocity. The speed is 2 m/sec. If the force is removed, how far will the block slide before it stops? The coefficient of kinetic friction is 0.8 and g = 10 m/s2

19



Example: A force is applied to an object, causing the object to slide on a table (with friction) at a constant velocity. The speed is 2 m/sec. If the force is removed, how far will the block slide before it stops? The coefficient of kinetic friction is 0.8 and g = 10 m/s2

F = μk mg → a = μk mg/m = μk g

Δv = a t → t = Δv/a

d = ½ a t2 = ½ a (Δv/a)2 = Δv2 / 2 μk g

d = (2 m/s)2 / (2 * 0.8 * 10 m/s2) = 0.25 m

20


Special Case: Pressure

P = F/A

21



P = F/A

F

22



P = F/A

F

Area

23



P = F/A

F

Area

MKS: N/m2

British: psi

1 motion – newton’s laws 1. every body continues in its state of rest or uniform motion unless...

Documents