Force and Motion

Mousetrap Education• Don’t be in the wrong place at the wrong time.

• Time and chance happen to us all.

• One thing leads to another.

• Actions have consequences.

• Causality– Law of cause and effect– Sewing and reaping (Gal. 6:7-9)

Newton’s 1st Law• An object at An object at rest remains at restrest remains at rest and an and an

object in object in motion remains in uniform straight motion remains in uniform straight line motionline motion unless acted on by an unless acted on by an unbalanced force.unbalanced force.

• InertiaInertia - Property of matter that resists - Property of matter that resists changes in motion.changes in motion.

Is inertia good or bad?

Newton’s 2nd Law

• Newton’s 2nd Law - The rate of change in momentum is equal to the net force acting on the body.

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• Ex. What force must be applied to a 40 kg ball for its velocity to be

Additional Info• Will use summation notation

for net force.

• Units for force– From dimensional analysis

– In metric will use a newton (N)

N

N

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mkgN

300 N

450 N

350 N

200 N

Which way will the ring move?

Fundamental Forces

• Gravitational - Masses attract• Electrical - Unlike charges attract• Magnetic - Unlike poles attract• Weak - Responsible for radioactive decay• Strong - Holds the nucleus of the atom together

Weight• Weight - The force due

to gravity.

• What is the weight of Alexeyev on the moon? On Jupiter?

gmW

At 157 kgand lifting246 kg

Newton’s 2nd Law and Weight

• Ex. You and your bike weigh 882 N. If your brakes can exert a force of 200 N, what stopping distance do you need if you are traveling at 11 m/s?

Elevator Ride• You stand on a scales while in an elevator. You

know you weigh 150 lbs, but the scales say you weigh 180 lbs. What is happening in the elevator?

•It is stationary.•It is moving upward at constant v.•It is moving downward at constant v.•It is moving upward at increasing v.•It is moving downward at increasing v.•It is accelerating upward.•It is accelerating downward.

Elevator (Cont.)• Ex. You stand on a scales while in an

elevator. You know you weigh 150 lbs., but the scales say you weigh 180 lbs. What is your acceleration?

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Newton’s 3rd Law

• For every action there is an equal and opposite reaction.

FABFBA

BAAB FF

Newton’s 3rd Law

mearth = 5.97 × 1024 kg

• Ex. A 110 kg skydiver accelerates towards the earth at 9.8 m/s². How does the earth respond to him?

Hooke’s Law• Hooke’s Law - The force exerted by a

spring is proportional to the distance the spring is compressed or stretched from its rest state.

xkF

k - spring constant (N/m)

• This is a restoring force. It opposes the displacement from its rest state.

Plan of Action1. Identify objects of interest.

2. Draw free-body diagram

3. Write Newton’s 2nd Law

4. Choose coordinate system.

5. Write out forces in chosen coordinates.

6. Solve equations symbolically.

7. Check the sense of the problem.

8. Insert the values for numerical answers.

Behind the Scenes• A 15 kg stage prop is suspended

from the ceiling by two ropes which are each run through a pulley and attached to a 20 kg sandbag. What angle does each rope make with respect to the horizontal?

(Assume no friction in pulleys and massless ropes.)

Down the Ramp• A 1.1 kg block is at the top of a 8.00 m long

ramp that is at an angle of 11.8°. If it starts at rest, how long does it take to get to the bottom of the ramp?

(Assume the surface is frictionless.)

“All the towering materialism which dominates the modern mind rests ultimately upon one assumption; a false assumption. It is supposed that if a thing goes on repeating itself it is probably dead; a piece of clockwork. People feel that if the universe was personal it would vary; if the sun were alive it would dance. This is a fallacy even in relation to known fact. For the variation in human affairs is generally brought into them, not by life, but by death; by the dying down or breaking off of their strength or desire. A man varies his movements because of some slight element of failure or fatigue.”

G.K. Chesterton (Orthodoxy)

Col. 1:16,17