Force, Motion, & Newton’s Laws Review Physical Science

Force, Motion, & Newton’s Laws

ReviewPhysical Science

• The rate of change of position• Speed = distance time• SI Units: cm/s, m/s, km/h• A scalar quantity (has magnitude, but

no direction)

Total distance divided by total time is:

A. Average speedB. Constant speedC. Variable speedD. Instantaneous speed

Total distance divided by total time is:

A. Average speedB. Constant speedC. Variable speedD. Instantaneous speed

Constant Speed

• Does not vary• Example?

Constant Speed

• Does not vary• Example: CRUISE CONTROL

Instantaneous Speed

• The speed at any given ____.

Instantaneous Speed

• The speed at any given instant.

A man drives 3 km east from home to the store and then 2 km west to a friend’s house. What is his displacement from his starting point at home?

A. 1 km westB. 1 km eastC. 5 km westD. 5 km east

A man drives 3 km east from home to the store and then 2 km west to a friend’s house. What is his displacement from his starting point at home?

A. 1 km westB. 1 km eastC. 5 km westD. 5 km east

Acceleration

• The rate of change in velocity• a = vf - vi = ∆v

tf - ti ∆t

The proper units of acceleration are:

A. s/km2

B. km/hC. m/s2

D. cm/s

The proper units of acceleration are:

A. s/km2

B. km/hC. m/s2

D. cm/s

Which of the following is NOT used in calculating acceleration?

A. initial velocityB. average speedC. time intervalD. final velocity

Which of the following is NOT used in calculating acceleration?

A. initial velocityB. average speedC. time intervalD. final velocity

In which of the following conditions does the car NOT

accelerate?A. A car moves at 80 km/h on a flat, straight

highway.B. The car slows from 80 km/h to 35 km/h.

C. The car turns a corner.

D. The car speeds up from 35 km/h to 80 km/h.

In which of the following conditions does the car NOT

accelerate?A. A car moves at 80 km/h on a flat, straight

highway.B. The car slows from 80 km/h to 35 km/h.

C. The car turns a corner.

D. The car speeds up from 35 km/h to 80 km/h.

A body accelerates if it_____.

a. speeds upb. slows downc. changes directiond. all of these

A body accelerates if it_____.

a. speeds upb. slows downc. changes directiond. all of these

A constant velocity means that acceleration is ____.

A. positiveB. negativeC. increasingD. decreasingE. constantF. zero

A constant velocity means that acceleration is ____.

A. positiveB. negativeC. increasingD. decreasingE. constantF. zero

Which of the terms below best describe the forces on an object

with a net force of zero?

A. inertiaB. balanced forcesC. accelerationD. unbalanced forces

Which of the terms below best describe the forces on an object

with a net force of zero?

A. inertiaB. balanced forcesC. accelerationD. unbalanced forces

The tendency of an object to resist any change in its motion:

A. Net forceB. AccelerationC. Balanced forceD. Inertia

The tendency of an object to resist any change in its motion:

A. Net forceB. AccelerationC. Balanced forceD. Inertia

Newton’s First Law of Motion

• An object in motion stays in motion and an object at rest stays at rest unless acted upon by an unbalanced force.

• the resistance of an object to change its motion

• also known as the Law of Inertia

Which of the objects has the greatest inertia?

A. a car parked on the side of the roadB. a baseball during a pop flyC. a computer sitting on a deskD. a woman running on a track

Which of the objects has the greatest inertia?

A. a car parked on the side of the roadB. a baseball during a pop flyC. a computer sitting on a deskD. a woman running on a track

An object of large mass has ______ than an object of small mass.A. less inertiaB. more inertiaC. less weightD. greater acceleration

Which answer best describes why a passenger who is not

wearing a seat belt will likely hit the windshield in a head-on

collision?A. forces acting on the windshieldB. inertia of the unbelted personC. acceleration of carD. gravity taking over

Which answer best describes why a passenger who is not

wearing a seat belt will likely hit the windshield in a head-on

collision?A. forces acting on the windshieldB. inertia of the unbelted personC. acceleration of carD. gravity taking over

Newton’s 2nd Law of Motion

• A net force causes an object to accelerate in the direction of the net force.

• Acceleration = Force mass

a = F/m m = F/a F = ma

• Units of Force= Newtons (N) or kg▪ m/s2

• Units of Mass = kg• Units of accel. = m/s2

Newton

• Unit of Force or weight• 1 N = 1 kg▪ m/s2

A weightlifter raises a 440-kg barbell with an acceleration of 2.00 m/s2. How much force does the weightlifter exert on the barbell?

F = maF = (440 kg)(2.00 m/s2)F = 880 kg▪ m/s2 = 880 N

Friction

• Is caused by microwelds that develop between the microscopic bumps on 2 surfaces.

• 3 types of friction: static, sliding, and rolling

Acceleration due to Gravity• All objects are attracted to Earth with

the same acceleration.• Air resistance exerts an upward force

on objects falling through the atmosphere

• Terminal velocity - when the force of air resistance equals the force of gravity and the falling object no longer accelerates

The Law of Gravitation

• Anything that has mass is attracted by the force of gravity.

• Any 2 masses exert an attractive force on each other.

• The attractive force depends on the mass of the 2 objects and the distance between them.

4 Basic Forces

• Gravity – a long-range force – NEVER disappears– Gravity is the force that gives a galaxy its

shape and the universe its structure• Electromagnetic Force • Strong nuclear force• Weak nuclear force

Gravitational Acceleration• Near Earth’s surface, the gravitational

attraction of the Earth causes all objects to have an acceleration of 9.8 m/s2

• According to Newton’s 2nd Law, F = ma• F = m x 9.8 m/s2

• The direction of gravitational force is downward

• When an object is only under the force of gravity, it is said to be in free-fall.

The gravitational force between two objects depends on_______.

A.their massesB.their velocitiesC.their shapesD.the distance between themE.more than one of the above

(Which ones?)

The gravitational force between two objects depends on_______.

A.their massesB.their velocitiesC.their shapesD.the distance between themE.more than one of the above

(A and D)

Weight• The gravitational force exerted on an object.• W = mg (same as F = ma)• W = mass (kg) x 9.8 m/s2

• What is the difference between weight and mass?

“Weightlessness” in space

• is caused by the orbiting space shuttle falling downward, and no force is pushing upward (remember, gravity is also a downward force), so they seem to be floating.

Projectile Motion• Projectile - anything shot or thrown

through the air.• Projectiles have both horizontal and

vertical motion

• See http://www.batesville.k12.in.us/Physics/PhyNet/Mechanics/Projectiles/HorProPath.html

Centripetal Force• Centripetal acceleration - acceleration toward

the center of a circular path• Centripetal force - an unbalanced force

toward the center acting on an object moving in a circular path.

• SEE http://hyperphysics.phy-astr.gsu.edu/hbase/cf.html

• ALSO SEE http://regentsprep.org/Regents/physics/phys06/bcentrif/default.htm

Newton’s 3rd Law of Motion

• Describes action - reaction pairs: when one object exerts a force on a second object, the second one exerts a force on the first that is equal and opposite in direction.

• “to every action force there is an equal and opposite reaction force.”

• For a description of Newton’s 3 Laws of motion, see http://csep10.phys.utk.edu/astr161/lect/history/newton3laws.html

• More in depth: http://electron9.phys.utk.edu/phys135d/modules/m4/3rdLaw.htm

Rocket Propulsion

• Rocket releases gases, the gases push back on the rocket in the opposite direction.

• http://www.glenbrook.k12.il.us/GBSSCI/PHYS/CLASS/newtlaws/u2l4a.html

Momentum

• P = m x v• Units are kg x m/s

• Do not confuse this with F = ma

= kg x m/s2 or Newtons

See: http://www.glenbrook.k12.il.us/gbssci/phys/Class/momentum/u4l2e.html

Law of Conservation of Momentum

• The momentum of an object does not change unless its mass, velocity, or both change.

• If a group of objects exerts forces only on each other, their total momentum does not change.

• m1v1 = m2v2

1. What will happen to an object when a net force greater than zero acts on it?a) Fallb) Stopc) Accelerated) Go in a circle

Answer: C

2. Which is Newton’s 2nd law?

• A) F = ½ ma2

• B) F = 2 ma• C) p = mv• D) a = F/m

2. Which is Newton’s 2nd law?

• A) F = ½ ma2

• B) F = 2 ma• C) p = mv• D) a = F/m

• Answer: D

3. The force of gravity on an object is known as ___.• A) centripetal force• B) friction• C) momentum• D) weight

• Answer: D

4. Which of the following is NOT a type of friction?• A. static• B. sliding• C. centripetal• D. rolling

• Answer: C

5. What is true about an object falling toward Earth?• A. It falls faster the heavier it is.• B. It falls faster the lighter it is.• C. Earth pulls on it, and it pulls on Earth• D. It has no weight.

• Answer: C

6. Why do projectiles follow a curved path?• A. They have a horizontal and a vertical

motion.• B. They have centripetal force.• C. They have momentum.• D. They have inertia.

6. Why do projectiles follow a curved path?• A. They have a horizontal and a vertical

motion.• B. They have centripetal force.• C. They have momentum.• D. They have inertia.

• Answer: A

7. The product of mass and velocity is known as:• A. gravity• B. momentum• C. friction• D. weight

• Answer: B

8. Which body exerts the weakest gravitational force on Earth?

• A. the Moon• B. Mars• C. Pluto• D. Venus

8. Which body exerts the weakest gravitational force on Earth?

• A. the Moon• B. Mars• C. Pluto• D. Venus

• Answer: C

9. When a leaf falls, what force opposes gravity?• A. air resistance• B. terminal velocity• C. friction• D. weight

• Answer: A

10. In circular motion, the centripetal force is in what direction?

• A. forward• B. backward• C. toward the center• D. toward the side

10. In circular motion, the centripetal force is in what direction?

• A. forward• B. backward• C. toward the center• D. toward the side

• Answer: C

11. What is the weight on Earth of a person with a mass of 65 kg?

• Solution: Use F = ma; therefore W = mg

(W = weight in Newtons; g = 9.8 m/s2)

W = m g

11. What is the weight on Earth of a person with a mass of 65 kg?

• Solution: Use F = ma; therefore W = mg

(W = weight in Newtons; g = 9.8 m/s2)

W = m gW = (65 kg)(9.8 m/s2)W = 637 N

12. Some people put chains on their tires in the winter. Why?

12. Some people put chains on their tires in the winter. Why?Answer:• The chains help provide traction on the

ice and snow by increasing the amount of friction between the tires and the ice and snow.

13. List some ways an astronaut could keep her supplies from floating away while orbiting around the Earth.

• Possible Answers:– Bolt down items– Use velcro– Keep them tethered to the astronaut

14. As you in-line skate around the block, what action and reaction forces keep you moving?

• Answer: The action force is the wheels pushing against the street or sidewalk. The reaction force is the Earth (street or sidewalk) pushing back on you.

15. Which of the following has the greatest momentum? Explain.• A charging elephant• A jumbo jet sitting on the runway• A baseball traveling at 100 km/h

• Answer: the elephant has much more momentum than the baseball because of its size. The jumbo jet has zero momentum because it is at rest.

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