Forces & Motion

Forces Forces & &

MotionMotion

MotionMotion• A change in the position of an object

• A change in motion is caused by force (a push or pull on an object caused by interaction of objects; either by contact or at a distance)

ForceForce• A push or pull on an object caused by the interaction of objects;

either by contact or at a distance.• Measured in units called newtons (N)• Measured with a spring scale• Forces act in pairs.

Types of Force:Types of Force:– gravitygravity– electricelectric– magneticmagnetic– frictionfriction– air resistanceair resistance

InertiaInertia• An object at rest will remain at rest until acted

upon by an unbalanced force.

• An object in motion will remain in motion until acted upon by an unbalanced force.

• When all forces acting on an object are equal (balanced)

BalancedBalanced ForcesForces• When all the forces acting on an object are equal.

(Net force is zero.)

• Balanced forces do not cause a change in motion.

• Balanced forces can change the physical properties of an object without changing its motion (shape).

• In your notes, describe an example of a balanced force.

How Can Balanced Forces How Can Balanced Forces Affect Objects?Affect Objects?

• Cause the shape of an object to change without changing its motion

• Cause an object at rest to stay at rest or an object in motion to stay in motion (inertia)

• Cause an object moving at a constant speed to continue at a constant speed

• In your notes, describe an example of a balanced force affecting an object.

UnbalancedUnbalanced ForcesForces• When all of the forces acting on an object are not

equal

• The forces can be in the same direction or in opposite directions.

• Unbalanced forces cause a change in motion.

• In your notes, describe an example of an unbalanced force.

How Can Unbalanced How Can Unbalanced Forces Affect Objects?Forces Affect Objects?

• Acceleration is caused by unbalanced forces:– slow down– speed up– stop – start – change direction– change shape

• In your notes, describe an example of an unbalanced force affecting an object.

Net ForceNet Force• The total of all forces acting on an object:

– Forces in the same direction are added.• The object will move in the direction of the

force.

– Forces in opposite directions are subtracted.• An object will move in the direction of the

greater force.

Calculating Net ForceCalculating Net Force

30 N 10 N

A child was playing with a jack in the box. The lid pushed down on the spring with 10 N of force, while the spring pushed up on the lid with 30 N of force. What is the net force applied by the spring?

In your notebook, calculate the net force and show all of your work.

http://commons.wikimedia.org/wiki/File:Jack-in-the-box.jpg

Check Your AnswerCheck Your Answer

30 N 10 N

A child was playing with a jack in the box. The lid pushed down on the spring with 10 N of force, while the spring pushed up on the lid with 30 N of force. What is the net force applied by the spring?

30 N up - 10 N down= 20 N up

http://commons.wikimedia.org/wiki/File:Jack-in-the-box.jpg

Calculating Net ForceCalculating Net Force

30 N 50 N

Two competitors are playing tug of war.

What is the net force? Which direction will the rope move?

In your notebook, calculate the net force and show all of your work.

Tug of War Game

Check Your AnswerCheck Your Answer

30 N 50 N

Two competitors are playing tug of war.

What is the net force? Which direction will the rope move?

50 N to the right – 30 N to the left = 20 N to the right

Tug of War Game

Mass and WeightMass and Weight• Mass is the amount of stuff (matter) in an object.

– The mass of an object will remain the same anywhere in the universe.– Mass is measured in grams or kilograms.

• Weight is the mass (kg) of an object multiplied by the acceleration of gravity (9.8 m/s2).– The weight of an object depends on the gravitational pull of the location in the universe.– The gravitational pull on the Moon is only 1/6 of Earth’s gravitational pull.– Weight is measured in newtons (N).

Mass and WeightMass and Weight Example: F=ma

A person with a mass of 25 kg x 9.8 m/s2 gravitational acceleration is equal to 245 N of weight force.

F= 25 kg x 9.8 m/s2

F = 245 N

The person exerts 245 N of force on the ground.If the person went to the Moon, they would only weigh 1/6 of 245 N.

ForceForceForce (N) = mass (kg) x acceleration (m/s2)

F ÷

m x a

F=ma

ForceForce Acceleration (m/s2) = force (N) ÷ mass (kg)

F ÷

m x a

a=F/m

ForceForce Mass (kg) = force (N) ÷ acceleration (m/s2)

F ÷

m x a

m=F/a

PracticePracticeThe acceleration of a sprinter is 10 m/s2. The forceexerted on the starting blocks is 650 N. What is themass of the sprinter?

Force = Mass = Acceleration =

Check Your AnswerCheck Your AnswerThe acceleration of a sprinter is 10 m/s2. The force exerted on the starting blocks is 650 N. What is themass of the sprinter?

Force = 650 N Mass = 65 kg Acceleration = 10 m/s2

Mass = force (N) ÷ acceleration (m/s2)

Mass = 650 (N) ÷ 10 (m/s2) Mass = 65 kg

PracticePracticeA 56 kg cart is accelerating at 15 m/s2. Calculatethe force exerted on the cart?

Force = Mass = Acceleration =

Check Your AnswerCheck Your AnswerA 56 kg cart is accelerating at 15 m/s2. Calculatethe force exerted on the cart?

Force = 840 N Mass = 56 kg Acceleration = 15 m/s2

Force (N) = mass (kg) x acceleration (m/s2) Force = 56 (kg) x 15 (m/s2) Force = 840 N

PracticePracticeA boat is pulling a 54 kg wake boarder. The forcethe boat is exerting on her is 108 N. Calculate heracceleration.

Force = Mass = Acceleration =

Check Your AnswerCheck Your AnswerA boat is pulling a 54 kg wake boarder. The forcethe boat is exerting on her is 108 N. Calculate heracceleration.

Force = 108 N Mass = 54 kg Acceleration = 2 m/s2

Acceleration = force (N) ÷ mass (kg) Acceleration = 108 (N) ÷ 54 (kg) Acceleration = 2 m/s2