Forces &

Motion

Motion

• A change in the position of an object

• Caused by force (a push or pull)

Inertia – tendency of an object to resist a change in motion

• 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.

• Balanced forces – when all forces acting on an object are equal.

Force• A push or pull on an object• Measured in units called newtons (N)• Measured with a spring scale• Forces act in pairs

Types of Force:– gravity– magnetic– friction– centripetal

Balanced Forces

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

• Act in opposite directions.

• Balanced forces do not cause a change in motion.

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

How Can Balanced Forces Affect Objects?

•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.

Unbalanced Forces• When all 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 Forces Affect Objects?

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

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

Net 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 Force

30 N 10 N

A child was playing with a jack in the box. The lid pushed down with 10 N of force, while the spring pushed up 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 Answer

30 N 10 N

A child was playing with a jack in the box. The lid pushed down with 10 N of force, while the spring pushed up 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 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 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 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 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.

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

F ÷

m x a

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

F ÷

m x a

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

F ÷

m x a

Practice

The acceleration of a sprinter is 10 m/s2. The force exerted

on the starting blocks is 650 N. What is the mass of the

sprinter?

Force =

Mass =

Acceleration =

Check Your Answer

The acceleration of a sprinter is 10 m/s2. The force exerted

on the starting blocks is 650 N. What is the mass of the

sprinter?

Force = 650 N

Mass = 65 kg

Acceleration = 10 m/s2

Mass = force ÷ acceleration

Mass = 650 ÷ 10

Mass = 65

Practice

A 56 kg cart is accelerating at 15 m/s2. Calculate the force

exerted on the cart?

Force =

Mass =

Acceleration =

Check Your Answer

A 56 kg cart is accelerating at 15 m/s2. Calculate the 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 x 15

Force = 840

Practice

A boat is pulling a 54 kg wake boarder. The force the boat

is exerting on her is 108 N. Calculate her acceleration.

Force =

Mass =

Acceleration =

Check Your Answer

A boat is pulling a 54 kg wake boarder. The force the boat

is exerting on her is 108 N. Calculate her acceleration.

Force = 108 N

Mass = 54 kg

Acceleration = 2 m/s2

Acceleration = force ÷ mass

Acceleration = 108 ÷ 54

Acceleration = 2