Forces & Motion

Motion

A change in the position of an object

Caused by force (a push or pull)

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› electric› magnetic› friction› Air resistance› Balanced› Unbalanced

Inertia

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)

Balanced Forces 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.

How Can Balanced Forces 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

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› change shape

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

The distance an object travels in a certain amount of time; describes a rate

› Average speed – Total distance divided by total time

› Constant speed - Speed that does not change

Speed equals distance divided by time.Unit of Measurement: meters per second (m/s), kilometers per hour (km/hr), or miles

per hour (mi/hr)

Speed Speed = distance ÷ time t = d/S

d = S*t

d ÷

S X t

A football field is about 100 meters long. If it takes a person 20 seconds to run its length, how fast was the football player running?

In your notebook, calculate this problem. Make sure you show all work!

A football field is about 100 meters long. If it takes a person 20 seconds to run its length, how fast was the football player running?

› Speed = Distance ÷ Time

› Speed = 100 m ÷ 20 s› Speed = 5m/s

Remember to label

the UNITS!!

An object’s speed and direction at a given time; describes a rate and how fast something moves› The wind is blowing 65 km/hr from the

North.

A change in the direction or speed (velocity) of an object over time – which may be:› A change in speed

Starting Stopping Speeding up (positive acceleration) Slowing down (negative acceleration)

› A change in direction Acceleration is caused by unbalanced forces.

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

PracticeThe acceleration of a sprinter is 10 m/s2. The force exertedon the starting blocks is 650 N. What is the mass of thesprinter?

Force = Mass = Acceleration =

Check Your AnswerThe 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

PracticeA 56 kg cart is accelerating at 15 m/s2. Calculate the forceexerted on the cart?

Force = Mass = Acceleration =

Check Your AnswerA 56 kg cart is accelerating at 15 m/s2. Calculate the

forceexerted 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

Check Your AnswerA boat is pulling a 54 kg wake boarder. The force the boatis 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

Exit SlipAcceleration Speed

Velocity