Forces and Motion

Demonstrate and calculate how unbalanced forces change the

speed or direction of an objects motion

Force

• Any push or pull on an object• Act in pairs• Measured in Newtons (N)

• Every force has a magnitude and direction• Forces can work together or against each

other• Forces can be balanced or unbalanced

Balanced Forces

• Occurs when there is NO CHANGE in an objects motion

• No acceleration – no change in motion

– objects are at rest or traveling at a constant speed and direction.

Balanced forces

Unbalanced Forces

• Always cause a change in motion - Acceleration

• Acceleration – change in speed or direction of an object

• Starting• Stopping• Speeding up• Slowing down• Turning

Net Force

• Combination of all forces acting on an object.

• Add forces working in the same direction.

• Subtract forces that are working in opposite directions.

Vectors

• Lines with arrows that show force.• Length of the line shows the amount

of force (magnitude)• The arrow shows the direction of the

force.

Add forces acting in the same direction

• 8 N + 6 N = 14 Newtons

Subtract forces acting in opposite directions

• 20 N – 18 N = 2 Newtons

Net Force = 0Forces are balanced

• 20 N – 20 N = 0 Newtons • Forces are balanced - No change in motion

Net force and direction

• What is the net force?

• What direction are they moving?

• Why?

Newton’s 1st Law of Motion

• Law of Inertia

• An object at rest will stay at rest unless acted on by an unbalanced force.

• An object in motion will stay in motion at the same speed and direction unless acted on by an unbalanced force.

In other words…

• An object will keep doing what it is doing unless forces become unbalanced

• Also known as the “Law of Inertia”

• Inertia – An object’s tendency to resist change

Law of Inertia (1st law)

Vehicle Restraints

• What is the purpose of vehicle restraints such as seat belts?

• Describe how Newton’s 1st Law of motion applies to vehicle restraints.

Newton’s 2nd Law of Motion

• Law of Acceleration

• The acceleration of an object by a force in inversely proportional to the mass of the object and directly proportional to the force applied.

In other words…

• The smaller the object, the greater the acceleration AND the larger the object, the smaller the acceleration.

• The smaller the force applied, the smaller the acceleration AND the larger the force applied the larger the acceleration.

Therefore

• Force = Mass X Acceleration• Force = 1,000 kg X .05 m/s/s• Force = 50 Newtons

Acceleration

• The rate of change in the velocity, speed and/or direction, of an object.

• Acceleration = Force/Mass

• Measured in meters/sec2

Force = Mass X Acceleration

• If the amount of force stays the same and the mass of the object increases, what will happen to the amount of acceleration?

• If the amount of force stays the same and the mass of the object decreases what will happen to the amount of acceleration?

What if the acceleration has to stay the same?

• What will have to happen to the amount of force needed if the mass of an object increases?

• What will happen to the amount of force needed if the mass of an object decreases?

Spring Scales

• Used to measure force.

• Use the spring scale to measure the amount of force needed to pull the CPO car up the ramp with different amounts of weight/mass.

• What happens to the amount of force when the mass increases?

Newton’s 3rd Law of Motion

• For every action, there is an equal but opposite reaction

In other words…

• All forces act in pairs.

• When one object exerts a force on a second object, the second object exerts a force back that is equal in size but opposite in direction

Newton’s 3rd Law

Space Shuttle Launch

Speed/Velocity

• How fast an object is moving• Rate at which an object covers a

distance• Speed = distance/time

(meters/second)

Calculating Speed

• While on vacation, Lisa Carr traveled a total distance of 440 miles. Her trip took her 8 hours. What was her average speed?

• Speed = 440 miles/8 hours

• Average speed = 55 miles/hour

Calculating Distance and Time

• Calculating the amount of time it takes to travel a distance.

•Time = distance/speed

• Calculating the distance traveled in a certain amount of time.

•Distance = speed X time

What’s the difference between Speed and

Velocity• Velocity - The rate at which an object

changes position. It’s the measurement of the rate and direction of motion.

Force = Mass x Acceleration

(F = MA)Which object would have the greatest

force?

– A 500 g object accelerating at 10m/s2

or– A 10 g object accelerating at 10m/s2

– Use your formula chart to calculate Force• Force = Mass x acceleration

Force = Mass x Acceleration

• 500g X 10 m/s2 = 5000 Newtons force

• 10g X 10 m/s2 = 100 Newtons of force

• The 500g object exerts a greater force

What is the motion of the object in the graph below?

• No motion • Moving away fast at a constant speed• Stopped (no motion)• Moving away more slowly at a constant speed• Stopped (no motion)

What is the motion of the object in the graph below?

• Moving away at a constant speed• Stopped moving (no motion)• Accelerating

Distance Time Graphs

What is the motion of the object in the graph below?

What is the speed of the object?

Speed = distance ÷ time

• 5 meters in 1 sec.• 25 meters in 5 sec.• 50 meters in 10 sec.

• 5 ÷ 1 = 5 m/s• 25 ÷ 5 = 5 m/s• 50 ÷ 10 = 5 m/s

This object is travelling at a constant speed of 5 m/s (meters per second)

Interpret the graph below

Kinetic vs Potential Energy

• Kinetic Energy – Energy of motion– The faster you are travelling the more

kinetic energy you have

• Potential Energy – Stored energy– The more stored energy you have built

up the more potential energy you have

Where does the rollercoaster have the greatest kinetic

energy vs. potential energy