Forces

Forces: Pushes and pulls

• Forces make things happen in the world. They also keep things together, like atoms held together

by the strong force. When you kick a soccer ball, your foot feels a push back from the ball. The
harder you kick, the more of a push you feel. This is another force called the reaction force. The
same force is experienced when you push on a wall and feel pressure on your hands.

• Gravity pulls all things down toward the earth. Since it is a pull, that means it is a type of

force. An example of a push force is the force of the ground pushing back up on you as you stand
in place. This is called the normal force. Even when an object is not moving, forces like gravity
and the normal force are acting on it. Since the object is not moving, the opposing forces are said
to be in equilibrium because they are equal but in opposite directions.

• Moving objects have at least 4 forces acting on them. Consider a sled sliding down a hill. Gravity

and the normal force are acting on it, just like with a stationary object. But there is also the force
of motion in the direction the sled is going. Then there is friction, a hidden force. Friction goes
against the direction of motion and is present whenever 2 substances are touching, in this case
the sled and the icy hill. Friction can occur between solid substances, or even liquids and gases

• Remember: forces are a vector which means they have a direction as well as a magnitude. When

opposing forces are balanced, there is no net movement. However when forces are unbalanced,
the object moves in the direction of the greatest net force.

BrainPOP Link

As complex as air travel may seem, it is
basically a struggle between forces:
Lift is needed to overcome the weight of
the plane in order for it to take flight. It
would depend on the speed of the plane
during take off as well as its design. To
simplify, each plane must reach a certain
speed on the runway in order to
experience take off. Conversely, a flying
plane will descend if it slows down to a
certain speed.
The thrust is what moves the aircraft in a
forward direction and it must be able to
overcome drag, which opposes motion.
Thrust is largely determined by the
power of the engine while drag depends
on the design of the aircraft, its speed,
and also the air itself.

Khan Academy Link

Balanced vs. Unbalanced Forces

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Motion: Speed, Velocity, and Acceleration

• Speed is how fast something moves over a certain time frame. We use

this word all the time in everyday language and know what it means. For
cars, we use the unit miles per hour (mph) which means how many miles
are traveled in a certain amount of hours.

• Velocity is the scientific way of saying "speed" and it includes a

direction. We use metric units so rather than mph we must use meters per
second (m/s). In diagrams, arrows are used to show the direction of
movement and a number shows the magnitude.

• Acceleration is the change in velocity over a period of time. In other

words, acceleration is speeding up or slowing down. When something is
moving with a constant velocity, the acceleration is 0. The unit we use is
meters per second squared (m/s2). When unbalanced forces are acting on
an object, the object will accelerate in the direction of the greater net
force, such as sliding down an icy hill.

BrainPOP Link

Worksheet: Velocity vs. Acceleration

• Use the formula V = d/t to calculate velocity. Remember

to use the correct units.

1)Find the velocity of a car that drives 250 meters in 7
seconds.

2)How fast is a baseball traveling that moves 19 meters in
.3 seconds?

3)A rocket is launched from the ground and after 5 seconds
it is moving 28 meters per second. How far did it travel?

4)A small fish can swim 3.5 meters per second to escape
predators and is 4 seconds away from its safe hiding
place. How fast must a predator swim to catch the small
fish if it starts out 10 meters behind the fish when the
chase begins? (Hint: Find the distance traveled by the
small fish to get to safety)

Key Idea: How can velocity be 0 meters per second?

• Use the formula a = (Vf – Vi)/t to calculate acceleration.

Remember to use the correct units.

1.A rocket is launched from the ground and reaches a
speed of 56 m/s after 9.6 seconds. How fast the it
accelerate?

2.A skate boarder is traveling 4.5 m/s and then accelerates
down a ramp for 7 seconds reaching 8.0 a final speed of
8 m/s. What was his acceleration?

3.A brick is dropped off a roof of a building. After 6.2
seconds it hits the ground traveling 60.7 meters per
second. What was its acceleration?

4.Find the acceleration of a track runner who crosses the
finish line at 8 meters per second and takes 4 seconds to
slow down to a stop.

Key Idea: How can acceleration be a negative number?

Newton's Laws: Describing
motion through forces

• Forces are measured with a unit called Newtons, named

after the famous Isaac Newton who first described the
motion of objects. He came up the following laws to
summarize how our universe works:

• 1) Objects in motion tend to stay in motion, until it is

affected by a force. Conversely, an object at rest will stay at
rest until an unbalanced for acts on it.

• 2) The force of an object is directly proportional to its mass

times acceleration (force = mass x acceleration). This law
explains how a force affects an object’s velocity.

• 3) For every action there is an equal but opposite reaction.

So when a force is applied, that object applied an equal
force in the opposite direction.

YouTube Link

Newton’s 1st Law of Motion:

Consider the following situation:
• A rocket that is waiting to be launched is at rest. It will stay at rest

until there is a force that causes it to launch. A moving bike will stay
in motion until an unbalanced force acts on it in the opposite
direction.

Khan Academy

1. What force will eventually cause the rocket to slow down, stop, and eventually fall back to earth?

2. How are the brakes of a car an example of Newton’s 1stlaw at work?

3. Why does a cyclist have to continue pedaling to keep the bicycle in motion?

Objects at rest
will stay at rest

Objects in
motion will
stay in motion

Newton's 2nd Law of Motion:

Consider the following situation:
• A student sets up 2 identical ramps (same height and same slope) to

make observations about the laws of motion. The student slides
down objects with 2 different masses.

BrainPOP Link

Mass of Object A = 2 kg

Mass of Object B = 4 kg

1. How would Newton explain the difference in force as each object arrives at the bottom of the ramp?

2. In which direction is the force of friction?

3. Which object would have a greater force of friction assuming both are made of the same material?

Using the formula

F = m

x

a

Example A: If a force of 10 Newtons being applied from the left on
object A which has a mass of 2 kg, what will the acceleration be?

F = m

x

a

10 = ( 2 ) x a
(2) ( 2 ) x a
5 = a

Direction of
acceleration

Newton's 2nd Law of Motion:

Khan Academy

A
Applied Net

force

Extension:

1. What would the acceleration be if the force was doubled?

2. What would the acceleration be if the mass was doubled?

Note: A Newton is defined as 1 kg · m/s2. The formula can be
rearranged when any 2 variables are given so that the unknown can
be calculated. It is helpful in the beginning to add units at the end.

Newton’s 3rd Law of Motion:

Consider the following situation:
• A car accident involves 1 moving car and 1 car stopped at a red light.

Damage is inflicted on both cars because they exert forces on each
other.

Khan Academy

1. How is it possible to hurt your foot by kicking a solid wall?

2. Why do we push down in order to jump?

3. Gravity pulls you down by a force we call weight. Why don’t you keep accelerating downward?

The parked car exerts a force on the car that hits it from behind.

The Science of NFL Football Note Catcher

Watch Video #1 Link
Newton’s 1stLaw

Watch Video #2 Link
Newton’s 2ndLaw

Watch Video #3 Link
Newton’s 3rdLaw

What is the goal of a defensive player?
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How is a force applied to a kicked ball?
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What is another name for this law?
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How is the mass of the player important
for inertia?
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What is momentum?
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How is mass different than weight?
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How is impulse calculated?
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What type of collision occurs in football?
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Newton's 2nd Law Independent Work

Practice Set A:

1)What force is needed to lift a 5000 kg helicopter with an acceleration of 2
m/s2?

2) How many Newtons of force is needed to accelerate a 150 gram (.15 kg)

baseball at a rate of 28 m/s2?

3) What mass baseball bat is needed to exert a force of 100 Newtons at an

acceleration of 40 m/s2?

4) How much force is needed to accelerate a bowling ball 3.5 m/s2if it has a

mass of 4.5 kg?

5)A kid with a mass of 40kg is riding in a wagon that has a mass of 8 kg. They
accelerate down a hill at 5.5 meters per second squared. What is the
combined force of the kid and the wagon?

6) A 16 kg boulder is placed on a catapult. When the catapult is released, it

applied a force of 752 N. What is the acceleration of the boulder at this
time?

7) A .180 kg soccer ball is kicked with a force of 14 Newtons. How fast does it

accelerate when it is first kicked?

8) How can an object have zero acceleration but still be moving? Explain.

Practice Set B:

9) .8 Newtons of force is applied to a 30 gram billiard ball. Assuming there is

no friction, what is the acceleration of the ball? Hint: Use correct units!

10) A golf ball with a mass of 35 grams is struck by a golfer with a force of 14

Newtons. What is the golf balls acceleration?

11) A 12.5 kg sled starts from a stationary position and is pushed down a hill

with an unknown force. After 10 seconds, the velocity of the sled is 21
meters per second.
a) How fast did it accelerate? b) How much force does the sled have at this
time?

12) a) How much force is needed to accelerate a 20,000 kilogram airplane 3.5

m/s2? b) What will be the final velocity after the plane has been accelerating
for 11 seconds?

13) A 275 gram box is pushed across a horizontal surface with an applied force of

4.25 Newtons. The box goes a distance of 5 meters in 1.25 seconds before
hitting a wall. Find the a) acceleration and b) the ending velocity before
hitting the wall.

14) A motorcycle is cruising at a constant velocity of 32 m/s but then accelerates

to a speed of 46 m/s in 12 seconds to pass another vehicle before slowing
down to 35 m/s again in 5 seconds. Find the a) acceleration , b) deceleration
and the c) force produced by the engine during the 12 second acceleration if
the mass of the motorcycle is 252 kg with the rider.