Motion and Force

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1. Motion 2. Force

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MotionA change in position in a certain amount of time is motion.

Motion is relative

All movement is described relative to a particular frame of reference.When you see a car drive by, it is moving with respect to the ground.The frame of reference you use depends on the type of movementand the position from which you are observing.

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MotionSpeed

Speed is the rate at which an object moves. You can find the speedof an object by dividing the distance it traveled by the time it took totravel that distance.

Distance is usually measured in meters (m) or kilometers (km). Timeis usually measure in seconds (s) or hours (h).

So the unit of speed is often meters per second (m/s) or kilometersper hour (km/hr).

For example, if a car went 120 kilometers in 2 hours, its averagespeed would be the distance of 120 km divided by the time of 2 hrequaling 60 km/hr.

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Motion

Velocity

Velocity is a measure of the speed in a given direction. Suppose an airplane is flying eastward at 300 kilometers per hour (kph). The airplane’s speed is 300kph, but its velocity is 300 kph in an eastward direction.

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Motion

Have you ever ridden on a roller coaster? As you go down a hill, your speed rapidly increases. You are experiencing a change in velocity.

Acceleration

The rate of change in velocity is known as acceleration. If something is accelerating, it is speeding up, slowing down, or changing direction.

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Motion

Acceleration

Mathematically, acceleration is the change in velocity divided by the time for the change:

Acceleration = (Final velocity - Original Velocity) / (Change in time)

For example, if an object speeds up from a velocity of 240 feet/second to 560 feet/second in a time period of 10 seconds, the acceleration is:

(560 - 240) / 10 = 320 / 10 = 32 feet/second/second

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Motion Momentum

All moving objects have momentum. And the more momentum an object has, the harder it is to stop.

Momentum depends on the mass of the object and the velocity with which it is traveling. If either of these measurements is large, the object will have a large momentum.

Momentum is equal to the mass of an object multiplied by its velocity:

Momentum = Mass x Velocity

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Motion Momentum

Example:

Which vehicle has greater momentum, a 1000 kg truck traveling 75 km/hr or a 500 kg car traveling 100 km/hr ?

Truck: 1000kg x 75 km/hr = 75,000 kg-km /hr

Car: 500 kg x 100 km/hr = 50,000 kg-km / hr

Therefore, the truck has greater momentum even though the car is traveling faster.

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Motion

Conservation of Momentum

The total momentum of any group of objects remains the same unless outside forces act on the objects. This is called the conservation of momentum.

For example, the momentum of a baseball bat is transferred to the ball when the bat and the ball meet. The more momentum the bat has, the more momentum is transferred to the ball.

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Force A force is a push or a pull. The wind pushes against a flag, and a magnet pulls iron toward it.

A force gives energy to an object when it causes it to start moving, stop moving, change direction, or changes the shape of the object.

If you want to open a door, you exert a force on it to cause it to move. Increasing your force will make it move faster. If you want to stop the door from opening, you also exert a force. This time the force stops the motion of the door.

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Force Combining forces

If you and a friend were both pushing a baby stroller, your forces would be exerted in the same direction. When two forces are acting in the same direction, they add together. The total force on the stroller would be the sum of both of your forces.

When the total force on an object is in one direction, the force is called unbalanced. An unbalanced force changes the direction of an object.

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Force Combining forces

If your friend pulled the stroller in the opposite direction, the forces would combine in a different way. Whey two forces act in opposite directions, they act by subtraction. If one force is greater than the other, the stroller would move in the direction of the greater force. The total force on the stroller would be the difference between the pulling and pushing forces.

If your force and your friend’s force were equal, the forces on the stroller would be balanced, or zero, and the stroller would not move. When forces are balanced, there is no change in motion.

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ForceFriction

Friction will cause a moving object to slow down and finally stop.

For example, if you roll a ball on the ground, it will eventually come to astop.

If there was no friction, an object would continue to travel forever.

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Newton's First Law - Law of Inertia An object at rest will remain at rest and an object in motion will remain in motion unless acted on by an external force.

Another way of stating this law in more detail is:

1. If an object is motionless, it will stay motionless unless acted upon by some force.

2. If an object is moving at a constant speed or velocity, it will continue at that speed unless acted upon by some force.

3. If an object is moving, it will move in a straight line unless acted upon at an angle by some force.

The Law of Inertia assumes there is no friction or other resistive force that can slow down an object.

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Law of Inertia

You feel the effects of inertia every day. For example:

When you are riding in a car and it stops suddenly, you keep moving forward. If you did not have a seat belt on, your inertia could send you through the windshield.

When a car is traveling along a road in a straight line and the driver must turn the steering wheel slightly to follow a curve in the road, people in the car will continue to move in a straight line and will bump into the sides of the car.

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Newton's Second Law – Law of Dynamics

Have you ever pushed a shopping cart at the grocery store?

If you push a cart, it begins to move. The harder you push the cart (or apply force), the faster the cart accelerates.

If a cart is filled with groceries, it is harder to push. This is because the filled cart has more mass, and therefore inertia. A greater force is required to accelerate an object with greater inertia.

Thus, force and acceleration are related to an object’s mass:

Force = mass x acceleration

Force is usually measured in newtons (N), where 1 N = 1 kg x 1m / sec / sec

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Newton's Second Law Newton’s second law of motion explains one reason why a small car has better gas mileage than a large car.

Example:

A large and a small car are both accelerating a 2 m/s/s. The mass of the small car is 750 kg. The mass of the large car is 1000 kg. According to Newton’s 2nd Law, the force required to accelerate the small car is:

750kg x 2 m / s / s = 1500 N

The force required to accelerate the large car is:

1000kg x 2 m / s / s = 2000 N

More gasoline will have to be burned in the engine of the large car to produce the additional force.

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Newton's Third Law – Law of Reciprocal Actions

The third law of motion states that for every action there is an equal and opposite reaction.

This means that whenever one body exerts force upon a second body, the second body exerts an equal and opposite force upon the first body. All forces are paired.

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Newton's Third Law

For example:

When you walk across a floor, your feet push against the floor. At the same time, the floor pushes with an equal but opposite force against your feet.

Imagine if you and a friend are on ice skates. If you push on your friend with a certain force, your friend will be accelerated backwards according to Newton’s 2nd Law. But because of Newton's Third Law, your push causes an equal but opposite push on you. So you will also accelerate backwards.

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Speed & Velocity

Acceleration & Momentum Force

Newton’s Laws

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Category1: 100 Question

Why is motion relative?

d) It depends on your point of reference

a) It depends on the speed

b) It depends on your relatives

c) None of the above

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A car travels 240 kilometers in 3 hours. What is the speed of the car?

a) 80 km/hr

c) 100 km/hr

b) 20 m/s

d) 50 mph

Category1: 200 Question

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What is the difference between velocity and speed?

b) Velocity has direction

a) Speed is faster than velocity

Category1: 300 Question

d) Speed has direction

c) There is no difference

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The speed of a cruise ship is 50 km/hr. How far will the ship travel

in 14 hours?

c) 700 km

Category1: 400 Question

d) 10,000 km

b) 500 kma) 70 m

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At what velocity did an airplane fly if it traveled westward 1760 meters in 8 seconds?

b) 220 m/sa) 160 m/s

Category1: 500 Question

d) None of the abovec) 260 m/s

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A car is traveling at 30 km/hr. It speeds up to 85 km/hr in 5 seconds. What is the

acceleration of the car?

b) 11 km/hr/hra) 55 km/hr/hr

Category2: 100 Question

d) 85 km/hrc) 10 km/hr/hr

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A roller coaster’s velocity at the top of the hill is 10 m/s. Two seconds later it reaches the bottom of the hill with a velocity of 26 m/s. What is the acceleration of the roller coaster?

b) 8 m/s/s a) 16 m/s/s

Category2: 200 Question

d) None of the abovec) 13 m/s/s

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Despite his mother’s warnings, Tim was playing ball in the house when his ball

bounced out the window. A free falling body accelerates at about 10 m/s/s. What is the

velocity of Tim’s ball after 2 seconds? (Hint: The original downward velocity of the

ball is zero m/s)

c) 20 m/s

a) 30 m/s

Category2: 300 Question

d) None of the above

b) 10 m/s

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What is the momentum of an 0.30 kg bluejay flying at 17 m/s?

b) 5.1 kg-m / s a) 56.7 kg-m /s

Category2: 400 Question

d) None of the abovec) 51 kg-m / s

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Which object most likely has more momentum: a car traveling at 10

km/hr or a baseball pitched at 150 km/hr?

b) cara) baseball

Category2: 500 Question

c) Their momentums would be the same

d) None of the above

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Force is?

d) All of these answers

b) The ability to change motion

Category3: 100 Question

c) A push

a) A pull

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Forces that are opposite and equal are called?

d) balanced

b) unbalanced

Category3: 200 Question

c) momentum

a) friction

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A force that slows down the motion of an object is called?

c) friction

b) density

Category3: 300 Question

a) acceleration

d) momentum

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True or False: A force can set an object in motion, stop its motion, or change the

speed and direction of its motion?

b) Truea) False

Category3: 400 Question

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True or False: The combined force of unbalanced forces is

always zero ?

b) Falsea) True

Category3: 400 Question

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Suppose a 15-N force is required to push a crate across a floor. If you exert a force of 7-N, what force must your friend exert so

that you can move the crate together?

c) 8 N

b) 22 N

Category3: 500 Question

d) None of the above

a) 10 N

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How does inertia affect riding a bicycle?

b) You can stop peddling and you will continue to roll

a) You need to peddle harder when going uphill

Category4: 100 Question

d) Bicycles don’t use inertia

c) You need to peddle harder when going downhill

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What happens when you double the force of an object (with no friction) ?

d) You double its acceleration

a) Nothing happens

Category4: 200 Question

c) You double its mass

b) You cut the acceleration in half

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If you are standing on a scale and jump off, what happens to its

reading?

c) It goes up accordingly to the force of the jump

a) Nothing

Category4: 300 Question

d) None of the above

b) It immediately goes down when you jump off

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How much force is needed to accelerate a 1400 kg car at 2/m/s/s ?

b) 2800 kg-m/s/s

or 2800 N a) 500 kg-m/s/s or 500 N

Category4: 400 Question

d) None of the above

c) 700 kg-m/s/s or 700 N

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What is the force on a 1000 kg elevator that is falling freely at

9.8 m/s/s?

c) 9800 N

a) 9000 N

Category4: 500 Question

b) 98 N

d) 900 N

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You are a champion in science!

Choose a Category:

Motion Force

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It is 1:00 pm. The local weather channel is tracking a thunder storm that is traveling at 25 km/hr. The storm is 75 km west of Ann Arbor. If school gets out at 3:00 pm, will Forsythe students be able to get home before the storm?

Yes - The storm will hit at 4:00 pm.

If the forces exerted on an object are 50 N in one direction and 30 N in the opposite direction, what is the net force exerted on the object?

20 N

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