newtons first law

overcoming inertia

To change the motion of an object, inertia must be overcome by an unbalanced force acting on the object. Until the soccer player kicks the ball in Figure 14.4, the ball remains motionless on the ground. However, when the ball is kicked, the force on it is suddenly unbalanced. The ball starts moving across the field because its inertia has been overcome.

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inertia and mass

The inertia of an object depends on its mass. Objects with greater mass also have greater inertia. Think how hard it would be to push a big box full of books, like the one in Figure 14.3. Then think how easy it would be to push the box if it was empty. The full box is harder to move because it has greater mass and therefore greater inertia.

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force and motion

Newtons first law of motion states that an objects motion will not change unless an unbalanced force acts on the object. If the object is at rest, it will stay at rest. If the object is in motion, it will stay in motion and its velocity will remain the same. In other words, neither the direction nor the speed of the object will change as long as the net force acting on it is zero. You can watch a video about Newtons first law at this URL: http://videos.howstuffworks.com/ Look at the pool balls in Figure 14.2. When a pool player pushes the pool stick against the white ball, the white ball is set into motion. Once the white ball is rolling, it rolls all the way across the table and stops moving only after it crashes into the cluster of colored balls. Then, the force of the collision starts the colored balls moving. Some may roll until they bounce off the raised sides of the table. Some may fall down into the holes at the edges of the table. None of these motions will occur, however, unless that initial push of the pool stick is applied. As long as the net force on the balls is zero, they will remain at rest.

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inertia

Newtons first law of motion is also called the law of inertia. Inertia is the tendency of an object to resist a change in its motion. If an object is already at rest, inertia will keep it at rest. If the object is already moving, inertia will keep it moving. Think about what happens when you are riding in a car that stops suddenly. Your body moves forward on the seat. Why? The brakes stop the car but not your body, so your body keeps moving forward because of inertia. Thats why its important to always wear a seat belt. Inertia also explains the amusement park ride in Figure 14.1. The car keeps changing direction, but the riders keep moving in the same direction as before. They slide to the opposite side of the car as a result. You can see an animation of inertia at this URL:

instructional diagrams

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questions

Newtons first law of motion is also called the law of

a. mass.

-->  b. inertia.

c. velocity.

d. unbalanced forces.

Newtons first law of motion states than an objects motion will not change unless

-->  a. the net force acting on it is greater than zero.

b. a force continues to be applied to the object.

c. its inertia is stronger than the applied force.

d. the object has no inertia.

Overcoming an objects inertia always requires a(n)

a. large mass.

b. massive force.

-->  c. unbalanced force.

d. two of the above

Once an applied force causes an object to start moving, the object keeps moving because

a. the force continues to be applied to it.

b. no other force is acting on it.

-->  c. it has inertia.

d. none of the above

It is more difficult to start a 50-kg box sliding across the floor than a 5-kg box because the 50-kg box has greater

a. size.

-->  b. inertia.

c. volume.

d. velocity.

If the net force acting on any object is zero, the object will

a. not move.

b. change its motion.

c. have zero velocity.

-->  d. none of the above

Once an object starts moving along a clear path, it would keep moving at the same velocity if it were not for

a. inerti

b. friction.

c. an unbalanced force.

-->  d. two of the above

If you run into a curb on your bike, you might fly forward over the handlebars because of

a. air resistance.

-->  b. inertia.

c. friction.

d. gravity.

An objects velocity will not change unless it is acted on by a(n)

a. net force.

b. strong force.

-->  c. unbalanced force.

d. opposite but equal force.

Inertia causes a stationary object to

a. start moving.

-->  b. remain stationary.

c. have an increase in velocity.

d. change its speed or direction.

The direction of a moving object will not change if the net force acting on it is

a. greater than zero.

b. less than zero.

-->  c. zero.

d. two of the above

Inertia is the tendency of an object to resist motion.

a. true

-->  b. false

Newtons first law of motion is also called the law of acceleration.

a. true

-->  b. false

If an object is at rest, inertia will keep it at rest.

-->  a. true

b. false

The inertia of an object is determined by its speed.

a. true

-->  b. false

Newtons first law relates motion to balanced and unbalanced forces.

-->  a. true

b. false

The speed of an object changes only when it is acted on by an unbalanced force.

-->  a. true

b. false

An object with greater mass has greater inertia.

-->  a. true

b. false

A stationary object resists movement only because of gravity.

a. true

-->  b. false

Balanced forces are needed to change an objects motion.

a. true

-->  b. false

The tendency of an object to resist a change in motion depends on its mass.

-->  a. true

b. false

If the net force acting on an object is zero, its inertia is also zero.

a. true

-->  b. false

A rolling ball will roll forever unless it runs into another object.

a. true

-->  b. false

The tendency of an object to resist a change in its motion depends on the objects size.

a. true

-->  b. false

When you are moving at a high rate of speed, inertia makes is hard to stop.

-->  a. true

b. false

Newtons first law of motion applies only to objects that are already moving.

a. true

-->  b. false

combination of all the forces acting on an object

a. inertia

b. unbalanced force

c. friction

d. law of inertia

e. mass

-->  f. net force

force that opposes the motion of any object

a. inertia

b. unbalanced force

-->  c. friction

d. law of inertia

e. mass

f. net force

an objects motion will not change unless an unbalanced force acts on it

a. inertia

b. unbalanced force

c. friction

-->  d. law of inertia

e. mass

f. net force

factor that determines the inertia of an object

a. inertia

b. unbalanced force

c. friction

d. law of inertia

-->  e. mass

f. net force

type of force needed to overcome inertia of an object

a. inertia

-->  b. unbalanced force

c. friction

d. law of inertia

e. mass

f. net force

tendency of an object to resist a change in motion

-->  a. inertia

b. unbalanced force

c. friction

d. law of inertia

e. mass

f. net force

diagram questions

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