Factors That Affect MotionObjective 4.05

Determine factors that affect motion including: Force Friction Inertia Momentum

What Affects Motion?• Motion is the change in an object’s

location over time. People and machines can make objects move. Unbalanced forces, like pushes or pulls, make objects move.

• We also understand that usually objects don’t keep moving forever. They can stop moving or change direction. They can speed up or slow down. Objects change their motion because unbalanced forces act on them. There are four main factors that affect the motion of objects.These factors are force, friction,inertia, and momentum.

Force• (Review) Ways that forces affect motion

are: * direction of application—the angle of a force determine the direction of the motion of the object. * strength of force—delivering a lot of force moves objects a long way. * mass of object—massive objects require more force.

• In Figure 15.1, the skater’s direction is determined by the angle of force he applies. How far he moves is determined by how hard he pushes. The lighter he is, the easier he can glide across the ice.

Figure 15.1 Skater

Friction• Friction acts in the opposite direction of

the object’s motion. Rubbing often produces friction. When objects touch, their surfaces interact. Moving one surface across another creates friction. It can be felt if you rub your hands together quickly. Repeated rubbing of your hands creates heat. This is one side effect of friction.

Figure 15.2 Rubbing Creates Friction

• Friction can also result from air resistance. Particles of air bump against the surface of an object. This slows its motion. Air resistance is a type of frictional force. Frictional force oppose the motion of objects.

• Think about this: a ball rolling across the floor. Where will friction occur? Some will come from the ball’s surface rubbing against the floor’s surface. This will slow the ball’s motion. Friction will also come from air resistance. These forces cancel the forward force (and thus the forward motion) of the ball.

Figure 15.3 Frictional Forces on a Ball

• Friction can be a good thing. It is what keeps your bike tires (and car tires) on the road as you go around corners or come up to a stop. It is also what allows your shoes to grip the basketball or tennis court when playing a game. Track runners and rock climbers use extra sticky shoes to increase friction and improve performance.

• There are only a few situations where friction is non-existent or a minor force. There are also many ways to reduce frictional forces. Grease or oil inside machinery is often used to reduce friction.

Inertia• You may have heard this one before: an object

at rest will remain at rest, and an object in motion will remain in motion. That is, unless an outside force acts on the object. When we put something down, it tends to stay in the same place unless someone or something moves it. The tendency of matter to remain at rest or in motion is another way of explaining inertia. Inertia is an object’s resistance to a change in motion. A massive object has a lot of inertia while a smaller less massive object has a little inertia.

• We see examples of inertia everyday. One common example of inertia is felt in a car. Have you ever been in a car that stops suddenly? If you have, you might have felt like you were being thrown forward. That is because your body resists the stopping motion of the car. Your body tends to remain in motion. The outside force, the stopping car and its seat belt, act to stop your forward motion.

• Figure 15.4 shows this very situation. The driver of this car forgot to stop at the stop sign. He slams on his brakes, causing the body of the car and himself lean forward. This is inertia at work. But remember, inertia is NOT a force. It is a property of the object.

Figure 15.4 Inertia Can be felt in a Car

• Perhaps you have tried to carry a glass of water quickly across a room. Stopping suddenly causes the water inside the glass to spill. This is because of the water has inertia. When you stop, the water wants to keep going, resisting the change in motion. In Figure 15.5, you can see some water behaving badly. Stopping the glass allows the liquid inside to keep moving according to inertia.

Momentum• Momentum is an objects mass multiplied by

its speed. In mathematical equations, we represent momentum with a “p.” The for momentum is: p = m (mass of an object) x v (velocity of the object)

• Let’s think about it like this: an object’s mass multiplied by its speed equals its momentum. How much matter you have times how fast it is moving equals the momentum of the object. In short, the amount of momentum depends on two factors: mass and speed. Large objects moving quickly have a lot of momentum. Smaller objects moving slowly have less momentum.

• Simply put, momentum is mass that is in motion. Any object in motion has momentum. Increasing the mass or speed will increase the momentum. Conversely, decreasing the mass or speed will decrease the momentum.

• Example, throwing a heavy bowling ball with a high acceleration will have more momentum than the same bowling ball thrown with a lower acceleration. Also, throwing a lighter ball (lower in mass) will result in less momentum.

Review1. According to momentum, which statement is true about a small ball rolled down a hallway?

A. It has force.

B. It has zero friction.

C. It needs a larger force.

D. It is very heavy.

2. What word best describes friction?

A. stoppingB. acceleratingC. harmfulD. quick

3. What best describes inertia?

A. the tendency to resist massB. the ability to moveC. the tendency to resist changes in motionD. the ability to decelerate

4. Which situation below best demonstrates friction?

A. Markus trips while running, falling forward.B. Josh throws a strike at the bowling alley. C. Francis pushes a box across the floor.D. Sam throws a ball through the air.

5. Kelly is riding her bike down the street. She runs into a pothole and falls forward off her bike. She hits the ground and dents her helmet. What causes her to dent her helmet?

A. gasoline

B. friction

C. inertia

D. momentum

The End