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Force and Motion
Force and Motion
Force• A force is a push or pull that one body
exerts on another
• Some forces you can feel, but others you can’t. Can you feel the force of the atmosphere pushing against you?
• Can you feel gravity pulling on your body towards the Earth?
Look at the arrows
Unbalanced Forces
Which way will the box move?
• What happens to the motion of an object when you exert a force on it? A force can cause the motion of an object to change. Think of a racket striking a ball with a force that causes the ball to stop and change directions.
What is another example of this?
– baseball
Balanced Forces• Forces do not always change
velocity. When two or more forces act on an object at the same time, the forces combine to form the NET FORCE
Balanced Forces• The net force on the box is
zero because the two forces cancel each other out.
• What do you notice about the arrows?
Unbalanced Force• Another example of how forces combine
is shown in Fig. 16B. (in the book)• When two students are pushing with
unequal forces in opposite directions, a net force occurs in the direction of the larger force.
• The student who pushes with a larger force will cause the box to move in the direction of the force.
Unequal Forces
Inertia and Mass• Inertia is the tendency of an object to
resist any change in its motion. If an object is moving, it will keep moving at the speed and in the same direction unless an outside (unbalanced) force acts on it.
Space the final frontier • How is outer space an example of
this?• Where • Is the• Golf• Ball• Now?
Questions• Does a large object have the same
inertia as a small object?• No! Does a bowling ball have
the same inertia has a table tennis ball?
• You can’t change the motion of a bowling ball much by swatting it with a table-tennis paddle.
• A greater force would be needed to change the motion of the bowling ball because it ahs greater inertia. Why?
• Because it has more MATTER!• The mass of an object is related to its
inertia. • The greater the mass of an object is, the
greater its inertia.
Which has more inertia?• a) b)
Newton’s First law of Motion• Sir Isaac Newton was able to state
rules that describe the effects of forces on the motion of objects.
• These rules are known as Newton’s laws of motion.
The Law of Inertia• According to Newton’s 1st law of
motion, an object moving at a constant velocity keeps moving at that velocity unless a net force (unbalanced) acts on it.
• If an object is at rest, it stays at rest unless a net force (unbalanced) acts on it.
Newton's first law• This is also called the law of
inertia.• (make sure you know this!!!)
• In the case of billiards, when the cue ball strikes the other balls, what are the forces involved? (the cue ball, gravity)
• Are they balanced of unbalanced?• (unbalanced) Write down • (Hint: if the object is moving, then it has
an unbalanced force)
Crash Test Dummies• What happens in a crash?• The law of inertia (Newton's 1st
law) can explain what happns in a car crash. When a car traveling about 50 km/hr collides head on with something solid, the car crumples, slows down, and stops.
• Any person not wearing their seatbelt will continue to move forward at the same speed the car was traveling.
• This is why if you are thrown out of a car during a wreck, the car will stop on top of you when it is finished rolling.
• When you turn a corner in the car, your body will resist the direction you are turning.
• So when you hit your window, dashboard, steering wheel, or the back of the front seat, you are hitting it with the same velocity that the car was traveling.
This is why you wear seatbelts
• About half the people who die in car crashes would survive if they wore seatbelts.
• Question #1• When a soccer player kicks a ball,
the ball accelerates. Explain what causes this acceleration in terms of a picture.
• Question #2• Do forces always cause motion?
• Describe three examples from sports in which a force changes the velocity of an object or person.
