GHS Integrated ScienceChapter 12 – Forces and Motion

12.1 Forces

force – a push or pull that acts on an object A force can cause a resting object to move, or it can accelerate a moving object by changing the object’s speed or direction. force is measured in newtons (N) one newton is the force that causes a 1-kg mass to accelerate at a rate of 1 meter per second per second equal to 1 kilogram-meter per second squared arrows are used to represent the direction and magnitude of the force

Relative motion – movement in relation to a frame of reference. Example: the students in the classroom are all sitting still in reference to each

other, but using the Sun as a reference we are actually travelling about 100,000 km/hr

Distance vs. Displacement

Distance – the length of a path between two points. Displacement – the length of a straight line from an object’s starting point to its

ending point Example: think of the student who leaves the classroom, walks up and down

every hallway, and then returns to the same classroom. His distance might be several hundred meters, but his displacement is zero because he ended up right where he started.

Combining Displacements

Displacement is a vector quantity. Vector – a quantity that has magnitude and direction.

Add displacements using vector addition Displacement along a straight line

When two displacements, represented by arrows, have the same direction, you can add their magnitudes

If two displacements are opposite, you subtract their magnitudes

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Displacement that isn’t along a straight line When two or more displacement vectors have different directions, they

may be combined by graphing (see below) Quick Lab Activity (page 330)

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11.2 Speed and Velocity

Speed

Speed – the distance an object travels divided by the time it took to travel that distance

Average speed – computed for the entire duration of a trip

– total distance/total time

Instantaneous speed – speed measured at a particular instant

Graphing Speed

A distance-time graph is a good way to describe motion. They show the distance an object has traveled versus the amount of time it

takes. The slope of a line on a distance-time graph is speed.

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Velocity

Velocity – the speed and direction in which an object is moving. Velocity is a vector.

Combining Velocities

Two or more velocities add by vector addition. Sometimes the motion of an object involves more than one velocity. If a boat is moving on a flowing river, the velocity of the river relative to the

riverbank and the velocity of the boat relative to the river combine. They yield the velocity of the boat relative to the riverbank.

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11.3 Acceleration

Acceleration – a change in velocity, which means a change in speed, direction, or both.

Acceleration is a vector. Can be caused by an increase or decrease in speed For example, when you are at a red light and speed up when it turns green, you

feel the acceleration as you increase your speed. Usually measured in meters per second per second m/s/s or m/s2

Question: Is a carousel going at a constant speed accelerating? Also, besides the gas pedal, what else in a car could be called the accelerator?

Complete Math Skills on page 346.

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Free Fall An example of acceleration is free fall Free fall – the movement of an object toward Earth

solely due to gravity. On Earth, objects in free fall accelerate at a rate of 9.8

m/s2

Graphs of Accelerated Motion You can use a graph to calculate acceleration. The slope of a speed-time graph is acceleration. This slope is change in speed divided by time.

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t = 0 sv = 0 m/s

t = 1 sv = 9.8 m/s

t = 2 sv = 19.6 m/s

t = 3 sv = 29.4 m/s

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