Explanation of how Force Affects Motion by Ron Kurtus - Succeed in Understanding Physics. Key words: physical science, push pull, change in velocity, accelerate, decelerate, force field, gravity, magnetism, airplane, wind, friction, Ron Kurtus, School for Champions. Copyright © Restrictions

# Force Affects Motion

by Ron Kurtus (30 October 2007)

A force is defined as an action on an object that affects its motion (or lack of motion).

The action is often called a push or pull. The push can come from direct contact, like when objects collide or from a force field like magnetsm. The pull seems to only come from a field at a distance, like gravity or magnetism.

The action from a force can cause an object to move or speed up (accelerate), to slow down (decelerate), to stop, or to change direction. Since any change in velocity is considered acceleration, it can be said that a force on an object results in the acceleration of an object.

Questions you may have include:

• How can a force accelerate an object?
• How can a force slow down an object?
• When can a force cause an object to change directions?

This lesson will answer those questions.

Useful tool: Metric-English Conversion

## Force can cause acceleration

When a force acts on an object that is stationary or not moving, the force will cause the object to move, provided there are no other forces preventing that movement. If you throw a ball, you are pushing on it to start its movement. If you drop an object, the force of gravity causes it to move.

If an object is initially stationary, it accelerates when it starts to move. Acceleration is the change in velocity over a period of time. The object is going from v = 0 to some other speed or velocity.

Likewise, if an object is already moving and a force is applied in the same direction, the object will speed up or accelerate. For example, a gust of wind can speed up a saliboat.

### Accelerates Until Force Stops

As long as the force is applied to a given object, it will to accelerate. Once the force is withdrawn, the object will continue to move at a constant velocity.

## Force can cause deceleration

If an object is moving and there is a force in the opposite direction of the motion, the object will decelerate or accelerate in the opposite direction. If you throw a ball up at a given velocity, it will slow down as it travels upward due to the force of gravity. Likewise, an airplane will decelerate if flying into a strong headwind.

A decelerating or resistive force can cause a moving object to stop. This can be seen when you apply the brakes on your car.

## Force can change direction

A force applied at an angle to the direction of motion of an object can cause it to change direction. A side wind will cause an airplane to change its direction.

It is possible that the object keeps going at the same speed, if the force is applied perpendicular to the direction of motion. But the velocity of the the object changes. Speed is how fast the object is going, while velocity is speed plus direction.

## Summary

A force is a push or pull on an object that affects its motion. The push can come from direct contact, like when objects collide or from a force field. The pull seems to only come from a field at a distance. The action from a force can cause an object to accelerate, to decelerate, to stop or to change direction.

Since any change in velocity is considered acceleration, it can be said that a force on an object results in the acceleration of an object.

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## Resources and references

Ron Kurtus' Credentials

### Websites

Physics Resources

### Books

Forces In Nature by Liz Sonneborn Rosen; Publishing Group (2004) \$25.25 - Understanding gravitational, electrical and magnetic force

The Science of Forces by Steve Parker; Heinemann (2005) \$29.29 - Projects with experiments with forces and machines

Glencoe Science: Motion, Forces, and Energy, by McGraw-Hill; Glencoe/McGraw-Hill (2001) \$19.32 - Student edition (Hardcover)

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