Read the following information on force. Once you have read the information, follow the link at the bottom to answer the force questions.
Force
Kick a football and it moves. Its path through the air is not a straight line – it curves due to gravity. Catch the ball and it stops. Most of the motion we see undergoes change. Most things start up, slow down, or curve as they move. Objects at rest or moving at constant velocity have no net force acting on them. Their motion is not changing. A more common case is that in which there is a change in motion – that is, accelerated motion.
Recall that acceleration describes how fast motion is changing. Specifically, it is the change in velocity per certain time interval. In shorthand notation,
Acceleration = (change in velocity)/(time interval)
This is the definition of acceleration. The cause of acceleration is a force.
Force causes acceleration
Consider an object at rest, such as a hockey puck on ice. Apply a force and it moves. Since it was not moving before, it has accelerated – changed its motion. When the stick is no longer in contact with the puck, it moves at constant velocity. Apply another force by striking it with the stick again, and the motion changes. Again, the puck has accelerated. Forces are what produce accelerations.
Most often, the force we apply is not the only force that acts on an object. Other forces may act as well. The combination of all the forces that act on an object is called the net force. It is the net force that accelerates an object.
Forces combine to produce net forces. When more than one force acts in the same direction on an object, the net force is the sum of the forces. When forces act in opposite directions, the net force is the difference of the forces. If you pull horizontally with a force of 10 N on an object that rests on a friction-free surface, an air track for example, then the net force acting on it is 10 N. If a friend assists you and pulls at the same time on the same object with a force of 5 N in the same direction, then the net force will be the sum of these forces, 15 N. The object will accelerate as if it were pulled with a single force of 15 N. If, however, your friend pulls with 5 N in the opposite direction, the net force will be the difference of these forces, 5 N. The acceleration of the object would be the same as if it were instead pulled with a single force of 5 N.
We find that the amount of acceleration depends on the amount of the net force. To increase the acceleration of an object, you must increase the net force. This makes good sense. Double the force on an object and you will double the acceleration. If you triple the force, you’ll triple the acceleration, and so on. We say that the acceleration produced is directly proportional to the net force. (As one goes up the other goes up, as one goes down, the other goes down)
Mass Resists Acceleration
Push on an empty shopping cart. Then push equally hard on a heavily loaded shopping cart, and you’ll produce much less acceleration. This is because acceleration depends on the mass being pushed upon. For objects of greater mass we find smaller accelerations. Twice as much mass for the same force results in only half the acceleration; three times the mass results in one third the acceleration, and so forth. In other words, for a given force the acceleration produced is inversely proportional to the mass. By inversely we mean that the two values change in opposite directions
(As one goes up, the other goes down).
Newton’s Second Law
Newton was the first to realize that the acceleration we produce when we move something depends not only on how hard we push or pull (the force) but on the mass as well. He came up with one of the most important rules of nature ever proposed, his second law of motion.
Newton’s second law states:
The acceleration produced by a net force on an object is directly proportional to the magnitude of the net force, is the same direction as the net force, and is inversely proportional to the mass of the body.
Or in shorter notation,
A = F/M or F = MA
The acceleration is equal to the net force divided by the mass. Rearranging the equation we get the net force equals the mass times the acceleration. From this relationship we can see that if the net force that acts on an object is doubled, the acceleration will be doubled. Suppose instead that the mass is doubled. Then the acceleration will be halved. If both the net force and the mass are doubled, then the acceleration will be unchanged.
Click on the following link to answer the force questions
Force Questions
