How Forces Affect Motion · Lesson 1 of 8
The Concept of Force
“A force is not merely a push or pull; it is a directed interaction that can rewrite an object’s motion or shape.”
• Describe the different effects a force can produce. • Explain why both magnitude and direction are needed to specify a force. • State and use the SI unit of force correctly. • Explain how a spring balance measures force. • Distinguish mass from weight.
A football lying on the ground does not begin to roll by itself. A player’s foot interacts with it, and the ball starts moving. A goalkeeper may then stop the same ball, a defender may turn it sideways, and a hard strike may briefly compress its surface. These apparently different events share one idea: an interaction has changed the state or shape of an object. We describe the strength and direction of that interaction using the physical quantity called force.
What A Force Can Do
A force can set an object at rest into motion, slow down or speed up a moving object, change the direction in which it moves, or change its shape. More than one effect can occur together. When a bat strikes a moving ball, the ball may leave with a different speed and in a different direction, while both bat and ball deform for a very short time. Therefore, the effect of force should be judged by observing the complete change, not by looking only for motion.
Force is a physical quantity that describes a push or pull arising from an interaction and capable of changing an object’s state of motion or shape.
Magnitude And Direction
Saying that a force is strong is incomplete. We must also say where it acts. A force of the same magnitude can produce entirely different results when its direction changes. Push a trolley forwards and it speeds up; apply the same force backwards and it slows down; apply it sideways and its direction changes. Force therefore has magnitude as well as direction.
The magnitude tells us the strength of the force. Direction tells us the way in which the force tends to accelerate or deform the object. If either magnitude or direction changes, the effect can change. This is why arrows are useful in force diagrams: the arrowhead shows direction and the arrow length can represent relative magnitude.
Unit Of Force
The newton is the SI unit of force. Its symbol is N. One newton is defined precisely through the relationship between force, mass, and acceleration developed later in the chapter.
The unit name is written as newton with a small first letter, while its symbol is the capital letter N. A statement such as “the force is five” is incomplete; a measured physical quantity must include its unit, so it should be written as “the force is 5 N”.
Measuring The Magnitude Of A Force
A spring balance contains a spring that stretches when pulled. Within its working range, a stronger pull produces a greater extension. The scale beside the spring is calibrated so that the extension can be read directly as force. Before measuring, the pointer should be checked at zero. The pull should be along the axis of the balance, and the reading should be taken only after the pointer becomes steady.
The same instrument can measure the pull needed to drag a block, the force exerted by a stretched elastic object, or the weight of an object. In every case, it reports the magnitude of force in newtons. It does not by itself provide a complete force description; the direction must still be stated from the arrangement.
Weight is the gravitational force with which the Earth attracts an object. It is a force, so it is measured in newtons and acts towards the Earth.
Mass and weight are related but not identical. Mass describes how much an object resists acceleration and is measured in kilograms. Weight is a force and depends on the gravitational field. A spring balance measures weight, whereas a weighing balance designed to compare masses reports mass.
Activity: Reading A Spring Balance
Hang a suitable object from a spring balance and keep the balance vertical. First note whether the pointer reads zero without a load. Add the object, allow oscillations to settle, and read the scale at eye level. Record both the number and N. Repeat the reading. Then pull the same object horizontally along a table with the balance. The two readings represent different forces: the vertical reading is weight, while the horizontal reading is the pulling force required in that situation.
Problem
A student says, “I applied a force of 12 N to a box.” Is the description complete?
- 1.Identify what has been given: the magnitude is 12 N.
- 2.Recall that force requires magnitude and direction.
- 3.Notice that the direction has not been stated.
- 4.Conclude that the description is incomplete. A complete statement could be: “A force of 12 N was applied horizontally towards the right.”
Quiz
Which description best matches Force?
Which description best matches Newton?
Which term matches this description: Force is a physical quantity that describes a push or pull arising from an interaction and capable of changing an object’s state of motion or shape.
Which term matches this description: The newton is the SI unit of force.
Which statement is a key takeaway from this lesson?
Key Takeaways
• A force can change motion, direction, speed, or shape. • Force requires both magnitude and direction. • The SI unit of force is the newton, symbol N. • A spring balance measures force magnitude. • Weight is a gravitational force, while mass is not a force.
Real objects usually experience several forces at once. The next lesson develops a way to combine them into one net force.
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Balanced and Unbalanced Forces