1.4 Newton's third law of motion

Describing newton’s third law of motion

If you have ever stubbed your toe, you have noticed that although your toe initiates the impact, the surface that you stub it on exerts a force back on your toe. Although the first thought that crosses your mind is probably “ouch, that hurt” rather than “this is a great example of Newton’s third law,” both statements are true.

This is exactly what happens whenever one object exerts a force on another—each object experiences a force that is the same strength as the force acting on the other object but that acts in the opposite direction. Everyday experiences, such as stubbing a toe or throwing a ball, are all perfect examples of Newton’s third law in action.

Newton’s third law of motion states that, whenever a first object exerts a force on a second object, the first object experiences a force equal in magnitude but opposite in direction to the force that it exerts

Newton’s third law of motion tells us that forces always occur in pairs, and one object cannot exert a force on another without experiencing the same strength force in return. We sometimes refer to these force pairs as “action-reaction” pairs, where the force exerted is the action, and the force experienced in return is the reaction (although which is which depends on your point of view).

Newton’s third law is useful for figuring out which forces are external to a system. Recall that identifying external forces is important when setting up a problem, because the external forces must be added together to find the net force.

Snap Lab

Mass and weight

In this activity you will use a scale to investigate mass and weight.

Materials
• 1 scale
• 1 table

Safety Warning
• Organisms: This lab uses organisms that can pass on diseases.

1. What do bathroom scales measure?
2. When you stand on a bathroom scale, what happens to the scale? It depresses slightly. The scale contains springs that compress in proportion to your weight—similar to rubber bands expanding when pulled. Newton’s 1st law: Fnet = 0 or (or ∑F = 01)
3. The springs provide a measure of your weight (provided you are not accelerating). This is a force in newtons (or pounds). In most countries, the measurement is now divided by 9.80 to give a reading in kilograms, which are units of mass. The scale detects weight but is calibrated to display mass.
4. If you went to the Moon and stood on your scale, would it detect the same “mass” as it did on Earth? How about: a. mars b. venus c.saturn

Planet	Mass	Weight (Fnet = 0 or (or ∑F = 01)
Mars	12kg	5N
Venus	20kg	90N
Saturn	1kg	13N

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Teacher Edition

Explain that, even though a scale gives a mass, it actually measures weight. Scales are calibrated to show the correct mass on Earth. They would give different results on the Moon, because the force of gravity is weaker on the Moon.