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Physics dynamics 2

# Physics dynamics 2 - Physics 101 Lecture 6 Dynamics II In...

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1 PHYS 101 - Lecture 6 1 In this lecture (Giancoli Chapter 4, continued): - Contact forces: can change quickly, usually normal to surface - Reaction forces: act on different bodies - Free-body diagrams: the way to start most force problems! - Forces applied by ropes & pulleys - Friction kinetic & static - Inclined planes Physics 101 – Lecture 6 Dynamics II PHYS 101 - Lecture 6 2 We were discussing reaction forces at the end of the previous lecture. Be careful to note that reaction forces act on different objects. Also important are contact forces that arise if two objects are in contact. For example, there is a force of gravity of some 750 N on me in the downward direction. I’m not accelerating downward, so there’s a balancing force (and thus no net force): The force of gravity is balanced by a contact force of the floor on me, of 750 N. F gravity F floor Reaction vs. Contact forces

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2 PHYS 101 - Lecture 6 3 Note that the contact force is NOT a reaction force. The contact force, like the force of gravity, acts on me . Here’s a simple illustration of the contact force NOT being the reaction force: suppose I jump up. What happens? v 0 g The contact force is often called a normal force since it is often normal (perpendicular) to the surface. PHYS 101 - Lecture 6 4 So, that begs some questions : Where is the reaction force ? What happened to the contact force? Where is the contact force coming from ? While I’m in the air, the reaction force hasn’t disappeared, but the contact force must have (I’m no longer in contact!). I have acceleration of -g and hence a net force on me of 750 N downwards. F g No contact: no contact force! v 0 g
3 PHYS 101 - Lecture 6 5 Firstly, the reaction force. Remember the 3 rd law: If an object exerts a force on another, the second object exerts an equal and opposite force on the first. We’re talking about the force of gravity on me by the Earth, so the reaction force is the force exerted by me on the Earth. This reaction force acts on the Earth, and doesn’t change when I jump in the air. Of course, in the general scheme of the motion of the planet, it’s small potatoes (my mass: ~10 2 kg, Earth’s mass: 6 x 10 24 kg). F gravity = F Earth F gravity = F me PHYS 101 - Lecture 6 6 However, let’s digress for a moment to consider the Earth-Moon system. In the Earth-Moon system, the Moon orbits the Earth because of the force of gravity from the Earth. But the Moon exerts a gravitational pull on the Earth that is equal in magnitude to the force on the Moon. What happens? The Earth has a mass ~81 times greater than the Moon’s, so the acceleration of the Earth will be 81 times smaller in magnitude than the acceleration of the Moon. But the Earth is in motion because of this force. In fact, the two objects orbit a common centre- of-mass (CM) that is NOT the centre of the Earth.

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