Forces and newton’s laws
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Transcript of Forces and newton’s laws
Forces and Newton’s Laws
Ch 6, 7.1
Force
• a push or a pull
• Measured in Newtons (N)
Types of Forces
Gravitational Force
• Attractive force that exists between all objects• Weakest of the 4 forces, but acts over the
longest distance
Electromagnetic
• Gives materials their strength, ability to bend, squeeze, stretch, etc.
Strong Nuclear Force
• Holds the particles in a nucleus together• strongest of the 4 forces, but acts over the
smallest distance.
Weak force
• Involved in the radioactive decay of some nuclei• form of the electromagnetic force
Newton’s Laws of Motion
1st Law - Inertia
• Any object will continue at constant velocity until acted on by some outside force.
2nd Law - F=ma
• The acceleration of an object is directly proportional to the net force on it and inversely proportional to its mass.
3rd Law - Action-Reaction pairs
• For every action there is an equal and opposite reaction.
Question
• The horse knows if he pulls with 100N of force and the carriage pulls back with 100 N of force then the carriage isn’t going to move. Is his logic right?
Net Force vs Action-Reaction Pairs
• Action-reaction pairs work on different objects and they are always equal to opposite; EX: I pull on the cart and the cart pulls on me.
• Net force is the sum of all forces and it causes acceleration; the forces work on the same object; EX: I pull on the cart and friction pulls on the cart.
Mass vs Weight
• Mass - amount of matter or inertia an object has.
• Weight - gravitational force exerted by the earth on an objects mass.
• W=mg
• Normal Force - force that is perpendicular to the contact surface.
Friction
• Force that opposes motion between 2 surfaces that are in contact.
Types of Friction
Static Friction
• Force that opposes the start of motion.
Sliding (Kinetic) Friction
• Force that opposes motion once the object is moving.
• Note: Static friction is greater than sliding friction because of Newton’s First Law
• Ff = μN
Example Problems
• A 12 kg box that is pulled across the floor at 2 m/s2 with a force of 40 Newtons. Find the coefficient of friction.
• A 400 kg crate is accelerated upward at 3 m/s2 by a crane. Find the tension in the cable.
Equilibrant
• Force equal and opposite to the resultant.
• Ex: Find the equilibrant for 20 N @ 125 degrees.
• Answer: 20 N @ 305 degrees
Example Problems
• A 30 kg box is on a 35 degree incline plane. Find:
• A. The force that causes the box to move down the incline.
• B. The normal force.
• C. The acceleration if μ = 0.25
• A 400 kg sign is hung by cables that are 110 degrees apart. Find the tension in each cable.
• A 50 kg box is pulled across the ground with a force of 300 Newtons @ 35 degrees and μ = 0.3. Find the acceleration.