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.