# Unit II Forces.

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Unit II Forces

Force = a push or pull on an object measured in Newtons (N)
4 Ways Force Effects an Object Increase v 2. Decrease v 3. Change Direction 4. Change Shape

What are the forces action on box being pushed?

Which box is moving? A B

Net Force = sum of forces acting on an object
Forces are vectors Same direction add Opposite direction subtract

Balanced vs. Unbalanced Forces
Unbalanced Forces: the forces are not equal and opposite -the object will change -the object will accelerate Balanced Forces: the forces on an object are equal and opposite. -no change to the motion -object does not accelerate

Friction: force that opposes motion of an object
1. Static Friction 2. Rolling Friction 3. Sliding Friction 4. Fluid Friction

Laws of Motion

1st Law of Motion An Object in motion will remain in motion, or an object at rest will remain at rest unless acted upon by an unbalanced force teachertech.rice.edu/.../Newton/law1.html

Inertia The tendency of an object to keep doing what it is already doing

2nd Law of Motion Force, mass and acceleration are all related

2nd Law Force = mass x acceleration F = ma

3rd Law of Motion Action and Reaction
For every action there is an equal and opposite reaction

How is weight different than mass?
Weight  pull of gravity acting on an object Weight = mass x acceleration due to gravity Mass  amount of matter in an object

Sample Problems 1. A boy pushes a cart of groceries forward with a total mass of 40 kg. What is the acceleration of the car if the net force on the cart is 60 N? 1.5 m/s²

2. What is the upward acceleration of a helicopter with a mass of 5000 kg if a force of N act on it in an upward direction? 2 m/s²

3. An automobile with a mass of 1200 kg accelerates at a rate of 3 m/s/s in the forward direction. What is the net force acting on the automobile? 3600 N

4. A 25 N force accelerates a boy in a wheelchair at 0. 5 m/s/s
4. A 25 N force accelerates a boy in a wheelchair at 0.5 m/s/s. What is the mass of the boy and the wheelchair? 50 kg

Sample Problems 5. A girl pushes a box of books forward with a total mass of 80 kg. What is the acceleration of the box if the net force on the box is 40 N? 0.5 m/s²

6. An truck with a mass of 3600 kg accelerates at a rate of 2 m/s/s in the forward direction. What is the net force acting on the automobile? 7200 N

7. A 30 N force accelerates a boy on a tricycle at 0. 5 m/s/s
7. A 30 N force accelerates a boy on a tricycle at 0.5 m/s/s. What is the mass of the boy and the tricycle? 60 kg

14.1 Work Definition: the product of distance and the force in the direction an object moves W = F x d SI unit for work: newton-meter (N·m) or joule (J) If there is no movement, no work is done

Power What is power? Power = Work SI Unit – Watt (W)
The rate of doing work Doing work faster requires more power Power = Work Time SI Unit – Watt (W)

Momentum Momentum = mass x velocity of an object
Momentum is large if product of mass and velocity is large Objects with large momentum’s are hard to stop

Momentum Momentum of an object at rest = 0 SI Units = kg*m s

Momentum Law of Conservation of Momentum
If no net force acts on the system, the total momentum of the system does not change. In a closed system, momentum is conserved.

Momentum Problem Which has more momentum, a kg golf ball with a velocity of 60 m/s or a 7.0 bowling ball with a velocity of 6 m/s? Remember: Momentum = mass x velocity

12.4 Universal Forces What are the 4 universal forces?
Electromagnetic Nuclear (Strong and Weak) Gravity How are these different from forces talked about in the ? All act over a distance between particles

Describe each universal force and what makes them unique.
Electromagnetic Force only force that can attract and repel Gravitational force An attractive force between 2 masses

Describe each universal force and what makes them unique.
Nuclear force: act within nucleus to hold it together Strong – acts only on neutrons and protons in the nucleus (100x stronger than charges) Weak – attractive force that acts over a short range

Force Can be represented by Can be Measured in units of unbalanced
Causes no change to the object Causes the object to Force unbalanced

Force Can be represented by Can be arrows Measured in units of
unbalanced Newtons balanced Causes the object to Causes no change to the object State of Motion Accelerate

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