# SPH3U Grade 11 Physics Forces Test

Unit 2: Forces Review Notes

Force

– A push or pull measured in Newtons (N), vector quantity.

– Objects need a force to accelerate or slowdown.

4 Fundamental Forces

Gravitational Force- force of attraction between all objects

Electromagnetic Force- force caused by electric c

harges

Strong Nuclear Force- force that holds protons and neutrons together in the nucleus

Weak Nuclear Force- force responsible for interactions, really small

Other Forces

–          Friction

–          Magnetic

–          Electrostatic

–          Normal

–          Buoyancy

–          Tension

–          Gravity

–          Applied

Contact Force

–          Forces that result from two objects in contact with one another

Net Force

–          If there is a net force, it could accelerate or decelerate an object

–          If an object is at rest or moving at a constant speed, the net force is 0

Free Body Diagrams –          A diagram that shows all forces on an object

Newton’s Laws

1. “If the net force on an object is zero, the object will maintain its state of rest or constant velocity.”

In summary,

–          Objects at rest stay at rest

–          Objects in motion tend to remain in motion

–          Object moving at constant velocity has a net force of 0

–          Object that changes its velocity is caused by change in net force

1. When the net external force on an object is not 0, object accelerates in the direction of the net force. Acceleration is proportional to net force. Acceleration is inversely proportional to object’s mass.

Gravitational Force

–          Gravitational field strength on Earth’s surface is 9.8m/s2 [down]

–          Fg= m x g ¬ Used to find the force of gravity on an object

Example: Calculate the force of gravity acting on a 5.0kg ball.

Given            M=5.0kg

g= 9.8m/s2[down]

Fg=?

Fg=m x g

Fg=5.0kg x 9.8m/s2[down]

Fg=49N[down]

Universal Gravitation

–          G is a constant= 6.67 x 10-11 (Nxm2)/kg2

Used in the equation:

Fg= (Gm1m2)/d2

Friction

–          Acts between two surfaces

–          Acts parallel to the surfaces and opposes forces that attempt to accelerate the object along the surface

–          The force of friction is proportional to the normal force acting on an object

• Ff= μ x FN

μ= co-efficient of friction

Depends on the two material that are in contact

Static Friction- A force that tends to prevent a stationary object from moving or starting to move

Kinetic Friction- A force that acts against an object’s motion in a direction opposite to the direction of motion.

μs= co-efficient of static friction

μk= co-efficient of kinetic friction

Force of static friction is only has high as it needs to be. Cannot be greater than the force that it is opposing.

Equations

FFS= μx FN

FFK= μK x FN

Fg= m x g

Fg= (Gm1m2)/d2