Forces & Motion
Which SI unit measures force?
SI unit of force: Newton (N)
Write the formula that gives work when a constant force acts along a displacement
Work formula: W=F x d
What is power (brief definition)?
Definition: Power is the rate at which work is done or energy is transferred (work per unit time)
What distinguishes velocity from speed? Give a one-sentence answer and include the vector/scalar idea.
Distinction: Speed is a scalar that gives how fast an object moves; velocity is a vector that gives speed and direction.
Define density and give its formula
Density is mass per unit volume.
Formula: density=mass/volume
State Newton’s First Law in your own short sentence
Newton’s First Law (short): An object at rest stays at rest and an object in motion stays in motion at constant velocity unless acted on by a net external force.
A landscaper pushes a 12 kg cart 6 m with a 25 N horizontal force. Calculate the work done (assume force and motion are along the same line).
Work by landscaper: Force 25 N over 6 m along the same line → W=F*d=25×6=150 J
Write the formula relating power, work, and time.
Power formula: Power=Work / time
A runner completes 200 m east in 25 s. Compute the runner’s average velocity (magnitude and direction).
Runner:
Average velocity = displacement / time = 200 m east/25 s =8 m/s east
Explain how conduction and convection differ in heat transfer.
Conduction transfers heat through direct contact between particles (solids conduct heat), while convection transfers heat by bulk movement of a fluid (liquids or gases) carrying warmer material upward and cooler material downward.
A 4 kg toy accelerates at 2 m/s2. Using F=ma, calculate the net force on the toy (show units).
F=ma: F=4 kg×2 m/s2=8 N
Define gravitational potential energy and give its formula using mass, gravity, and height.
Gravitational potential energy: PE=m x g x h
Raul exerted 50 N over 0.25 m in 2 s. Calculate the power in watts (W or J/s) used during the shove. (Show main step.)
Raul’s shove:
Work = W=F*d=50 N×0.25 m=12.5 J
Power = P=W/t=12.5 J / 2 s=6.25 Watts (W or J/s)
Define acceleration and list the three types of change acceleration can describe
Acceleration is the rate of change of velocity. It can mean increasing speed, decreasing speed (deceleration), or change of direction.
Explain why the tilt of Earth causes seasons
Earth’s tilt causes seasons because as Earth orbits the Sun, different hemispheres tilt toward or away from the Sun, changing the angle and duration of sunlight and therefore seasonal temperatures.
Two horizontal forces act on a block: 20 N left and 10 N right. What is the net force and its direction?
Net force: 20 N left and 10 N right → net = 20−10=10 N to the left.
Is carrying a rock “work”? Give a short explanation that uses the definition of work.
Is carrying a rock work? Answer: No, if carrying the rock horizontally at constant height with force vertical balancing gravity (no displacement in direction of applied force), then by physics definition no work is done by that applied force on the rock in the direction of the force; if you lift it (vertical displacement in direction of force) then yes. (Accept clear explanation referencing displacement and force direction.)
A forklift applies 12,000 N to lift a crate 4 m in 3 s. Compute the average power output.
Forklift:
Work = W=F*d=12,000 N×4 m=48,000 J
Power = P=48,000 J / 3 s=16,000 W
A toy car moves from x=2 mx=2 m to x=−3 mx=−3 m in 4 s. Determine the average velocity and the average speed.
Toy car from x=2x=2 to x=−3x=−3 in 4 s:
A balloon is inflated and then released; it flies around the room. Which of Newton’s laws most directly explains this motion? Name the law and give a brief justification.
Newton’s Third Law: for every action there is an equal and opposite reaction — the escaping air pushes back on the balloon and the balloon is pushed forward.
Explain, using Newton’s Second Law, why a car and a truck pushed with the same force accelerate differently (mention mass).
Explanation (Newton’s 2nd): For the same applied force, acceleration is a=F/m so the more massive truck has a smaller acceleration than the less massive car
A skier of mass 70 kg descends 20 m vertically. Assuming no energy losses, calculate the change in gravitational potential energy (use g=9.8 m/s2).
Change in gravitational PE for skier:
ΔPE=m*g*h=70 kg×9.8 m/s2×20 m=13,720 J
An elevator raises an 800 kg load 12 m in 15 s. Assuming no losses, calculate the average power required (use g=9.8 m/s2). Then state one real factor that would make the motor need more power in practice.
Elevator:
W=m*g*h=800kg×9.8m/s2×12 m=94,080 J
Power = P=94,080 J / 15 s ≈6,272 W or J/s
Practical factor increasing required power: frictional losses, motor inefficiency, cable/machine heating, or additional safety margin
A skateboarder’s velocity changes from 3 m/s to 11 m/s in 4 s with constant acceleration. Find the acceleration (a=vfinal−vinitial / t).
Skateboarder: Acceleration a=vf−vi / t = 11−3 /4= 2 m/s2
What particle in an atom has a negative charge?
The electron has a negative charge.